waitgroup: remove races, make platform independent, document

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Richard Warburton 2020-06-22 21:07:34 +12:00 committed by GitHub
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1 changed files with 41 additions and 23 deletions

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// Copyright (c) 2019-2020 Alexander Medvednikov. All rights reserved.
// Copyright (c) 2019 Alexander Medvednikov. All rights reserved.
// Use of this source code is governed by an MIT license
// that can be found in the LICENSE file.
module sync
// WaitGroup implementation. wait() blocks until all tasks complete.
// Do not copy an instance of WaitGroup, use a ref instead.
//
// Two mutexes are required so that wait() doesn't unblock on done()/add() before
// task_count becomes zero.
//
// [init_with=new_waitgroup] // TODO: implement support for init_with struct attribute, and disallow WaitGroup{} from outside the sync.new_waitgroup() function.
[ref_only]
pub struct WaitGroup {
struct WaitGroup {
mut:
mu &Mutex = &Mutex(0)
active int
task_count int // current task count
task_count_mutex &Mutex = &Mutex(0) // This mutex protects the task_count count in add()
wait_blocker &Mutex = &Mutex(0) // This mutex blocks the wait() until released by add()
}
pub fn new_waitgroup() &WaitGroup {
return &WaitGroup{mu: sync.new_mutex() }
}
pub fn (mut wg WaitGroup) add(delta int) {
wg.mu.lock()
wg.active += delta
wg.mu.unlock()
if wg.active < 0 {
panic('Negative number of jobs in waitgroup')
return &WaitGroup{
task_count_mutex: new_mutex()
wait_blocker: new_mutex()
}
}
// add increments (+ve delta) or decrements (-ve delta) task count by delta
// and unblocks any wait() calls if task count becomes zero.
// add panics if task count drops below zero.
pub fn (mut wg WaitGroup) add(delta int) {
// protect task_count
wg.task_count_mutex.lock()
defer {
wg.task_count_mutex.unlock()
}
// If task_count likely to leave zero, set wait() to block
if wg.task_count == 0 {
wg.wait_blocker.lock()
}
wg.task_count += delta
if wg.task_count < 0 {
panic('Negative number of jobs in waitgroup')
}
if wg.task_count == 0 { // if no more task_count tasks
wg.wait_blocker.unlock() // unblock wait()
}
}
// done is a convenience fn for add(-1)
pub fn (mut wg WaitGroup) done() {
wg.add(-1)
}
pub fn (wg &WaitGroup) wait() {
for wg.active > 0 {
// Do not remove this, busy empty loops are optimized
// with -prod by some compilers, see issue #2874
$if windows {
C.Sleep(1)
} $else {
C.usleep(1000)
}
}
// wait blocks until all tasks are done (task count becomes zero)
pub fn (mut wg WaitGroup) wait() {
wg.wait_blocker.lock() // blocks until task_count becomes 0
wg.wait_blocker.unlock() // allow other wait()s to unblock or reuse wait group
}