waiter may double-release (if closed/times out)

internal/semaphore.go

@@ -107,17 +107,6 @@ func (s *FastSemaphore) dequeue() *waiter {
 	return w
 }
 
-// notifyOne wakes up the first waiter in the queue if any.
-func (s *FastSemaphore) notifyOne() {
-	s.lock.Lock()
-	w := s.dequeue()
-	s.lock.Unlock()
-
-	if w != nil {
-		close(w.ready)
-	}
-}
-
 // Acquire acquires a token, blocking if necessary until one is available or the context is cancelled.
 // Returns an error if the context is cancelled or the timeout expires.
 // Returns timeoutErr when the timeout expires.
@@ -273,16 +262,47 @@ func (s *FastSemaphore) AcquireBlocking() {
 // This should be called when a waiter was notified but then cancelled/timed out.
 // We need to pass the token to another waiter if any, otherwise put it back in the channel.
 func (s *FastSemaphore) releaseToPool() {
+	// Try to give the token to a waiter first
+	for {
 		s.lock.Lock()
 		w := s.dequeue()
 		s.lock.Unlock()
 
-	if w != nil {
-		// Transfer the token to another waiter
-		close(w.ready)
-	} else {
+		if w == nil {
 			// No waiters, put the token back in the channel
-		s.tokens <- struct{}{}
+			// Use select to avoid blocking (should never block, but just in case)
+			select {
+			case s.tokens <- struct{}{}:
+				return
+			default:
+				// Channel is full - this should never happen!
+				// It means we have a logic error in token accounting
+				panic("semaphore: releaseToPool: channel is full")
+			}
+		}
+
+		// Try to claim this waiter by setting notified flag
+		// If the waiter is being cancelled concurrently, one of us will win
+		if !w.notified.CompareAndSwap(false, true) {
+			// Someone else (the waiter itself) already claimed it
+			// This means the waiter is cancelling, skip to next
+			close(w.ready) // Still need to close to unblock them
+			continue
+		}
+
+		// We successfully claimed the waiter
+		// Check if it was cancelled after we claimed it (shouldn't happen, but check anyway)
+		if w.cancelled.Load() {
+			// Waiter was cancelled, but we claimed it first
+			// We need to release the token
+			close(w.ready) // Unblock the waiter
+			// Continue to try next waiter
+			continue
+		}
+
+		// We successfully claimed the waiter, transfer the token
+		close(w.ready)
+		return
 	}
 }
 
@@ -301,14 +321,6 @@ func (s *FastSemaphore) Release() {
 			return
 		}
 
-		// Check if this waiter was cancelled before we notify them
-		if w.cancelled.Load() {
-			// This waiter was cancelled, skip them and try the next one
-			// We still have the token, so continue the loop
-			close(w.ready) // Still need to close to unblock them
-			continue
-		}
-
 		// Try to claim this waiter by setting notified flag
 		// If the waiter is being cancelled concurrently, one of us will win
 		if !w.notified.CompareAndSwap(false, true) {
@@ -318,6 +330,16 @@ func (s *FastSemaphore) Release() {
 			continue
 		}
 
+		// We successfully claimed the waiter
+		// Check if it was cancelled after we claimed it (shouldn't happen, but check anyway)
+		if w.cancelled.Load() {
+			// Waiter was cancelled, but we claimed it first
+			// We need to release the token
+			close(w.ready) // Unblock the waiter
+			// Continue to try next waiter
+			continue
+		}
+
 		// We successfully claimed the waiter, transfer the token
 		close(w.ready)
 		return