sync: simplify `WaitGroup` and `PoolProcessor` and use atomic counters (#8715)

vlib/sync/pool/pool.v

@@ -4,6 +4,9 @@ import sync
 
 import runtime
 
+[trusted]
+fn C.atomic_fetch_add_u32(voidptr, u32) u32
+
 pub const (
 	no_result = voidptr(0)
 )
@@ -14,9 +17,8 @@ mut:
 	njobs           int
 	items           []voidptr
 	results         []voidptr
-	ntask           int // writing to this should be locked by ntask_mtx.
-	ntask_mtx       &sync.Mutex
-	waitgroup       &sync.WaitGroup
+	ntask           u32 // reading/writing to this should be atomic
+	waitgroup       sync.WaitGroup
 	shared_context  voidptr
 	thread_contexts []voidptr
 }
@@ -44,17 +46,16 @@ pub fn new_pool_processor(context PoolProcessorConfig) &PoolProcessor {
 	if isnil(context.callback) {
 		panic('You need to pass a valid callback to new_pool_processor.')
 	}
-	pool := &PoolProcessor {
+	mut pool := &PoolProcessor {
 		items: []
 		results: []
 		shared_context: voidptr(0)
 		thread_contexts: []
 		njobs: context.maxjobs
 		ntask: 0
-		ntask_mtx: sync.new_mutex()
-		waitgroup: sync.new_waitgroup()
 		thread_cb: voidptr(context.callback)
 	}
+	pool.waitgroup.init()
 	return pool
 }
 
@@ -104,16 +105,9 @@ pub fn (mut pool PoolProcessor) work_on_pointers(items []voidptr) {
 // method in a callback.
 fn process_in_thread(mut pool PoolProcessor, task_id int) {
 	cb := ThreadCB(pool.thread_cb)
-	mut idx := 0
 	ilen := pool.items.len
 	for {
-		if pool.ntask >= ilen {
-			break
-		}
-		pool.ntask_mtx.@lock()
-		idx = pool.ntask
-		pool.ntask++
-		pool.ntask_mtx.unlock()
+		idx := int(C.atomic_fetch_add_u32(&pool.ntask, 1))
 		if idx >= ilen {
 			break
 		}

vlib/sync/waitgroup.v

@@ -3,47 +3,49 @@
 // that can be found in the LICENSE file.
 module sync
 
-// WaitGroup implementation. wait() blocks until all tasks complete.
+[trusted]
+fn C.atomic_fetch_add_u32(voidptr, u32) u32
+
+// WaitGroup
 // 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.
+// usage: in main thread:
+// `wg := sync.new_waitgroup()
+// `wg.add(nr_jobs)` before starting jobs with `go ...`
+// `wg.wait()` to wait for all jobs to have finished
+//
+// in each parallel job:
+// `wg.done()` when finished 
 //
 // [init_with=new_waitgroup] // TODO: implement support for init_with struct attribute, and disallow WaitGroup{} from outside the sync.new_waitgroup() function.
 [ref_only]
 struct WaitGroup {
 mut:
-	task_count       int // current task count
-	task_count_mutex Mutex // This mutex protects the task_count count in add()
-	wait_blocker     Semaphore // This blocks the wait() until released by add()
+	task_count u32 // current task count - reading/writing should be atomic
+	sem        Semaphore // This blocks wait() until tast_countreleased by add()
 }
 
 pub fn new_waitgroup() &WaitGroup {
 	mut wg := &WaitGroup{}
-	wg.task_count_mutex.init()
-	wg.wait_blocker.init(1)
+	wg.init()
 	return wg
 }
 
+pub fn (mut wg WaitGroup) init() {
+	wg.sem.init(0)
+}
+
 // 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.wait()
-	}
-	wg.task_count += delta
-	if wg.task_count < 0 {
+	old_nrjobs := int(C.atomic_fetch_add_u32(&wg.task_count, u32(delta)))
+	new_nrjobs := old_nrjobs + delta
+	if new_nrjobs < 0 {
 		panic('Negative number of jobs in waitgroup')
 	}
-	if wg.task_count == 0 { // if no more task_count tasks
-		wg.wait_blocker.post() // unblock wait()
+	if new_nrjobs == 0 {
+		wg.sem.post()
 	}
 }
 
@@ -54,6 +56,5 @@ pub fn (mut wg WaitGroup) done() {
 
 // wait blocks until all tasks are done (task count becomes zero)
 pub fn (mut wg WaitGroup) wait() {
-	wg.wait_blocker.wait() // blocks until task_count becomes 0
-	wg.wait_blocker.post() // allow other wait()s to unblock or reuse wait group
+	wg.sem.wait() // blocks until task_count becomes 0
 }