add comment and modify test

Author: zhiqiangxu

semaphore/semaphore.go

@@ -69,6 +69,7 @@ func (s *Weighted) Acquire(ctx context.Context, n int64) error {
 default:
 isFront := s.waiters.Front() == elem
 s.waiters.Remove(elem)
+// If we're at the front and there're extra tokens left, notify other waiters.
 if isFront && s.size > s.cur {
 s.notifyWaiters()
 }

semaphore/semaphore_test.go

@@ -174,39 +174,29 @@ func TestLargeAcquireDoesntStarve(t *testing.T) {
 func TestAllocCancelDoesntStarve(t *testing.T) {
 sem := semaphore.NewWeighted(10)
 
-// hold 1 read lock
+// Block off a portion of the semaphore so that Acquire(_, 10) can eventually succeed.
 sem.Acquire(context.Background(), 1)
 
+// In the background, Acquire(_, 10).
+ctx, cancel := context.WithCancel(context.Background())
+defer cancel()
 go func() {
-ctx, cancelFunc := context.WithTimeout(context.Background(), time.Millisecond*300)
-defer cancelFunc()
-// start a write lock request that will giveup after 300ms
-err := sem.Acquire(ctx, 10)
-if err == nil {
-t.FailNow()
-}
+sem.Acquire(ctx, 10)
 }()
 
-// sleep 100ms, long enough for the Lock request to be queued
-time.Sleep(time.Millisecond * 100)
-
-// this channel will be closed if the following RLock succeeded
-doneCh := make(chan struct{})
-go func() {
-// try to grab a read lock, it will be queued after the Lock request
-// but should be notified when the Lock request is canceled
-err := sem.Acquire(context.Background(), 1)
-if err != nil {
-t.FailNow()
-}
+// Wait until the Acquire(_, 10) call blocks.
+for sem.TryAcquire(1) {
 sem.Release(1)
-close(doneCh)
-}()
+runtime.Gosched()
+}
+
+// Now try to grab a read lock, and simultaneously unblock the Acquire(_, 10) call.
+// Both Acquire calls should unblock and return, in either order.
+go cancel()
 
-// doneCh should be closed if the above RLock succeeded
-select {
-case <-doneCh:
-case <-time.After(time.Second):
-t.FailNow()
+err := sem.Acquire(context.Background(), 1)
+if err != nil {
+t.Fatalf("Acquire(_, 1) failed unexpectedly: %v", err)
 }
+sem.Release(1)
 }