libdispatch-703.50.37.tar.gz

[apple/libdispatch.git] / src / io.c

/*
 * Copyright (c) 2009-2013 Apple Inc. All rights reserved.
 *
 * @APPLE_APACHE_LICENSE_HEADER_START@
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * @APPLE_APACHE_LICENSE_HEADER_END@
 */

#include "internal.h"

#ifndef DISPATCH_IO_DEBUG
#define DISPATCH_IO_DEBUG DISPATCH_DEBUG
#endif

#ifndef PAGE_SIZE
#define PAGE_SIZE getpagesize()
#endif

#if DISPATCH_DATA_IS_BRIDGED_TO_NSDATA
#define _dispatch_io_data_retain(x) _dispatch_objc_retain(x)
#define _dispatch_io_data_release(x) _dispatch_objc_release(x)
#else
#define _dispatch_io_data_retain(x) dispatch_retain(x)
#define _dispatch_io_data_release(x) dispatch_release(x)
#endif

typedef void (^dispatch_fd_entry_init_callback_t)(dispatch_fd_entry_t fd_entry);

DISPATCH_EXPORT DISPATCH_NOTHROW
void _dispatch_iocntl(uint32_t param, uint64_t value);

static dispatch_operation_t _dispatch_operation_create(
                dispatch_op_direction_t direction, dispatch_io_t channel, off_t offset,
                size_t length, dispatch_data_t data, dispatch_queue_t queue,
                dispatch_io_handler_t handler);
static void _dispatch_operation_enqueue(dispatch_operation_t op,
                dispatch_op_direction_t direction, dispatch_data_t data);
static dispatch_source_t _dispatch_operation_timer(dispatch_queue_t tq,
                dispatch_operation_t op);
static inline void _dispatch_fd_entry_retain(dispatch_fd_entry_t fd_entry);
static inline void _dispatch_fd_entry_release(dispatch_fd_entry_t fd_entry);
static void _dispatch_fd_entry_init_async(dispatch_fd_t fd,
                dispatch_fd_entry_init_callback_t completion_callback);
static dispatch_fd_entry_t _dispatch_fd_entry_create_with_fd(dispatch_fd_t fd,
                uintptr_t hash);
static dispatch_fd_entry_t _dispatch_fd_entry_create_with_path(
                dispatch_io_path_data_t path_data, dev_t dev, mode_t mode);
static int _dispatch_fd_entry_open(dispatch_fd_entry_t fd_entry,
                dispatch_io_t channel);
static void _dispatch_fd_entry_cleanup_operations(dispatch_fd_entry_t fd_entry,
                dispatch_io_t channel);
static void _dispatch_stream_init(dispatch_fd_entry_t fd_entry,
                dispatch_queue_t tq);
static void _dispatch_stream_dispose(dispatch_fd_entry_t fd_entry,
                dispatch_op_direction_t direction);
static void _dispatch_disk_init(dispatch_fd_entry_t fd_entry, dev_t dev);
static void _dispatch_stream_enqueue_operation(dispatch_stream_t stream,
                dispatch_operation_t operation, dispatch_data_t data);
static void _dispatch_disk_enqueue_operation(dispatch_disk_t dsk,
                dispatch_operation_t operation, dispatch_data_t data);
static void _dispatch_stream_cleanup_operations(dispatch_stream_t stream,
                dispatch_io_t channel);
static void _dispatch_disk_cleanup_inactive_operations(dispatch_disk_t disk,
                dispatch_io_t channel);
static void _dispatch_stream_source_handler(void *ctx);
static void _dispatch_stream_queue_handler(void *ctx);
static void _dispatch_stream_handler(void *ctx);
static void _dispatch_disk_handler(void *ctx);
static void _dispatch_disk_perform(void *ctxt);
static void _dispatch_operation_advise(dispatch_operation_t op,
                size_t chunk_size);
static int _dispatch_operation_perform(dispatch_operation_t op);
static void _dispatch_operation_deliver_data(dispatch_operation_t op,
                dispatch_op_flags_t flags);

// Macros to wrap syscalls which return -1 on error, and retry on EINTR
#define _dispatch_io_syscall_switch_noerr(_err, _syscall, ...) do { \
                switch (((_err) = (((_syscall) == -1) ? errno : 0))) { \
                case EINTR: continue; \
                __VA_ARGS__ \
                } \
                break; \
        } while (1)
#define _dispatch_io_syscall_switch(__err, __syscall, ...) do { \
                _dispatch_io_syscall_switch_noerr(__err, __syscall, \
                case 0: break; \
                __VA_ARGS__ \
                ); \
        } while (0)
#define _dispatch_io_syscall(__syscall) do { int __err; \
                _dispatch_io_syscall_switch(__err, __syscall); \
        } while (0)

enum {
        DISPATCH_OP_COMPLETE = 1,
        DISPATCH_OP_DELIVER,
        DISPATCH_OP_DELIVER_AND_COMPLETE,
        DISPATCH_OP_COMPLETE_RESUME,
        DISPATCH_OP_RESUME,
        DISPATCH_OP_ERR,
        DISPATCH_OP_FD_ERR,
};

#define _dispatch_io_Block_copy(x) \
                ((typeof(x))_dispatch_Block_copy((dispatch_block_t)(x)))

#pragma mark -
#pragma mark dispatch_io_debug

#if DISPATCH_IO_DEBUG
#if !DISPATCH_DEBUG
#define _dispatch_io_log(x, ...) do { \
                        _dispatch_log("%llu\t%p\t" x, _dispatch_absolute_time(), \
                        (void *)_dispatch_thread_self(), ##__VA_ARGS__); \
                } while (0)
#ifdef _dispatch_object_debug
#undef _dispatch_object_debug
#define _dispatch_object_debug dispatch_debug
#pragma clang diagnostic ignored "-Wdeprecated-declarations"
#endif
#else
#define _dispatch_io_log(x, ...) _dispatch_debug(x, ##__VA_ARGS__)
#endif // DISPATCH_DEBUG
#else
#define _dispatch_io_log(x, ...)
#endif // DISPATCH_IO_DEBUG

#define _dispatch_fd_debug(msg, fd, ...) \
                _dispatch_io_log("fd[0x%x]: " msg, fd, ##__VA_ARGS__)
#define _dispatch_op_debug(msg, op, ...) \
                _dispatch_io_log("op[%p]: " msg, op, ##__VA_ARGS__)
#define _dispatch_channel_debug(msg, channel, ...) \
                _dispatch_io_log("channel[%p]: " msg, channel, ##__VA_ARGS__)
#define _dispatch_fd_entry_debug(msg, fd_entry, ...) \
                _dispatch_io_log("fd_entry[%p]: " msg, fd_entry, ##__VA_ARGS__)
#define _dispatch_disk_debug(msg, disk, ...) \
                _dispatch_io_log("disk[%p]: " msg, disk, ##__VA_ARGS__)

#pragma mark -
#pragma mark dispatch_io_hashtables

// Global hashtable of dev_t -> disk_s mappings
DISPATCH_CACHELINE_ALIGN
static TAILQ_HEAD(, dispatch_disk_s) _dispatch_io_devs[DIO_HASH_SIZE];
// Global hashtable of fd -> fd_entry_s mappings
DISPATCH_CACHELINE_ALIGN
static TAILQ_HEAD(, dispatch_fd_entry_s) _dispatch_io_fds[DIO_HASH_SIZE];

static dispatch_once_t  _dispatch_io_devs_lockq_pred;
static dispatch_queue_t _dispatch_io_devs_lockq;
static dispatch_queue_t _dispatch_io_fds_lockq;

static char const * const _dispatch_io_key = "io";

static void
_dispatch_io_fds_lockq_init(void *context DISPATCH_UNUSED)
{
        _dispatch_io_fds_lockq = dispatch_queue_create(
                        "com.apple.libdispatch-io.fd_lockq", NULL);
        unsigned int i;
        for (i = 0; i < DIO_HASH_SIZE; i++) {
                TAILQ_INIT(&_dispatch_io_fds[i]);
        }
}

static void
_dispatch_io_devs_lockq_init(void *context DISPATCH_UNUSED)
{
        _dispatch_io_devs_lockq = dispatch_queue_create(
                        "com.apple.libdispatch-io.dev_lockq", NULL);
        unsigned int i;
        for (i = 0; i < DIO_HASH_SIZE; i++) {
                TAILQ_INIT(&_dispatch_io_devs[i]);
        }
}

#pragma mark -
#pragma mark dispatch_io_defaults

enum {
        DISPATCH_IOCNTL_CHUNK_PAGES = 1,
        DISPATCH_IOCNTL_LOW_WATER_CHUNKS,
        DISPATCH_IOCNTL_INITIAL_DELIVERY,
        DISPATCH_IOCNTL_MAX_PENDING_IO_REQS,
};

static struct dispatch_io_defaults_s {
        size_t chunk_size, low_water_chunks, max_pending_io_reqs;
        bool initial_delivery;
} dispatch_io_defaults = {
        .chunk_size = DIO_MAX_CHUNK_SIZE,
        .low_water_chunks = DIO_DEFAULT_LOW_WATER_CHUNKS,
        .max_pending_io_reqs = DIO_MAX_PENDING_IO_REQS,
};

#define _dispatch_iocntl_set_default(p, v) do { \
                dispatch_io_defaults.p = (typeof(dispatch_io_defaults.p))(v); \
        } while (0)

void
_dispatch_iocntl(uint32_t param, uint64_t value)
{
        switch (param) {
        case DISPATCH_IOCNTL_CHUNK_PAGES:
                _dispatch_iocntl_set_default(chunk_size, value * PAGE_SIZE);
                break;
        case DISPATCH_IOCNTL_LOW_WATER_CHUNKS:
                _dispatch_iocntl_set_default(low_water_chunks, value);
                break;
        case DISPATCH_IOCNTL_INITIAL_DELIVERY:
                _dispatch_iocntl_set_default(initial_delivery, value);
        case DISPATCH_IOCNTL_MAX_PENDING_IO_REQS:
                _dispatch_iocntl_set_default(max_pending_io_reqs, value);
                break;
        }
}

#pragma mark -
#pragma mark dispatch_io_t

static dispatch_io_t
_dispatch_io_create(dispatch_io_type_t type)
{
        dispatch_io_t channel = _dispatch_alloc(DISPATCH_VTABLE(io),
                        sizeof(struct dispatch_io_s));
        channel->do_next = DISPATCH_OBJECT_LISTLESS;
        channel->do_targetq = _dispatch_get_root_queue(_DISPATCH_QOS_CLASS_DEFAULT,
                        true);
        channel->params.type = type;
        channel->params.high = SIZE_MAX;
        channel->params.low = dispatch_io_defaults.low_water_chunks *
                        dispatch_io_defaults.chunk_size;
        channel->queue = dispatch_queue_create("com.apple.libdispatch-io.channelq",
                        NULL);
        return channel;
}

static void
_dispatch_io_init(dispatch_io_t channel, dispatch_fd_entry_t fd_entry,
                dispatch_queue_t queue, int err, void (^cleanup_handler)(int))
{
        // Enqueue the cleanup handler on the suspended close queue
        if (cleanup_handler) {
                _dispatch_retain(queue);
                dispatch_async(!err ? fd_entry->close_queue : channel->queue, ^{
                        dispatch_async(queue, ^{
                                _dispatch_channel_debug("cleanup handler invoke: err %d",
                                                channel, err);
                                cleanup_handler(err);
                        });
                        _dispatch_release(queue);
                });
        }
        if (fd_entry) {
                channel->fd_entry = fd_entry;
                dispatch_retain(fd_entry->barrier_queue);
                dispatch_retain(fd_entry->barrier_group);
                channel->barrier_queue = fd_entry->barrier_queue;
                channel->barrier_group = fd_entry->barrier_group;
        } else {
                // Still need to create a barrier queue, since all operations go
                // through it
                channel->barrier_queue = dispatch_queue_create(
                                "com.apple.libdispatch-io.barrierq", NULL);
                channel->barrier_group = dispatch_group_create();
        }
}

void
_dispatch_io_dispose(dispatch_io_t channel)
{
        _dispatch_object_debug(channel, "%s", __func__);
        if (channel->fd_entry &&
                        !(channel->atomic_flags & (DIO_CLOSED|DIO_STOPPED))) {
                if (channel->fd_entry->path_data) {
                        // This modification is safe since path_data->channel is checked
                        // only on close_queue (which is still suspended at this point)
                        channel->fd_entry->path_data->channel = NULL;
                }
                // Cleanup handlers will only run when all channels related to this
                // fd are complete
                _dispatch_fd_entry_release(channel->fd_entry);
        }
        if (channel->queue) {
                dispatch_release(channel->queue);
        }
        if (channel->barrier_queue) {
                dispatch_release(channel->barrier_queue);
        }
        if (channel->barrier_group) {
                dispatch_release(channel->barrier_group);
        }
}

static int
_dispatch_io_validate_type(dispatch_io_t channel, mode_t mode)
{
        int err = 0;
        if (S_ISDIR(mode)) {
                err = EISDIR;
        } else if (channel->params.type == DISPATCH_IO_RANDOM &&
                        (S_ISFIFO(mode) || S_ISSOCK(mode))) {
                err = ESPIPE;
        }
        return err;
}

static int
_dispatch_io_get_error(dispatch_operation_t op, dispatch_io_t channel,
                bool ignore_closed)
{
        // On _any_ queue
        int err;
        if (op) {
                channel = op->channel;
        }
        if (channel->atomic_flags & (DIO_CLOSED|DIO_STOPPED)) {
                if (!ignore_closed || channel->atomic_flags & DIO_STOPPED) {
                        err = ECANCELED;
                } else {
                        err = 0;
                }
        } else {
                err = op ? op->fd_entry->err : channel->err;
        }
        return err;
}

#pragma mark -
#pragma mark dispatch_io_channels

dispatch_io_t
dispatch_io_create(dispatch_io_type_t type, dispatch_fd_t fd,
                dispatch_queue_t queue, void (^cleanup_handler)(int))
{
        if (type != DISPATCH_IO_STREAM && type != DISPATCH_IO_RANDOM) {
                return DISPATCH_BAD_INPUT;
        }
        dispatch_io_t channel = _dispatch_io_create(type);
        channel->fd = fd;
        _dispatch_channel_debug("create", channel);
        channel->fd_actual = fd;
        dispatch_suspend(channel->queue);
        _dispatch_retain(queue);
        _dispatch_retain(channel);
        _dispatch_fd_entry_init_async(fd, ^(dispatch_fd_entry_t fd_entry) {
                // On barrier queue
                int err = fd_entry->err;
                if (!err) {
                        err = _dispatch_io_validate_type(channel, fd_entry->stat.mode);
                }
                if (!err && type == DISPATCH_IO_RANDOM) {
                        off_t f_ptr;
                        _dispatch_io_syscall_switch_noerr(err,
                                f_ptr = lseek(fd_entry->fd, 0, SEEK_CUR),
                                case 0: channel->f_ptr = f_ptr; break;
                                default: (void)dispatch_assume_zero(err); break;
                        );
                }
                channel->err = err;
                _dispatch_fd_entry_retain(fd_entry);
                _dispatch_io_init(channel, fd_entry, queue, err, cleanup_handler);
                dispatch_resume(channel->queue);
                _dispatch_object_debug(channel, "%s", __func__);
                _dispatch_release(channel);
                _dispatch_release(queue);
        });
        _dispatch_object_debug(channel, "%s", __func__);
        return channel;
}

dispatch_io_t
dispatch_io_create_f(dispatch_io_type_t type, dispatch_fd_t fd,
                dispatch_queue_t queue, void *context,
                void (*cleanup_handler)(void *context, int error))
{
        return dispatch_io_create(type, fd, queue, !cleanup_handler ? NULL :
                        ^(int error){ cleanup_handler(context, error); });
}

dispatch_io_t
dispatch_io_create_with_path(dispatch_io_type_t type, const char *path,
                int oflag, mode_t mode, dispatch_queue_t queue,
                void (^cleanup_handler)(int error))
{
        if ((type != DISPATCH_IO_STREAM && type != DISPATCH_IO_RANDOM) ||
                        !(*path == '/')) {
                return DISPATCH_BAD_INPUT;
        }
        size_t pathlen = strlen(path);
        dispatch_io_path_data_t path_data = malloc(sizeof(*path_data) + pathlen+1);
        if (!path_data) {
                return DISPATCH_OUT_OF_MEMORY;
        }
        dispatch_io_t channel = _dispatch_io_create(type);
        channel->fd = -1;
        _dispatch_channel_debug("create with path %s", channel, path);
        channel->fd_actual = -1;
        path_data->channel = channel;
        path_data->oflag = oflag;
        path_data->mode = mode;
        path_data->pathlen = pathlen;
        memcpy(path_data->path, path, pathlen + 1);
        _dispatch_retain(queue);
        _dispatch_retain(channel);
        dispatch_async(channel->queue, ^{
                int err = 0;
                struct stat st;
                _dispatch_io_syscall_switch_noerr(err,
                        (path_data->oflag & O_NOFOLLOW) == O_NOFOLLOW
#ifndef __linux__
                                        || (path_data->oflag & O_SYMLINK) == O_SYMLINK
#endif
                                        ? lstat(path_data->path, &st) : stat(path_data->path, &st),
                        case 0:
                                err = _dispatch_io_validate_type(channel, st.st_mode);
                                break;
                        default:
                                if ((path_data->oflag & O_CREAT) &&
                                                (*(path_data->path + path_data->pathlen - 1) != '/')) {
                                        // Check parent directory
                                        char *c = strrchr(path_data->path, '/');
                                        dispatch_assert(c);
                                        *c = 0;
                                        int perr;
                                        _dispatch_io_syscall_switch_noerr(perr,
                                                stat(path_data->path, &st),
                                                case 0:
                                                        // Since the parent directory exists, open() will
                                                        // create a regular file after the fd_entry has
                                                        // been filled in
                                                        st.st_mode = S_IFREG;
                                                        err = 0;
                                                        break;
                                        );
                                        *c = '/';
                                }
                                break;
                );
                channel->err = err;
                if (err) {
                        free(path_data);
                        _dispatch_io_init(channel, NULL, queue, err, cleanup_handler);
                        _dispatch_release(channel);
                        _dispatch_release(queue);
                        return;
                }
                dispatch_suspend(channel->queue);
                dispatch_once_f(&_dispatch_io_devs_lockq_pred, NULL,
                                _dispatch_io_devs_lockq_init);
                dispatch_async(_dispatch_io_devs_lockq, ^{
                        dispatch_fd_entry_t fd_entry = _dispatch_fd_entry_create_with_path(
                                        path_data, st.st_dev, st.st_mode);
                        _dispatch_io_init(channel, fd_entry, queue, 0, cleanup_handler);
                        dispatch_resume(channel->queue);
                        _dispatch_object_debug(channel, "%s", __func__);
                        _dispatch_release(channel);
                        _dispatch_release(queue);
                });
        });
        _dispatch_object_debug(channel, "%s", __func__);
        return channel;
}

dispatch_io_t
dispatch_io_create_with_path_f(dispatch_io_type_t type, const char *path,
                int oflag, mode_t mode, dispatch_queue_t queue, void *context,
                void (*cleanup_handler)(void *context, int error))
{
        return dispatch_io_create_with_path(type, path, oflag, mode, queue,
                        !cleanup_handler ? NULL :
                        ^(int error){ cleanup_handler(context, error); });
}

dispatch_io_t
dispatch_io_create_with_io(dispatch_io_type_t type, dispatch_io_t in_channel,
                dispatch_queue_t queue, void (^cleanup_handler)(int error))
{
        if (type != DISPATCH_IO_STREAM && type != DISPATCH_IO_RANDOM) {
                return DISPATCH_BAD_INPUT;
        }
        dispatch_io_t channel = _dispatch_io_create(type);
        _dispatch_channel_debug("create with channel %p", channel, in_channel);
        dispatch_suspend(channel->queue);
        _dispatch_retain(queue);
        _dispatch_retain(channel);
        _dispatch_retain(in_channel);
        dispatch_async(in_channel->queue, ^{
                int err0 = _dispatch_io_get_error(NULL, in_channel, false);
                if (err0) {
                        channel->err = err0;
                        _dispatch_io_init(channel, NULL, queue, err0, cleanup_handler);
                        dispatch_resume(channel->queue);
                        _dispatch_release(channel);
                        _dispatch_release(in_channel);
                        _dispatch_release(queue);
                        return;
                }
                dispatch_async(in_channel->barrier_queue, ^{
                        int err = _dispatch_io_get_error(NULL, in_channel, false);
                        // If there is no error, the fd_entry for the in_channel is valid.
                        // Since we are running on in_channel's queue, the fd_entry has been
                        // fully resolved and will stay valid for the duration of this block
                        if (!err) {
                                err = in_channel->err;
                                if (!err) {
                                        err = in_channel->fd_entry->err;
                                }
                        }
                        if (!err) {
                                err = _dispatch_io_validate_type(channel,
                                                in_channel->fd_entry->stat.mode);
                        }
                        if (!err && type == DISPATCH_IO_RANDOM && in_channel->fd != -1) {
                                off_t f_ptr;
                                _dispatch_io_syscall_switch_noerr(err,
                                        f_ptr = lseek(in_channel->fd_entry->fd, 0, SEEK_CUR),
                                        case 0: channel->f_ptr = f_ptr; break;
                                        default: (void)dispatch_assume_zero(err); break;
                                );
                        }
                        channel->err = err;
                        if (err) {
                                _dispatch_io_init(channel, NULL, queue, err, cleanup_handler);
                                dispatch_resume(channel->queue);
                                _dispatch_release(channel);
                                _dispatch_release(in_channel);
                                _dispatch_release(queue);
                                return;
                        }
                        if (in_channel->fd == -1) {
                                // in_channel was created from path
                                channel->fd = -1;
                                channel->fd_actual = -1;
                                mode_t mode = in_channel->fd_entry->stat.mode;
                                dev_t dev = in_channel->fd_entry->stat.dev;
                                size_t path_data_len = sizeof(struct dispatch_io_path_data_s) +
                                                in_channel->fd_entry->path_data->pathlen + 1;
                                dispatch_io_path_data_t path_data = malloc(path_data_len);
                                memcpy(path_data, in_channel->fd_entry->path_data,
                                                path_data_len);
                                path_data->channel = channel;
                                // lockq_io_devs is known to already exist
                                dispatch_async(_dispatch_io_devs_lockq, ^{
                                        dispatch_fd_entry_t fd_entry;
                                        fd_entry = _dispatch_fd_entry_create_with_path(path_data,
                                                        dev, mode);
                                        _dispatch_io_init(channel, fd_entry, queue, 0,
                                                        cleanup_handler);
                                        dispatch_resume(channel->queue);
                                        _dispatch_release(channel);
                                        _dispatch_release(queue);
                                });
                        } else {
                                dispatch_fd_entry_t fd_entry = in_channel->fd_entry;
                                channel->fd = in_channel->fd;
                                channel->fd_actual = in_channel->fd_actual;
                                _dispatch_fd_entry_retain(fd_entry);
                                _dispatch_io_init(channel, fd_entry, queue, 0, cleanup_handler);
                                dispatch_resume(channel->queue);
                                _dispatch_release(channel);
                                _dispatch_release(queue);
                        }
                        _dispatch_release(in_channel);
                        _dispatch_object_debug(channel, "%s", __func__);
                });
        });
        _dispatch_object_debug(channel, "%s", __func__);
        return channel;
}

dispatch_io_t
dispatch_io_create_with_io_f(dispatch_io_type_t type, dispatch_io_t in_channel,
                dispatch_queue_t queue, void *context,
                void (*cleanup_handler)(void *context, int error))
{
        return dispatch_io_create_with_io(type, in_channel, queue,
                        !cleanup_handler ? NULL :
                        ^(int error){ cleanup_handler(context, error); });
}

#pragma mark -
#pragma mark dispatch_io_accessors

void
dispatch_io_set_high_water(dispatch_io_t channel, size_t high_water)
{
        _dispatch_retain(channel);
        dispatch_async(channel->queue, ^{
                _dispatch_channel_debug("set high water: %zu", channel, high_water);
                if (channel->params.low > high_water) {
                        channel->params.low = high_water;
                }
                channel->params.high = high_water ? high_water : 1;
                _dispatch_release(channel);
        });
}

void
dispatch_io_set_low_water(dispatch_io_t channel, size_t low_water)
{
        _dispatch_retain(channel);
        dispatch_async(channel->queue, ^{
                _dispatch_channel_debug("set low water: %zu", channel, low_water);
                if (channel->params.high < low_water) {
                        channel->params.high = low_water ? low_water : 1;
                }
                channel->params.low = low_water;
                _dispatch_release(channel);
        });
}

void
dispatch_io_set_interval(dispatch_io_t channel, uint64_t interval,
                unsigned long flags)
{
        _dispatch_retain(channel);
        dispatch_async(channel->queue, ^{
                _dispatch_channel_debug("set interval: %llu", channel, interval);
                channel->params.interval = interval < INT64_MAX ? interval : INT64_MAX;
                channel->params.interval_flags = flags;
                _dispatch_release(channel);
        });
}

void
_dispatch_io_set_target_queue(dispatch_io_t channel, dispatch_queue_t dq)
{
        _dispatch_retain(dq);
        _dispatch_retain(channel);
        dispatch_async(channel->queue, ^{
                dispatch_queue_t prev_dq = channel->do_targetq;
                channel->do_targetq = dq;
                _dispatch_release(prev_dq);
                _dispatch_object_debug(channel, "%s", __func__);
                _dispatch_release(channel);
        });
}

dispatch_fd_t
dispatch_io_get_descriptor(dispatch_io_t channel)
{
        if (channel->atomic_flags & (DIO_CLOSED|DIO_STOPPED)) {
                return -1;
        }
        dispatch_fd_t fd = channel->fd_actual;
        if (fd == -1 && !_dispatch_io_get_error(NULL, channel, false)) {
                dispatch_thread_context_t ctxt =
                                _dispatch_thread_context_find(_dispatch_io_key);
                if (ctxt && ctxt->dtc_io_in_barrier == channel) {
                        (void)_dispatch_fd_entry_open(channel->fd_entry, channel);
                }
        }
        return channel->fd_actual;
}

#pragma mark -
#pragma mark dispatch_io_operations

static void
_dispatch_io_stop(dispatch_io_t channel)
{
        _dispatch_channel_debug("stop", channel);
        (void)os_atomic_or2o(channel, atomic_flags, DIO_STOPPED, relaxed);
        _dispatch_retain(channel);
        dispatch_async(channel->queue, ^{
                dispatch_async(channel->barrier_queue, ^{
                        _dispatch_object_debug(channel, "%s", __func__);
                        dispatch_fd_entry_t fd_entry = channel->fd_entry;
                        if (fd_entry) {
                                _dispatch_channel_debug("stop cleanup", channel);
                                _dispatch_fd_entry_cleanup_operations(fd_entry, channel);
                                if (!(channel->atomic_flags & DIO_CLOSED)) {
                                        channel->fd_entry = NULL;
                                        _dispatch_fd_entry_release(fd_entry);
                                }
                        } else if (channel->fd != -1) {
                                // Stop after close, need to check if fd_entry still exists
                                _dispatch_retain(channel);
                                dispatch_async(_dispatch_io_fds_lockq, ^{
                                        _dispatch_object_debug(channel, "%s", __func__);
                                        _dispatch_channel_debug("stop cleanup after close",
                                                        channel);
                                        dispatch_fd_entry_t fdi;
                                        uintptr_t hash = DIO_HASH(channel->fd);
                                        TAILQ_FOREACH(fdi, &_dispatch_io_fds[hash], fd_list) {
                                                if (fdi->fd == channel->fd) {
                                                        _dispatch_fd_entry_cleanup_operations(fdi, channel);
                                                        break;
                                                }
                                        }
                                        _dispatch_release(channel);
                                });
                        }
                        _dispatch_release(channel);
                });
        });
}

void
dispatch_io_close(dispatch_io_t channel, unsigned long flags)
{
        if (flags & DISPATCH_IO_STOP) {
                // Don't stop an already stopped channel
                if (channel->atomic_flags & DIO_STOPPED) {
                        return;
                }
                return _dispatch_io_stop(channel);
        }
        // Don't close an already closed or stopped channel
        if (channel->atomic_flags & (DIO_CLOSED|DIO_STOPPED)) {
                return;
        }
        _dispatch_retain(channel);
        dispatch_async(channel->queue, ^{
                dispatch_async(channel->barrier_queue, ^{
                        _dispatch_object_debug(channel, "%s", __func__);
                        _dispatch_channel_debug("close", channel);
                        if (!(channel->atomic_flags & (DIO_CLOSED|DIO_STOPPED))) {
                                (void)os_atomic_or2o(channel, atomic_flags, DIO_CLOSED,
                                                relaxed);
                                dispatch_fd_entry_t fd_entry = channel->fd_entry;
                                if (fd_entry) {
                                        if (!fd_entry->path_data) {
                                                channel->fd_entry = NULL;
                                        }
                                        _dispatch_fd_entry_release(fd_entry);
                                }
                        }
                        _dispatch_release(channel);
                });
        });
}

void
dispatch_io_barrier(dispatch_io_t channel, dispatch_block_t barrier)
{
        _dispatch_retain(channel);
        dispatch_async(channel->queue, ^{
                dispatch_queue_t io_q = channel->do_targetq;
                dispatch_queue_t barrier_queue = channel->barrier_queue;
                dispatch_group_t barrier_group = channel->barrier_group;
                dispatch_async(barrier_queue, ^{
                        dispatch_suspend(barrier_queue);
                        dispatch_group_notify(barrier_group, io_q, ^{
                                dispatch_thread_context_s io_ctxt = {
                                        .dtc_key = _dispatch_io_key,
                                        .dtc_io_in_barrier = channel,
                                };

                                _dispatch_object_debug(channel, "%s", __func__);
                                _dispatch_thread_context_push(&io_ctxt);
                                barrier();
                                _dispatch_thread_context_pop(&io_ctxt);
                                dispatch_resume(barrier_queue);
                                _dispatch_release(channel);
                        });
                });
        });
}

void
dispatch_io_barrier_f(dispatch_io_t channel, void *context,
                dispatch_function_t barrier)
{
        return dispatch_io_barrier(channel, ^{ barrier(context); });
}

void
dispatch_io_read(dispatch_io_t channel, off_t offset, size_t length,
                dispatch_queue_t queue, dispatch_io_handler_t handler)
{
        _dispatch_retain(channel);
        _dispatch_retain(queue);
        dispatch_async(channel->queue, ^{
                dispatch_operation_t op;
                op = _dispatch_operation_create(DOP_DIR_READ, channel, offset,
                                length, dispatch_data_empty, queue, handler);
                if (op) {
                        dispatch_queue_t barrier_q = channel->barrier_queue;
                        dispatch_async(barrier_q, ^{
                                _dispatch_operation_enqueue(op, DOP_DIR_READ,
                                                dispatch_data_empty);
                        });
                }
                _dispatch_release(channel);
                _dispatch_release(queue);
        });
}

void
dispatch_io_read_f(dispatch_io_t channel, off_t offset, size_t length,
                dispatch_queue_t queue, void *context,
                dispatch_io_handler_function_t handler)
{
        return dispatch_io_read(channel, offset, length, queue,
                        ^(bool done, dispatch_data_t d, int error){
                handler(context, done, d, error);
        });
}

void
dispatch_io_write(dispatch_io_t channel, off_t offset, dispatch_data_t data,
                dispatch_queue_t queue, dispatch_io_handler_t handler)
{
        _dispatch_io_data_retain(data);
        _dispatch_retain(channel);
        _dispatch_retain(queue);
        dispatch_async(channel->queue, ^{
                dispatch_operation_t op;
                op = _dispatch_operation_create(DOP_DIR_WRITE, channel, offset,
                                dispatch_data_get_size(data), data, queue, handler);
                if (op) {
                        dispatch_queue_t barrier_q = channel->barrier_queue;
                        dispatch_async(barrier_q, ^{
                                _dispatch_operation_enqueue(op, DOP_DIR_WRITE, data);
                                _dispatch_io_data_release(data);
                        });
                } else {
                        _dispatch_io_data_release(data);
                }
                _dispatch_release(channel);
                _dispatch_release(queue);
        });
}

void
dispatch_io_write_f(dispatch_io_t channel, off_t offset, dispatch_data_t data,
                dispatch_queue_t queue, void *context,
                dispatch_io_handler_function_t handler)
{
        return dispatch_io_write(channel, offset, data, queue,
                        ^(bool done, dispatch_data_t d, int error){
                handler(context, done, d, error);
        });
}

void
dispatch_read(dispatch_fd_t fd, size_t length, dispatch_queue_t queue,
                void (^handler)(dispatch_data_t, int))
{
        _dispatch_retain(queue);
        _dispatch_fd_entry_init_async(fd, ^(dispatch_fd_entry_t fd_entry) {
                // On barrier queue
                if (fd_entry->err) {
                        int err = fd_entry->err;
                        dispatch_async(queue, ^{
                                _dispatch_fd_debug("convenience handler invoke", fd);
                                handler(dispatch_data_empty, err);
                        });
                        _dispatch_release(queue);
                        return;
                }
                // Safe to access fd_entry on barrier queue
                dispatch_io_t channel = fd_entry->convenience_channel;
                if (!channel) {
                        channel = _dispatch_io_create(DISPATCH_IO_STREAM);
                        channel->fd = fd;
                        channel->fd_actual = fd;
                        channel->fd_entry = fd_entry;
                        dispatch_retain(fd_entry->barrier_queue);
                        dispatch_retain(fd_entry->barrier_group);
                        channel->barrier_queue = fd_entry->barrier_queue;
                        channel->barrier_group = fd_entry->barrier_group;
                        fd_entry->convenience_channel = channel;
                }
                __block dispatch_data_t deliver_data = dispatch_data_empty;
                __block int err = 0;
                dispatch_async(fd_entry->close_queue, ^{
                        dispatch_async(queue, ^{
                                _dispatch_fd_debug("convenience handler invoke", fd);
                                handler(deliver_data, err);
                                _dispatch_io_data_release(deliver_data);
                        });
                        _dispatch_release(queue);
                });
                dispatch_operation_t op =
                        _dispatch_operation_create(DOP_DIR_READ, channel, 0,
                                        length, dispatch_data_empty,
                                        _dispatch_get_root_queue(_DISPATCH_QOS_CLASS_DEFAULT,false),
                                        ^(bool done, dispatch_data_t data, int error) {
                                if (data) {
                                        data = dispatch_data_create_concat(deliver_data, data);
                                        _dispatch_io_data_release(deliver_data);
                                        deliver_data = data;
                                }
                                if (done) {
                                        err = error;
                                }
                        });
                if (op) {
                        _dispatch_operation_enqueue(op, DOP_DIR_READ, dispatch_data_empty);
                }
        });
}

void
dispatch_read_f(dispatch_fd_t fd, size_t length, dispatch_queue_t queue,
                void *context, void (*handler)(void *, dispatch_data_t, int))
{
        return dispatch_read(fd, length, queue, ^(dispatch_data_t d, int error){
                handler(context, d, error);
        });
}

void
dispatch_write(dispatch_fd_t fd, dispatch_data_t data, dispatch_queue_t queue,
                void (^handler)(dispatch_data_t, int))
{
        _dispatch_io_data_retain(data);
        _dispatch_retain(queue);
        _dispatch_fd_entry_init_async(fd, ^(dispatch_fd_entry_t fd_entry) {
                // On barrier queue
                if (fd_entry->err) {
                        int err = fd_entry->err;
                        dispatch_async(queue, ^{
                                _dispatch_fd_debug("convenience handler invoke", fd);
                                handler(NULL, err);
                        });
                        _dispatch_release(queue);
                        return;
                }
                // Safe to access fd_entry on barrier queue
                dispatch_io_t channel = fd_entry->convenience_channel;
                if (!channel) {
                        channel = _dispatch_io_create(DISPATCH_IO_STREAM);
                        channel->fd = fd;
                        channel->fd_actual = fd;
                        channel->fd_entry = fd_entry;
                        dispatch_retain(fd_entry->barrier_queue);
                        dispatch_retain(fd_entry->barrier_group);
                        channel->barrier_queue = fd_entry->barrier_queue;
                        channel->barrier_group = fd_entry->barrier_group;
                        fd_entry->convenience_channel = channel;
                }
                __block dispatch_data_t deliver_data = NULL;
                __block int err = 0;
                dispatch_async(fd_entry->close_queue, ^{
                        dispatch_async(queue, ^{
                                _dispatch_fd_debug("convenience handler invoke", fd);
                                handler(deliver_data, err);
                                if (deliver_data) {
                                        _dispatch_io_data_release(deliver_data);
                                }
                        });
                        _dispatch_release(queue);
                });
                dispatch_operation_t op =
                        _dispatch_operation_create(DOP_DIR_WRITE, channel, 0,
                                        dispatch_data_get_size(data), data,
                                        _dispatch_get_root_queue(_DISPATCH_QOS_CLASS_DEFAULT,false),
                                        ^(bool done, dispatch_data_t d, int error) {
                                if (done) {
                                        if (d) {
                                                _dispatch_io_data_retain(d);
                                                deliver_data = d;
                                        }
                                        err = error;
                                }
                        });
                if (op) {
                        _dispatch_operation_enqueue(op, DOP_DIR_WRITE, data);
                }
                _dispatch_io_data_release(data);
        });
}

void
dispatch_write_f(dispatch_fd_t fd, dispatch_data_t data, dispatch_queue_t queue,
                void *context, void (*handler)(void *, dispatch_data_t, int))
{
        return dispatch_write(fd, data, queue, ^(dispatch_data_t d, int error){
                handler(context, d, error);
        });
}

#pragma mark -
#pragma mark dispatch_operation_t

static dispatch_operation_t
_dispatch_operation_create(dispatch_op_direction_t direction,
                dispatch_io_t channel, off_t offset, size_t length,
                dispatch_data_t data, dispatch_queue_t queue,
                dispatch_io_handler_t handler)
{
        // On channel queue
        dispatch_assert(direction < DOP_DIR_MAX);
        // Safe to call _dispatch_io_get_error() with channel->fd_entry since
        // that can only be NULL if atomic_flags are set rdar://problem/8362514
        int err = _dispatch_io_get_error(NULL, channel, false);
        if (err || !length) {
                _dispatch_io_data_retain(data);
                _dispatch_retain(queue);
                dispatch_async(channel->barrier_queue, ^{
                        dispatch_async(queue, ^{
                                dispatch_data_t d = data;
                                if (direction == DOP_DIR_READ && err) {
                                        d = NULL;
                                } else if (direction == DOP_DIR_WRITE && !err) {
                                        d = NULL;
                                }
                                _dispatch_channel_debug("IO handler invoke: err %d", channel,
                                                err);
                                handler(true, d, err);
                                _dispatch_io_data_release(data);
                        });
                        _dispatch_release(queue);
                });
                return NULL;
        }
        dispatch_operation_t op = _dispatch_alloc(DISPATCH_VTABLE(operation),
                        sizeof(struct dispatch_operation_s));
        _dispatch_channel_debug("operation create: %p", channel, op);
        op->do_next = DISPATCH_OBJECT_LISTLESS;
        op->do_xref_cnt = -1; // operation object is not exposed externally
        op->op_q = dispatch_queue_create("com.apple.libdispatch-io.opq", NULL);
        op->op_q->do_targetq = queue;
        _dispatch_retain(queue);
        op->active = false;
        op->direction = direction;
        op->offset = offset + channel->f_ptr;
        op->length = length;
        op->handler = _dispatch_io_Block_copy(handler);
        _dispatch_retain(channel);
        op->channel = channel;
        op->params = channel->params;
        // Take a snapshot of the priority of the channel queue. The actual I/O
        // for this operation will be performed at this priority
        dispatch_queue_t targetq = op->channel->do_targetq;
        while (fastpath(targetq->do_targetq)) {
                targetq = targetq->do_targetq;
        }
        op->do_targetq = targetq;
        _dispatch_object_debug(op, "%s", __func__);
        return op;
}

void
_dispatch_operation_dispose(dispatch_operation_t op)
{
        _dispatch_object_debug(op, "%s", __func__);
        _dispatch_op_debug("dispose", op);
        // Deliver the data if there's any
        if (op->fd_entry) {
                _dispatch_operation_deliver_data(op, DOP_DONE);
                dispatch_group_leave(op->fd_entry->barrier_group);
                _dispatch_fd_entry_release(op->fd_entry);
        }
        if (op->channel) {
                _dispatch_release(op->channel);
        }
        if (op->timer) {
                dispatch_release(op->timer);
        }
        // For write operations, op->buf is owned by op->buf_data
        if (op->buf && op->direction == DOP_DIR_READ) {
                free(op->buf);
        }
        if (op->buf_data) {
                _dispatch_io_data_release(op->buf_data);
        }
        if (op->data) {
                _dispatch_io_data_release(op->data);
        }
        if (op->op_q) {
                dispatch_release(op->op_q);
        }
        Block_release(op->handler);
        _dispatch_op_debug("disposed", op);
}

static void
_dispatch_operation_enqueue(dispatch_operation_t op,
                dispatch_op_direction_t direction, dispatch_data_t data)
{
        // Called from the barrier queue
        _dispatch_io_data_retain(data);
        // If channel is closed or stopped, then call the handler immediately
        int err = _dispatch_io_get_error(NULL, op->channel, false);
        if (err) {
                dispatch_io_handler_t handler = op->handler;
                dispatch_async(op->op_q, ^{
                        dispatch_data_t d = data;
                        if (direction == DOP_DIR_READ && err) {
                                d = NULL;
                        } else if (direction == DOP_DIR_WRITE && !err) {
                                d = NULL;
                        }
                        handler(true, d, err);
                        _dispatch_io_data_release(data);
                });
                _dispatch_op_debug("release -> %d, err %d", op, op->do_ref_cnt, err);
                _dispatch_release(op);
                return;
        }
        // Finish operation init
        op->fd_entry = op->channel->fd_entry;
        _dispatch_fd_entry_retain(op->fd_entry);
        dispatch_group_enter(op->fd_entry->barrier_group);
        dispatch_disk_t disk = op->fd_entry->disk;
        if (!disk) {
                dispatch_stream_t stream = op->fd_entry->streams[direction];
                dispatch_async(stream->dq, ^{
                        _dispatch_stream_enqueue_operation(stream, op, data);
                        _dispatch_io_data_release(data);
                });
        } else {
                dispatch_async(disk->pick_queue, ^{
                        _dispatch_disk_enqueue_operation(disk, op, data);
                        _dispatch_io_data_release(data);
                });
        }
}

static bool
_dispatch_operation_should_enqueue(dispatch_operation_t op,
                dispatch_queue_t tq, dispatch_data_t data)
{
        // On stream queue or disk queue
        _dispatch_op_debug("enqueue", op);
        _dispatch_io_data_retain(data);
        op->data = data;
        int err = _dispatch_io_get_error(op, NULL, true);
        if (err) {
                op->err = err;
                // Final release
                _dispatch_op_debug("release -> %d, err %d", op, op->do_ref_cnt, err);
                _dispatch_release(op);
                return false;
        }
        if (op->params.interval) {
                dispatch_resume(_dispatch_operation_timer(tq, op));
        }
        return true;
}

static dispatch_source_t
_dispatch_operation_timer(dispatch_queue_t tq, dispatch_operation_t op)
{
        // On stream queue or pick queue
        if (op->timer) {
                return op->timer;
        }
        dispatch_source_t timer = dispatch_source_create(
                        DISPATCH_SOURCE_TYPE_TIMER, 0, 0, tq);
        dispatch_source_set_timer(timer, dispatch_time(DISPATCH_TIME_NOW,
                        (int64_t)op->params.interval), op->params.interval, 0);
        dispatch_source_set_event_handler(timer, ^{
                // On stream queue or pick queue
                if (dispatch_source_testcancel(timer)) {
                        // Do nothing. The operation has already completed
                        return;
                }
                dispatch_op_flags_t flags = DOP_DEFAULT;
                if (op->params.interval_flags & DISPATCH_IO_STRICT_INTERVAL) {
                        // Deliver even if there is less data than the low-water mark
                        flags |= DOP_DELIVER;
                }
                // If the operation is active, dont deliver data
                if ((op->active) && (flags & DOP_DELIVER)) {
                        op->flags = flags;
                } else {
                        _dispatch_operation_deliver_data(op, flags);
                }
        });
        op->timer = timer;
        return op->timer;
}

#pragma mark -
#pragma mark dispatch_fd_entry_t

#if DISPATCH_USE_GUARDED_FD_CHANGE_FDGUARD
static void
_dispatch_fd_entry_guard(dispatch_fd_entry_t fd_entry)
{
        guardid_t guard = fd_entry;
        const unsigned int guard_flags = GUARD_CLOSE;
        int err, fd_flags = 0;
        _dispatch_io_syscall_switch_noerr(err,
                change_fdguard_np(fd_entry->fd, NULL, 0, &guard, guard_flags,
                                &fd_flags),
                case 0:
                        fd_entry->guard_flags = guard_flags;
                        fd_entry->orig_fd_flags = fd_flags;
                        break;
                case EPERM: break;
                default: (void)dispatch_assume_zero(err); break;
        );
}

static void
_dispatch_fd_entry_unguard(dispatch_fd_entry_t fd_entry)
{
        if (!fd_entry->guard_flags) {
                return;
        }
        guardid_t guard = fd_entry;
        int err, fd_flags = fd_entry->orig_fd_flags;
        _dispatch_io_syscall_switch(err,
                change_fdguard_np(fd_entry->fd, &guard, fd_entry->guard_flags, NULL, 0,
                                &fd_flags),
                default: (void)dispatch_assume_zero(err); break;
        );
}
#else
static inline void
_dispatch_fd_entry_guard(dispatch_fd_entry_t fd_entry) { (void)fd_entry; }
static inline void
_dispatch_fd_entry_unguard(dispatch_fd_entry_t fd_entry) { (void)fd_entry; }
#endif // DISPATCH_USE_GUARDED_FD

static inline int
_dispatch_fd_entry_guarded_open(dispatch_fd_entry_t fd_entry, const char *path,
                int oflag, mode_t mode) {
#if DISPATCH_USE_GUARDED_FD
        guardid_t guard = (uintptr_t)fd_entry;
        const unsigned int guard_flags = GUARD_CLOSE | GUARD_DUP |
                        GUARD_SOCKET_IPC | GUARD_FILEPORT;
        int fd = guarded_open_np(path, &guard, guard_flags, oflag | O_CLOEXEC,
                        mode);
        if (fd != -1) {
                fd_entry->guard_flags = guard_flags;
                return fd;
        }
        errno = 0;
#endif
        return open(path, oflag, mode);
        (void)fd_entry;
}

static inline int
_dispatch_fd_entry_guarded_close(dispatch_fd_entry_t fd_entry, int fd) {
#if DISPATCH_USE_GUARDED_FD
        if (fd_entry->guard_flags) {
                guardid_t guard = (uintptr_t)fd_entry;
                return guarded_close_np(fd, &guard);
        } else
#endif
        {
                return close(fd);
        }
        (void)fd_entry;
}

static inline void
_dispatch_fd_entry_retain(dispatch_fd_entry_t fd_entry) {
        dispatch_suspend(fd_entry->close_queue);
}

static inline void
_dispatch_fd_entry_release(dispatch_fd_entry_t fd_entry) {
        dispatch_resume(fd_entry->close_queue);
}

static void
_dispatch_fd_entry_init_async(dispatch_fd_t fd,
                dispatch_fd_entry_init_callback_t completion_callback)
{
        static dispatch_once_t _dispatch_io_fds_lockq_pred;
        dispatch_once_f(&_dispatch_io_fds_lockq_pred, NULL,
                        _dispatch_io_fds_lockq_init);
        dispatch_async(_dispatch_io_fds_lockq, ^{
                dispatch_fd_entry_t fd_entry = NULL;
                // Check to see if there is an existing entry for the given fd
                uintptr_t hash = DIO_HASH(fd);
                TAILQ_FOREACH(fd_entry, &_dispatch_io_fds[hash], fd_list) {
                        if (fd_entry->fd == fd) {
                                // Retain the fd_entry to ensure it cannot go away until the
                                // stat() has completed
                                _dispatch_fd_entry_retain(fd_entry);
                                break;
                        }
                }
                if (!fd_entry) {
                        // If we did not find an existing entry, create one
                        fd_entry = _dispatch_fd_entry_create_with_fd(fd, hash);
                }
                _dispatch_fd_entry_debug("init", fd_entry);
                dispatch_async(fd_entry->barrier_queue, ^{
                        _dispatch_fd_entry_debug("init completion", fd_entry);
                        completion_callback(fd_entry);
                        // stat() is complete, release reference to fd_entry
                        _dispatch_fd_entry_release(fd_entry);
                });
        });
}

static dispatch_fd_entry_t
_dispatch_fd_entry_create(dispatch_queue_t q)
{
        dispatch_fd_entry_t fd_entry;
        fd_entry = _dispatch_calloc(1ul, sizeof(struct dispatch_fd_entry_s));
        fd_entry->close_queue = dispatch_queue_create(
                        "com.apple.libdispatch-io.closeq", NULL);
        // Use target queue to ensure that no concurrent lookups are going on when
        // the close queue is running
        fd_entry->close_queue->do_targetq = q;
        _dispatch_retain(q);
        // Suspend the cleanup queue until closing
        _dispatch_fd_entry_retain(fd_entry);
        return fd_entry;
}

static dispatch_fd_entry_t
_dispatch_fd_entry_create_with_fd(dispatch_fd_t fd, uintptr_t hash)
{
        // On fds lock queue
        dispatch_fd_entry_t fd_entry = _dispatch_fd_entry_create(
                        _dispatch_io_fds_lockq);
        _dispatch_fd_entry_debug("create: fd %d", fd_entry, fd);
        fd_entry->fd = fd;
        TAILQ_INSERT_TAIL(&_dispatch_io_fds[hash], fd_entry, fd_list);
        fd_entry->barrier_queue = dispatch_queue_create(
                        "com.apple.libdispatch-io.barrierq", NULL);
        fd_entry->barrier_group = dispatch_group_create();
        dispatch_async(fd_entry->barrier_queue, ^{
                _dispatch_fd_entry_debug("stat", fd_entry);
                int err, orig_flags, orig_nosigpipe = -1;
                struct stat st;
                _dispatch_io_syscall_switch(err,
                        fstat(fd, &st),
                        default: fd_entry->err = err; return;
                );
                fd_entry->stat.dev = st.st_dev;
                fd_entry->stat.mode = st.st_mode;
                _dispatch_fd_entry_guard(fd_entry);
                _dispatch_io_syscall_switch(err,
                        orig_flags = fcntl(fd, F_GETFL),
                        default: (void)dispatch_assume_zero(err); break;
                );
#if DISPATCH_USE_SETNOSIGPIPE // rdar://problem/4121123
                if (S_ISFIFO(st.st_mode)) {
                        _dispatch_io_syscall_switch(err,
                                orig_nosigpipe = fcntl(fd, F_GETNOSIGPIPE),
                                default: (void)dispatch_assume_zero(err); break;
                        );
                        if (orig_nosigpipe != -1) {
                                _dispatch_io_syscall_switch(err,
                                        orig_nosigpipe = fcntl(fd, F_SETNOSIGPIPE, 1),
                                        default:
                                                orig_nosigpipe = -1;
                                                (void)dispatch_assume_zero(err);
                                                break;
                                );
                        }
                }
#endif
                if (S_ISREG(st.st_mode)) {
                        if (orig_flags != -1) {
                                _dispatch_io_syscall_switch(err,
                                        fcntl(fd, F_SETFL, orig_flags & ~O_NONBLOCK),
                                        default:
                                                orig_flags = -1;
                                                (void)dispatch_assume_zero(err);
                                                break;
                                );
                        }
                        int32_t dev = major(st.st_dev);
                        // We have to get the disk on the global dev queue. The
                        // barrier queue cannot continue until that is complete
                        dispatch_suspend(fd_entry->barrier_queue);
                        dispatch_once_f(&_dispatch_io_devs_lockq_pred, NULL,
                                        _dispatch_io_devs_lockq_init);
                        dispatch_async(_dispatch_io_devs_lockq, ^{
                                _dispatch_disk_init(fd_entry, dev);
                                dispatch_resume(fd_entry->barrier_queue);
                        });
                } else {
                        if (orig_flags != -1) {
                                _dispatch_io_syscall_switch(err,
                                        fcntl(fd, F_SETFL, orig_flags | O_NONBLOCK),
                                        default:
                                                orig_flags = -1;
                                                (void)dispatch_assume_zero(err);
                                                break;
                                );
                        }
                        _dispatch_stream_init(fd_entry, _dispatch_get_root_queue(
                                        _DISPATCH_QOS_CLASS_DEFAULT, false));
                }
                fd_entry->orig_flags = orig_flags;
                fd_entry->orig_nosigpipe = orig_nosigpipe;
        });
        // This is the first item run when the close queue is resumed, indicating
        // that all channels associated with this entry have been closed and that
        // all operations associated with this entry have been freed
        dispatch_async(fd_entry->close_queue, ^{
                if (!fd_entry->disk) {
                        _dispatch_fd_entry_debug("close queue cleanup", fd_entry);
                        dispatch_op_direction_t dir;
                        for (dir = 0; dir < DOP_DIR_MAX; dir++) {
                                _dispatch_stream_dispose(fd_entry, dir);
                        }
                } else {
                        dispatch_disk_t disk = fd_entry->disk;
                        dispatch_async(_dispatch_io_devs_lockq, ^{
                                _dispatch_release(disk);
                        });
                }
                // Remove this entry from the global fd list
                TAILQ_REMOVE(&_dispatch_io_fds[hash], fd_entry, fd_list);
        });
        // If there was a source associated with this stream, disposing of the
        // source cancels it and suspends the close queue. Freeing the fd_entry
        // structure must happen after the source cancel handler has finished
        dispatch_async(fd_entry->close_queue, ^{
                _dispatch_fd_entry_debug("close queue release", fd_entry);
                dispatch_release(fd_entry->close_queue);
                _dispatch_fd_entry_debug("barrier queue release", fd_entry);
                dispatch_release(fd_entry->barrier_queue);
                _dispatch_fd_entry_debug("barrier group release", fd_entry);
                dispatch_release(fd_entry->barrier_group);
                if (fd_entry->orig_flags != -1) {
                        _dispatch_io_syscall(
                                fcntl(fd, F_SETFL, fd_entry->orig_flags)
                        );
                }
#if DISPATCH_USE_SETNOSIGPIPE // rdar://problem/4121123
                if (fd_entry->orig_nosigpipe != -1) {
                        _dispatch_io_syscall(
                                fcntl(fd, F_SETNOSIGPIPE, fd_entry->orig_nosigpipe)
                        );
                }
#endif
                _dispatch_fd_entry_unguard(fd_entry);
                if (fd_entry->convenience_channel) {
                        fd_entry->convenience_channel->fd_entry = NULL;
                        dispatch_release(fd_entry->convenience_channel);
                }
                free(fd_entry);
        });
        return fd_entry;
}

static dispatch_fd_entry_t
_dispatch_fd_entry_create_with_path(dispatch_io_path_data_t path_data,
                dev_t dev, mode_t mode)
{
        // On devs lock queue
        dispatch_fd_entry_t fd_entry = _dispatch_fd_entry_create(
                        path_data->channel->queue);
        _dispatch_fd_entry_debug("create: path %s", fd_entry, path_data->path);
        if (S_ISREG(mode)) {
                _dispatch_disk_init(fd_entry, major(dev));
        } else {
                _dispatch_stream_init(fd_entry, _dispatch_get_root_queue(
                                _DISPATCH_QOS_CLASS_DEFAULT, false));
        }
        fd_entry->fd = -1;
        fd_entry->orig_flags = -1;
        fd_entry->path_data = path_data;
        fd_entry->stat.dev = dev;
        fd_entry->stat.mode = mode;
        fd_entry->barrier_queue = dispatch_queue_create(
                        "com.apple.libdispatch-io.barrierq", NULL);
        fd_entry->barrier_group = dispatch_group_create();
        // This is the first item run when the close queue is resumed, indicating
        // that the channel associated with this entry has been closed and that
        // all operations associated with this entry have been freed
        dispatch_async(fd_entry->close_queue, ^{
                _dispatch_fd_entry_debug("close queue cleanup", fd_entry);
                if (!fd_entry->disk) {
                        dispatch_op_direction_t dir;
                        for (dir = 0; dir < DOP_DIR_MAX; dir++) {
                                _dispatch_stream_dispose(fd_entry, dir);
                        }
                }
                if (fd_entry->fd != -1) {
                        _dispatch_fd_entry_guarded_close(fd_entry, fd_entry->fd);
                }
                if (fd_entry->path_data->channel) {
                        // If associated channel has not been released yet, mark it as
                        // no longer having an fd_entry (for stop after close).
                        // It is safe to modify channel since we are on close_queue with
                        // target queue the channel queue
                        fd_entry->path_data->channel->fd_entry = NULL;
                }
        });
        dispatch_async(fd_entry->close_queue, ^{
                _dispatch_fd_entry_debug("close queue release", fd_entry);
                dispatch_release(fd_entry->close_queue);
                dispatch_release(fd_entry->barrier_queue);
                dispatch_release(fd_entry->barrier_group);
                free(fd_entry->path_data);
                free(fd_entry);
        });
        return fd_entry;
}

static int
_dispatch_fd_entry_open(dispatch_fd_entry_t fd_entry, dispatch_io_t channel)
{
        if (!(fd_entry->fd == -1 && fd_entry->path_data)) {
                return 0;
        }
        if (fd_entry->err) {
                return fd_entry->err;
        }
        int fd = -1;
        int oflag = fd_entry->disk ? fd_entry->path_data->oflag & ~O_NONBLOCK :
                        fd_entry->path_data->oflag | O_NONBLOCK;
open:
        fd = _dispatch_fd_entry_guarded_open(fd_entry, fd_entry->path_data->path,
                        oflag, fd_entry->path_data->mode);
        if (fd == -1) {
                int err = errno;
                if (err == EINTR) {
                        goto open;
                }
                (void)os_atomic_cmpxchg2o(fd_entry, err, 0, err, relaxed);
                return err;
        }
        if (!os_atomic_cmpxchg2o(fd_entry, fd, -1, fd, relaxed)) {
                // Lost the race with another open
                _dispatch_fd_entry_guarded_close(fd_entry, fd);
        } else {
                channel->fd_actual = fd;
        }
        _dispatch_object_debug(channel, "%s", __func__);
        return 0;
}

static void
_dispatch_fd_entry_cleanup_operations(dispatch_fd_entry_t fd_entry,
                dispatch_io_t channel)
{
        if (fd_entry->disk) {
                if (channel) {
                        _dispatch_retain(channel);
                }
                _dispatch_fd_entry_retain(fd_entry);
                dispatch_async(fd_entry->disk->pick_queue, ^{
                        _dispatch_disk_cleanup_inactive_operations(fd_entry->disk, channel);
                        _dispatch_fd_entry_release(fd_entry);
                        if (channel) {
                                _dispatch_release(channel);
                        }
                });
        } else {
                dispatch_op_direction_t direction;
                for (direction = 0; direction < DOP_DIR_MAX; direction++) {
                        dispatch_stream_t stream = fd_entry->streams[direction];
                        if (!stream) {
                                continue;
                        }
                        if (channel) {
                                _dispatch_retain(channel);
                        }
                        _dispatch_fd_entry_retain(fd_entry);
                        dispatch_async(stream->dq, ^{
                                _dispatch_stream_cleanup_operations(stream, channel);
                                _dispatch_fd_entry_release(fd_entry);
                                if (channel) {
                                        _dispatch_release(channel);
                                }
                        });
                }
        }
}

#pragma mark -
#pragma mark dispatch_stream_t/dispatch_disk_t

static void
_dispatch_stream_init(dispatch_fd_entry_t fd_entry, dispatch_queue_t tq)
{
        dispatch_op_direction_t direction;
        for (direction = 0; direction < DOP_DIR_MAX; direction++) {
                dispatch_stream_t stream;
                stream = _dispatch_calloc(1ul, sizeof(struct dispatch_stream_s));
                stream->dq = dispatch_queue_create("com.apple.libdispatch-io.streamq",
                                NULL);
                dispatch_set_context(stream->dq, stream);
                _dispatch_retain(tq);
                stream->dq->do_targetq = tq;
                TAILQ_INIT(&stream->operations[DISPATCH_IO_RANDOM]);
                TAILQ_INIT(&stream->operations[DISPATCH_IO_STREAM]);
                fd_entry->streams[direction] = stream;
        }
}

static void
_dispatch_stream_dispose(dispatch_fd_entry_t fd_entry,
                dispatch_op_direction_t direction)
{
        // On close queue
        dispatch_stream_t stream = fd_entry->streams[direction];
        if (!stream) {
                return;
        }
        dispatch_assert(TAILQ_EMPTY(&stream->operations[DISPATCH_IO_STREAM]));
        dispatch_assert(TAILQ_EMPTY(&stream->operations[DISPATCH_IO_RANDOM]));
        if (stream->source) {
                // Balanced by source cancel handler:
                _dispatch_fd_entry_retain(fd_entry);
                dispatch_source_cancel(stream->source);
                dispatch_resume(stream->source);
                dispatch_release(stream->source);
        }
        dispatch_set_context(stream->dq, NULL);
        dispatch_release(stream->dq);
        free(stream);
}

static void
_dispatch_disk_init(dispatch_fd_entry_t fd_entry, dev_t dev)
{
        // On devs lock queue
        dispatch_disk_t disk;
        // Check to see if there is an existing entry for the given device
        uintptr_t hash = DIO_HASH(dev);
        TAILQ_FOREACH(disk, &_dispatch_io_devs[hash], disk_list) {
                if (disk->dev == dev) {
                        _dispatch_retain(disk);
                        goto out;
                }
        }
        // Otherwise create a new entry
        size_t pending_reqs_depth = dispatch_io_defaults.max_pending_io_reqs;
        disk = _dispatch_alloc(DISPATCH_VTABLE(disk),
                        sizeof(struct dispatch_disk_s) +
                        (pending_reqs_depth * sizeof(dispatch_operation_t)));
        disk->do_next = DISPATCH_OBJECT_LISTLESS;
        disk->do_xref_cnt = -1;
        disk->advise_list_depth = pending_reqs_depth;
        disk->do_targetq = _dispatch_get_root_queue(_DISPATCH_QOS_CLASS_DEFAULT,
                        false);
        disk->dev = dev;
        TAILQ_INIT(&disk->operations);
        disk->cur_rq = TAILQ_FIRST(&disk->operations);
        char label[45];
        snprintf(label, sizeof(label), "com.apple.libdispatch-io.deviceq.%d",
                        (int)dev);
        disk->pick_queue = dispatch_queue_create(label, NULL);
        TAILQ_INSERT_TAIL(&_dispatch_io_devs[hash], disk, disk_list);
out:
        fd_entry->disk = disk;
        TAILQ_INIT(&fd_entry->stream_ops);
}

void
_dispatch_disk_dispose(dispatch_disk_t disk)
{
        uintptr_t hash = DIO_HASH(disk->dev);
        TAILQ_REMOVE(&_dispatch_io_devs[hash], disk, disk_list);
        dispatch_assert(TAILQ_EMPTY(&disk->operations));
        size_t i;
        for (i=0; i<disk->advise_list_depth; ++i) {
                dispatch_assert(!disk->advise_list[i]);
        }
        dispatch_release(disk->pick_queue);
}

#pragma mark -
#pragma mark dispatch_stream_operations/dispatch_disk_operations

static inline bool
_dispatch_stream_operation_avail(dispatch_stream_t stream)
{
        return  !(TAILQ_EMPTY(&stream->operations[DISPATCH_IO_RANDOM])) ||
                        !(TAILQ_EMPTY(&stream->operations[DISPATCH_IO_STREAM]));
}

static void
_dispatch_stream_enqueue_operation(dispatch_stream_t stream,
                dispatch_operation_t op, dispatch_data_t data)
{
        if (!_dispatch_operation_should_enqueue(op, stream->dq, data)) {
                return;
        }
        _dispatch_object_debug(op, "%s", __func__);
        bool no_ops = !_dispatch_stream_operation_avail(stream);
        TAILQ_INSERT_TAIL(&stream->operations[op->params.type], op, operation_list);
        if (no_ops) {
                dispatch_async_f(stream->dq, stream->dq,
                                _dispatch_stream_queue_handler);
        }
}

static void
_dispatch_disk_enqueue_operation(dispatch_disk_t disk, dispatch_operation_t op,
                dispatch_data_t data)
{
        if (!_dispatch_operation_should_enqueue(op, disk->pick_queue, data)) {
                return;
        }
        _dispatch_object_debug(op, "%s", __func__);
        if (op->params.type == DISPATCH_IO_STREAM) {
                if (TAILQ_EMPTY(&op->fd_entry->stream_ops)) {
                        TAILQ_INSERT_TAIL(&disk->operations, op, operation_list);
                }
                TAILQ_INSERT_TAIL(&op->fd_entry->stream_ops, op, stream_list);
        } else {
                TAILQ_INSERT_TAIL(&disk->operations, op, operation_list);
        }
        _dispatch_disk_handler(disk);
}

static void
_dispatch_stream_complete_operation(dispatch_stream_t stream,
                dispatch_operation_t op)
{
        // On stream queue
        _dispatch_object_debug(op, "%s", __func__);
        _dispatch_op_debug("complete: stream %p", op, stream);
        TAILQ_REMOVE(&stream->operations[op->params.type], op, operation_list);
        if (op == stream->op) {
                stream->op = NULL;
        }
        if (op->timer) {
                dispatch_source_cancel(op->timer);
        }
        // Final release will deliver any pending data
        _dispatch_op_debug("release -> %d (stream complete)", op, op->do_ref_cnt);
        _dispatch_release(op);
}

static void
_dispatch_disk_complete_operation(dispatch_disk_t disk, dispatch_operation_t op)
{
        // On pick queue
        _dispatch_object_debug(op, "%s", __func__);
        _dispatch_op_debug("complete: disk %p", op, disk);
        // Current request is always the last op returned
        if (disk->cur_rq == op) {
                disk->cur_rq = TAILQ_PREV(op, dispatch_disk_operations_s,
                                operation_list);
        }
        if (op->params.type == DISPATCH_IO_STREAM) {
                // Check if there are other pending stream operations behind it
                dispatch_operation_t op_next = TAILQ_NEXT(op, stream_list);
                TAILQ_REMOVE(&op->fd_entry->stream_ops, op, stream_list);
                if (op_next) {
                        TAILQ_INSERT_TAIL(&disk->operations, op_next, operation_list);
                }
        }
        TAILQ_REMOVE(&disk->operations, op, operation_list);
        if (op->timer) {
                dispatch_source_cancel(op->timer);
        }
        // Final release will deliver any pending data
        _dispatch_op_debug("release -> %d (disk complete)", op, op->do_ref_cnt);
        _dispatch_release(op);
}

static dispatch_operation_t
_dispatch_stream_pick_next_operation(dispatch_stream_t stream,
                dispatch_operation_t op)
{
        // On stream queue
        if (!op) {
                // On the first run through, pick the first operation
                if (!_dispatch_stream_operation_avail(stream)) {
                        return op;
                }
                if (!TAILQ_EMPTY(&stream->operations[DISPATCH_IO_STREAM])) {
                        op = TAILQ_FIRST(&stream->operations[DISPATCH_IO_STREAM]);
                } else if (!TAILQ_EMPTY(&stream->operations[DISPATCH_IO_RANDOM])) {
                        op = TAILQ_FIRST(&stream->operations[DISPATCH_IO_RANDOM]);
                }
                return op;
        }
        if (op->params.type == DISPATCH_IO_STREAM) {
                // Stream operations need to be serialized so continue the current
                // operation until it is finished
                return op;
        }
        // Get the next random operation (round-robin)
        if (op->params.type == DISPATCH_IO_RANDOM) {
                op = TAILQ_NEXT(op, operation_list);
                if (!op) {
                        op = TAILQ_FIRST(&stream->operations[DISPATCH_IO_RANDOM]);
                }
                return op;
        }
        return NULL;
}

static dispatch_operation_t
_dispatch_disk_pick_next_operation(dispatch_disk_t disk)
{
        // On pick queue
        dispatch_operation_t op;
        if (!TAILQ_EMPTY(&disk->operations)) {
                if (disk->cur_rq == NULL) {
                        op = TAILQ_FIRST(&disk->operations);
                } else {
                        op = disk->cur_rq;
                        do {
                                op = TAILQ_NEXT(op, operation_list);
                                if (!op) {
                                        op = TAILQ_FIRST(&disk->operations);
                                }
                                // TODO: more involved picking algorithm rdar://problem/8780312
                        } while (op->active && op != disk->cur_rq);
                }
                if (!op->active) {
                        disk->cur_rq = op;
                        return op;
                }
        }
        return NULL;
}

static void
_dispatch_stream_cleanup_operations(dispatch_stream_t stream,
                dispatch_io_t channel)
{
        // On stream queue
        dispatch_operation_t op, tmp;
        typeof(*stream->operations) *operations;
        operations = &stream->operations[DISPATCH_IO_RANDOM];
        TAILQ_FOREACH_SAFE(op, operations, operation_list, tmp) {
                if (!channel || op->channel == channel) {
                        _dispatch_stream_complete_operation(stream, op);
                }
        }
        operations = &stream->operations[DISPATCH_IO_STREAM];
        TAILQ_FOREACH_SAFE(op, operations, operation_list, tmp) {
                if (!channel || op->channel == channel) {
                        _dispatch_stream_complete_operation(stream, op);
                }
        }
        if (stream->source_running && !_dispatch_stream_operation_avail(stream)) {
                dispatch_suspend(stream->source);
                stream->source_running = false;
        }
}

static inline void
_dispatch_disk_cleanup_specified_operations(dispatch_disk_t disk,
                dispatch_io_t channel, bool inactive_only)
{
        // On pick queue
        dispatch_operation_t op, tmp;
        TAILQ_FOREACH_SAFE(op, &disk->operations, operation_list, tmp) {
                if (inactive_only && op->active) continue;
                if (!channel || op->channel == channel) {
                        _dispatch_op_debug("cleanup: disk %p", op, disk);
                        _dispatch_disk_complete_operation(disk, op);
                }
        }
}

static void
_dispatch_disk_cleanup_operations(dispatch_disk_t disk, dispatch_io_t channel)
{
        _dispatch_disk_cleanup_specified_operations(disk, channel, false);
}

static void
_dispatch_disk_cleanup_inactive_operations(dispatch_disk_t disk,
                dispatch_io_t channel)
{
        _dispatch_disk_cleanup_specified_operations(disk, channel, true);
}

#pragma mark -
#pragma mark dispatch_stream_handler/dispatch_disk_handler

static dispatch_source_t
_dispatch_stream_source(dispatch_stream_t stream, dispatch_operation_t op)
{
        // On stream queue
        if (stream->source) {
                return stream->source;
        }
        dispatch_fd_t fd = op->fd_entry->fd;
        _dispatch_op_debug("stream source create", op);
        dispatch_source_t source = NULL;
        if (op->direction == DOP_DIR_READ) {
                source = dispatch_source_create(DISPATCH_SOURCE_TYPE_READ,
                                (uintptr_t)fd, 0, stream->dq);
        } else if (op->direction == DOP_DIR_WRITE) {
                source = dispatch_source_create(DISPATCH_SOURCE_TYPE_WRITE,
                                (uintptr_t)fd, 0, stream->dq);
        } else {
                dispatch_assert(op->direction < DOP_DIR_MAX);
                return NULL;
        }
        dispatch_set_context(source, stream);
        dispatch_source_set_event_handler_f(source,
                        _dispatch_stream_source_handler);
        // Close queue must not run user cleanup handlers until sources are fully
        // unregistered
        dispatch_queue_t close_queue = op->fd_entry->close_queue;
        dispatch_source_set_cancel_handler(source, ^{
                _dispatch_op_debug("stream source cancel", op);
                dispatch_resume(close_queue);
        });
        stream->source = source;
        return stream->source;
}

static void
_dispatch_stream_source_handler(void *ctx)
{
        // On stream queue
        dispatch_stream_t stream = (dispatch_stream_t)ctx;
        dispatch_suspend(stream->source);
        stream->source_running = false;
        return _dispatch_stream_handler(stream);
}

static void
_dispatch_stream_queue_handler(void *ctx)
{
        // On stream queue
        dispatch_stream_t stream = (dispatch_stream_t)dispatch_get_context(ctx);
        if (!stream) {
                // _dispatch_stream_dispose has been called
                return;
        }
        return _dispatch_stream_handler(stream);
}

static void
_dispatch_stream_handler(void *ctx)
{
        // On stream queue
        dispatch_stream_t stream = (dispatch_stream_t)ctx;
        dispatch_operation_t op;
pick:
        op = _dispatch_stream_pick_next_operation(stream, stream->op);
        if (!op) {
                _dispatch_debug("no operation found: stream %p", stream);
                return;
        }
        int err = _dispatch_io_get_error(op, NULL, true);
        if (err) {
                op->err = err;
                _dispatch_stream_complete_operation(stream, op);
                goto pick;
        }
        stream->op = op;
        _dispatch_op_debug("stream handler", op);
        dispatch_fd_entry_t fd_entry = op->fd_entry;
        _dispatch_fd_entry_retain(fd_entry);
        // For performance analysis
        if (!op->total && dispatch_io_defaults.initial_delivery) {
                // Empty delivery to signal the start of the operation
                _dispatch_op_debug("initial delivery", op);
                _dispatch_operation_deliver_data(op, DOP_DELIVER);
        }
        // TODO: perform on the operation target queue to get correct priority
        int result = _dispatch_operation_perform(op);
        dispatch_op_flags_t flags = ~0u;
        switch (result) {
        case DISPATCH_OP_DELIVER:
                flags = DOP_DEFAULT;
                // Fall through
        case DISPATCH_OP_DELIVER_AND_COMPLETE:
                flags = (flags != DOP_DEFAULT) ? DOP_DELIVER | DOP_NO_EMPTY :
                                DOP_DEFAULT;
                _dispatch_operation_deliver_data(op, flags);
                // Fall through
        case DISPATCH_OP_COMPLETE:
                if (flags != DOP_DEFAULT) {
                        _dispatch_stream_complete_operation(stream, op);
                }
                if (_dispatch_stream_operation_avail(stream)) {
                        dispatch_async_f(stream->dq, stream->dq,
                                        _dispatch_stream_queue_handler);
                }
                break;
        case DISPATCH_OP_COMPLETE_RESUME:
                _dispatch_stream_complete_operation(stream, op);
                // Fall through
        case DISPATCH_OP_RESUME:
                if (_dispatch_stream_operation_avail(stream)) {
                        stream->source_running = true;
                        dispatch_resume(_dispatch_stream_source(stream, op));
                }
                break;
        case DISPATCH_OP_ERR:
                _dispatch_stream_cleanup_operations(stream, op->channel);
                break;
        case DISPATCH_OP_FD_ERR:
                _dispatch_fd_entry_retain(fd_entry);
                dispatch_async(fd_entry->barrier_queue, ^{
                        _dispatch_fd_entry_cleanup_operations(fd_entry, NULL);
                        _dispatch_fd_entry_release(fd_entry);
                });
                break;
        default:
                break;
        }
        _dispatch_fd_entry_release(fd_entry);
        return;
}

static void
_dispatch_disk_handler(void *ctx)
{
        // On pick queue
        dispatch_disk_t disk = (dispatch_disk_t)ctx;
        if (disk->io_active) {
                return;
        }
        _dispatch_disk_debug("disk handler", disk);
        dispatch_operation_t op;
        size_t i = disk->free_idx, j = disk->req_idx;
        if (j <= i) {
                j += disk->advise_list_depth;
        }
        while (i <= j) {
                if ((!disk->advise_list[i%disk->advise_list_depth]) &&
                                (op = _dispatch_disk_pick_next_operation(disk))) {
                        int err = _dispatch_io_get_error(op, NULL, true);
                        if (err) {
                                op->err = err;
                                _dispatch_disk_complete_operation(disk, op);
                                continue;
                        }
                        _dispatch_retain(op);
                        _dispatch_op_debug("retain -> %d", op, op->do_ref_cnt + 1);
                        disk->advise_list[i%disk->advise_list_depth] = op;
                        op->active = true;
                        _dispatch_op_debug("activate: disk %p", op, disk);
                        _dispatch_object_debug(op, "%s", __func__);
                } else {
                        // No more operations to get
                        break;
                }
                i++;
        }
        disk->free_idx = (i%disk->advise_list_depth);
        op = disk->advise_list[disk->req_idx];
        if (op) {
                disk->io_active = true;
                _dispatch_op_debug("async perform: disk %p", op, disk);
                dispatch_async_f(op->do_targetq, disk, _dispatch_disk_perform);
        }
}

static void
_dispatch_disk_perform(void *ctxt)
{
        dispatch_disk_t disk = ctxt;
        _dispatch_disk_debug("disk perform", disk);
        size_t chunk_size = dispatch_io_defaults.chunk_size;
        dispatch_operation_t op;
        size_t i = disk->advise_idx, j = disk->free_idx;
        if (j <= i) {
                j += disk->advise_list_depth;
        }
        do {
                op = disk->advise_list[i%disk->advise_list_depth];
                if (!op) {
                        // Nothing more to advise, must be at free_idx
                        dispatch_assert(i%disk->advise_list_depth == disk->free_idx);
                        break;
                }
                if (op->direction == DOP_DIR_WRITE) {
                        // TODO: preallocate writes ? rdar://problem/9032172
                        continue;
                }
                if (op->fd_entry->fd == -1 && _dispatch_fd_entry_open(op->fd_entry,
                                op->channel)) {
                        continue;
                }
                // For performance analysis
                if (!op->total && dispatch_io_defaults.initial_delivery) {
                        // Empty delivery to signal the start of the operation
                        _dispatch_op_debug("initial delivery", op);
                        _dispatch_operation_deliver_data(op, DOP_DELIVER);
                }
                // Advise two chunks if the list only has one element and this is the
                // first advise on the operation
                if ((j-i) == 1 && !disk->advise_list[disk->free_idx] &&
                                !op->advise_offset) {
                        chunk_size *= 2;
                }
                _dispatch_operation_advise(op, chunk_size);
        } while (++i < j);
        disk->advise_idx = i%disk->advise_list_depth;
        op = disk->advise_list[disk->req_idx];
        int result = _dispatch_operation_perform(op);
        disk->advise_list[disk->req_idx] = NULL;
        disk->req_idx = (++disk->req_idx)%disk->advise_list_depth;
        _dispatch_op_debug("async perform completion: disk %p", op, disk);
        dispatch_async(disk->pick_queue, ^{
                _dispatch_op_debug("perform completion", op);
                switch (result) {
                case DISPATCH_OP_DELIVER:
                        _dispatch_operation_deliver_data(op, DOP_DEFAULT);
                        break;
                case DISPATCH_OP_COMPLETE:
                        _dispatch_disk_complete_operation(disk, op);
                        break;
                case DISPATCH_OP_DELIVER_AND_COMPLETE:
                        _dispatch_operation_deliver_data(op, DOP_DELIVER | DOP_NO_EMPTY);
                        _dispatch_disk_complete_operation(disk, op);
                        break;
                case DISPATCH_OP_ERR:
                        _dispatch_disk_cleanup_operations(disk, op->channel);
                        break;
                case DISPATCH_OP_FD_ERR:
                        _dispatch_disk_cleanup_operations(disk, NULL);
                        break;
                default:
                        dispatch_assert(result);
                        break;
                }
                _dispatch_op_debug("deactivate: disk %p", op, disk);
                op->active = false;
                disk->io_active = false;
                _dispatch_disk_handler(disk);
                // Balancing the retain in _dispatch_disk_handler. Note that op must be
                // released at the very end, since it might hold the last reference to
                // the disk
                _dispatch_op_debug("release -> %d (disk perform complete)", op,
                                op->do_ref_cnt);
                _dispatch_release(op);
        });
}

#pragma mark -
#pragma mark dispatch_operation_perform

static void
_dispatch_operation_advise(dispatch_operation_t op, size_t chunk_size)
{
        _dispatch_op_debug("advise", op);
        if (_dispatch_io_get_error(op, NULL, true)) return;
#ifdef __linux__
        // linux does not support fcntl (F_RDAVISE)
        // define necessary datastructure and use readahead
        struct radvisory {
                off_t ra_offset;
                int   ra_count;
        };
#endif
        int err;
        struct radvisory advise;
        // No point in issuing a read advise for the next chunk if we are already
        // a chunk ahead from reading the bytes
        if (op->advise_offset > (off_t)(((size_t)op->offset + op->total) +
                        chunk_size + PAGE_SIZE)) {
                return;
        }
        _dispatch_object_debug(op, "%s", __func__);
        advise.ra_count = (int)chunk_size;
        if (!op->advise_offset) {
                op->advise_offset = op->offset;
                // If this is the first time through, align the advised range to a
                // page boundary
                size_t pg_fraction = ((size_t)op->offset + chunk_size) % PAGE_SIZE;
                advise.ra_count += (int)(pg_fraction ? PAGE_SIZE - pg_fraction : 0);
        }
        advise.ra_offset = op->advise_offset;
        op->advise_offset += advise.ra_count;
#ifdef __linux__
        _dispatch_io_syscall_switch(err,
                readahead(op->fd_entry->fd, advise.ra_offset, advise.ra_count),
                case EINVAL: break; // fd does refer to a non-supported filetype
                default: (void)dispatch_assume_zero(err); break;
        );
#else
        _dispatch_io_syscall_switch(err,
                fcntl(op->fd_entry->fd, F_RDADVISE, &advise),
                case EFBIG: break; // advised past the end of the file rdar://10415691
                case ENOTSUP: break; // not all FS support radvise rdar://13484629
                // TODO: set disk status on error
                default: (void)dispatch_assume_zero(err); break;
        );
#endif
}

static int
_dispatch_operation_perform(dispatch_operation_t op)
{
        _dispatch_op_debug("perform", op);
        int err = _dispatch_io_get_error(op, NULL, true);
        if (err) {
                goto error;
        }
        _dispatch_object_debug(op, "%s", __func__);
        if (!op->buf) {
                size_t max_buf_siz = op->params.high;
                size_t chunk_siz = dispatch_io_defaults.chunk_size;
                if (op->direction == DOP_DIR_READ) {
                        // If necessary, create a buffer for the ongoing operation, large
                        // enough to fit chunk_size but at most high-water
                        size_t data_siz = dispatch_data_get_size(op->data);
                        if (data_siz) {
                                dispatch_assert(data_siz < max_buf_siz);
                                max_buf_siz -= data_siz;
                        }
                        if (max_buf_siz > chunk_siz) {
                                max_buf_siz = chunk_siz;
                        }
                        if (op->length < SIZE_MAX) {
                                op->buf_siz = op->length - op->total;
                                if (op->buf_siz > max_buf_siz) {
                                        op->buf_siz = max_buf_siz;
                                }
                        } else {
                                op->buf_siz = max_buf_siz;
                        }
                        op->buf = valloc(op->buf_siz);
                        _dispatch_op_debug("buffer allocated", op);
                } else if (op->direction == DOP_DIR_WRITE) {
                        // Always write the first data piece, if that is smaller than a
                        // chunk, accumulate further data pieces until chunk size is reached
                        if (chunk_siz > max_buf_siz) {
                                chunk_siz = max_buf_siz;
                        }
                        op->buf_siz = 0;
                        dispatch_data_apply(op->data,
                                        ^(dispatch_data_t region DISPATCH_UNUSED,
                                        size_t offset DISPATCH_UNUSED,
                                        const void* buf DISPATCH_UNUSED, size_t len) {
                                size_t siz = op->buf_siz + len;
                                if (!op->buf_siz || siz <= chunk_siz) {
                                        op->buf_siz = siz;
                                }
                                return (bool)(siz < chunk_siz);
                        });
                        if (op->buf_siz > max_buf_siz) {
                                op->buf_siz = max_buf_siz;
                        }
                        dispatch_data_t d;
                        d = dispatch_data_create_subrange(op->data, 0, op->buf_siz);
                        op->buf_data = dispatch_data_create_map(d, (const void**)&op->buf,
                                        NULL);
                        _dispatch_io_data_release(d);
                        _dispatch_op_debug("buffer mapped", op);
                }
        }
        if (op->fd_entry->fd == -1) {
                err = _dispatch_fd_entry_open(op->fd_entry, op->channel);
                if (err) {
                        goto error;
                }
        }
        void *buf = op->buf + op->buf_len;
        size_t len = op->buf_siz - op->buf_len;
        off_t off = (off_t)((size_t)op->offset + op->total);
        ssize_t processed = -1;
syscall:
        if (op->direction == DOP_DIR_READ) {
                if (op->params.type == DISPATCH_IO_STREAM) {
                        processed = read(op->fd_entry->fd, buf, len);
                } else if (op->params.type == DISPATCH_IO_RANDOM) {
                        processed = pread(op->fd_entry->fd, buf, len, off);
                }
        } else if (op->direction == DOP_DIR_WRITE) {
                if (op->params.type == DISPATCH_IO_STREAM) {
                        processed = write(op->fd_entry->fd, buf, len);
                } else if (op->params.type == DISPATCH_IO_RANDOM) {
                        processed = pwrite(op->fd_entry->fd, buf, len, off);
                }
        }
        // Encountered an error on the file descriptor
        if (processed == -1) {
                err = errno;
                if (err == EINTR) {
                        goto syscall;
                }
                goto error;
        }
        // EOF is indicated by two handler invocations
        if (processed == 0) {
                _dispatch_op_debug("performed: EOF", op);
                return DISPATCH_OP_DELIVER_AND_COMPLETE;
        }
        op->buf_len += (size_t)processed;
        op->total += (size_t)processed;
        if (op->total == op->length) {
                // Finished processing all the bytes requested by the operation
                return DISPATCH_OP_COMPLETE;
        } else {
                // Deliver data only if we satisfy the filters
                return DISPATCH_OP_DELIVER;
        }
error:
        if (err == EAGAIN || err == EWOULDBLOCK) {
                // For disk based files with blocking I/O we should never get EAGAIN
                dispatch_assert(!op->fd_entry->disk);
                _dispatch_op_debug("performed: EAGAIN/EWOULDBLOCK", op);
                if (op->direction == DOP_DIR_READ && op->total &&
                                op->channel == op->fd_entry->convenience_channel) {
                        // Convenience read with available data completes on EAGAIN
                        return DISPATCH_OP_COMPLETE_RESUME;
                }
                return DISPATCH_OP_RESUME;
        }
        _dispatch_op_debug("performed: err %d", op, err);
        op->err = err;
        switch (err) {
        case ECANCELED:
                return DISPATCH_OP_ERR;
        case EBADF:
                (void)os_atomic_cmpxchg2o(op->fd_entry, err, 0, err, relaxed);
                return DISPATCH_OP_FD_ERR;
        default:
                return DISPATCH_OP_COMPLETE;
        }
}

static void
_dispatch_operation_deliver_data(dispatch_operation_t op,
                dispatch_op_flags_t flags)
{
        // Either called from stream resp. pick queue or when op is finalized
        dispatch_data_t data = NULL;
        int err = 0;
        size_t undelivered = op->undelivered + op->buf_len;
        bool deliver = (flags & (DOP_DELIVER|DOP_DONE)) ||
                        (op->flags & DOP_DELIVER);
        op->flags = DOP_DEFAULT;
        if (!deliver) {
                // Don't deliver data until low water mark has been reached
                if (undelivered >= op->params.low) {
                        deliver = true;
                } else if (op->buf_len < op->buf_siz) {
                        // Request buffer is not yet used up
                        _dispatch_op_debug("buffer data: undelivered %zu", op, undelivered);
                        return;
                }
        } else {
                err = op->err;
                if (!err && (op->channel->atomic_flags & DIO_STOPPED)) {
                        err = ECANCELED;
                        op->err = err;
                }
        }
        // Deliver data or buffer used up
        if (op->direction == DOP_DIR_READ) {
                if (op->buf_len) {
                        void *buf = op->buf;
                        data = dispatch_data_create(buf, op->buf_len, NULL,
                                        DISPATCH_DATA_DESTRUCTOR_FREE);
                        op->buf = NULL;
                        op->buf_len = 0;
                        dispatch_data_t d = dispatch_data_create_concat(op->data, data);
                        _dispatch_io_data_release(op->data);
                        _dispatch_io_data_release(data);
                        data = d;
                } else {
                        data = op->data;
                }
                op->data = deliver ? dispatch_data_empty : data;
        } else if (op->direction == DOP_DIR_WRITE) {
                if (deliver) {
                        data = dispatch_data_create_subrange(op->data, op->buf_len,
                                        op->length);
                }
                if (op->buf_data && op->buf_len == op->buf_siz) {
                        _dispatch_io_data_release(op->buf_data);
                        op->buf_data = NULL;
                        op->buf = NULL;
                        op->buf_len = 0;
                        // Trim newly written buffer from head of unwritten data
                        dispatch_data_t d;
                        if (deliver) {
                                _dispatch_io_data_retain(data);
                                d = data;
                        } else {
                                d = dispatch_data_create_subrange(op->data, op->buf_siz,
                                                op->length);
                        }
                        _dispatch_io_data_release(op->data);
                        op->data = d;
                }
        } else {
                dispatch_assert(op->direction < DOP_DIR_MAX);
                return;
        }
        if (!deliver || ((flags & DOP_NO_EMPTY) && !dispatch_data_get_size(data))) {
                op->undelivered = undelivered;
                _dispatch_op_debug("buffer data: undelivered %zu", op, undelivered);
                return;
        }
        op->undelivered = 0;
        _dispatch_object_debug(op, "%s", __func__);
        _dispatch_op_debug("deliver data", op);
        dispatch_op_direction_t direction = op->direction;
        dispatch_io_handler_t handler = op->handler;
        dispatch_fd_entry_t fd_entry = op->fd_entry;
        _dispatch_fd_entry_retain(fd_entry);
        dispatch_io_t channel = op->channel;
        _dispatch_retain(channel);
        // Note that data delivery may occur after the operation is freed
        dispatch_async(op->op_q, ^{
                bool done = (flags & DOP_DONE);
                dispatch_data_t d = data;
                if (done) {
                        if (direction == DOP_DIR_READ && err) {
                                if (dispatch_data_get_size(d)) {
                                        _dispatch_op_debug("IO handler invoke", op);
                                        handler(false, d, 0);
                                }
                                d = NULL;
                        } else if (direction == DOP_DIR_WRITE && !err) {
                                d = NULL;
                        }
                }
                _dispatch_op_debug("IO handler invoke: err %d", op, err);
                handler(done, d, err);
                _dispatch_release(channel);
                _dispatch_fd_entry_release(fd_entry);
                _dispatch_io_data_release(data);
        });
}

#pragma mark -
#pragma mark dispatch_io_debug

static size_t
_dispatch_io_debug_attr(dispatch_io_t channel, char* buf, size_t bufsiz)
{
        dispatch_queue_t target = channel->do_targetq;
        return dsnprintf(buf, bufsiz, "type = %s, fd = 0x%x, %sfd_entry = %p, "
                        "queue = %p, target = %s[%p], barrier_queue = %p, barrier_group = "
                        "%p, err = 0x%x, low = 0x%zx, high = 0x%zx, interval%s = %llu ",
                        channel->params.type == DISPATCH_IO_STREAM ? "stream" : "random",
                        channel->fd_actual, channel->atomic_flags & DIO_STOPPED ?
                        "stopped, " : channel->atomic_flags & DIO_CLOSED ? "closed, " : "",
                        channel->fd_entry, channel->queue, target && target->dq_label ?
                        target->dq_label : "", target, channel->barrier_queue,
                        channel->barrier_group, channel->err, channel->params.low,
                        channel->params.high, channel->params.interval_flags &
                        DISPATCH_IO_STRICT_INTERVAL ? "(strict)" : "",
                        (unsigned long long) channel->params.interval);
}

size_t
_dispatch_io_debug(dispatch_io_t channel, char* buf, size_t bufsiz)
{
        size_t offset = 0;
        offset += dsnprintf(&buf[offset], bufsiz - offset, "%s[%p] = { ",
                        dx_kind(channel), channel);
        offset += _dispatch_object_debug_attr(channel, &buf[offset],
                        bufsiz - offset);
        offset += _dispatch_io_debug_attr(channel, &buf[offset], bufsiz - offset);
        offset += dsnprintf(&buf[offset], bufsiz - offset, "}");
        return offset;
}

static size_t
_dispatch_operation_debug_attr(dispatch_operation_t op, char* buf,
                size_t bufsiz)
{
        dispatch_queue_t target = op->do_targetq;
        dispatch_queue_t oqtarget = op->op_q ? op->op_q->do_targetq : NULL;
        return dsnprintf(buf, bufsiz, "type = %s %s, fd = 0x%x, fd_entry = %p, "
                        "channel = %p, queue = %p -> %s[%p], target = %s[%p], "
                        "offset = %lld, length = %zu, done = %zu, undelivered = %zu, "
                        "flags = %u, err = 0x%x, low = 0x%zx, high = 0x%zx, "
                        "interval%s = %llu ", op->params.type == DISPATCH_IO_STREAM ?
                        "stream" : "random", op->direction == DOP_DIR_READ ? "read" :
                        "write", op->fd_entry ? op->fd_entry->fd : -1, op->fd_entry,
                        op->channel, op->op_q, oqtarget && oqtarget->dq_label ?
                        oqtarget->dq_label : "", oqtarget, target && target->dq_label ?
                        target->dq_label : "", target, (long long)op->offset, op->length,
                        op->total, op->undelivered + op->buf_len, op->flags, op->err,
                        op->params.low, op->params.high, op->params.interval_flags &
                        DISPATCH_IO_STRICT_INTERVAL ? "(strict)" : "",
                        (unsigned long long)op->params.interval);
}

size_t
_dispatch_operation_debug(dispatch_operation_t op, char* buf, size_t bufsiz)
{
        size_t offset = 0;
        offset += dsnprintf(&buf[offset], bufsiz - offset, "%s[%p] = { ",
                        dx_kind(op), op);
        offset += _dispatch_object_debug_attr(op, &buf[offset], bufsiz - offset);
        offset += _dispatch_operation_debug_attr(op, &buf[offset], bufsiz - offset);
        offset += dsnprintf(&buf[offset], bufsiz - offset, "}");
        return offset;
}