From 1dfa805f311aa31d22cf08ea2f68deba7f21d9ec Mon Sep 17 00:00:00 2001
From: Nedyalko Dyakov <nedyalko.dyakov@gmail.com>
Date: Mon, 10 Nov 2025 03:21:36 +0200
Subject: [PATCH] fix fast semaphore that could have been starved

---
 internal/semaphore.go | 31 +++++++++++--------------------
 1 file changed, 11 insertions(+), 20 deletions(-)

diff --git a/internal/semaphore.go b/internal/semaphore.go
index 091b6635..f00b3a4b 100644
--- a/internal/semaphore.go
+++ b/internal/semaphore.go
@@ -89,8 +89,6 @@ func (s *FastSemaphore) Acquire(ctx context.Context, timeout time.Duration, time
 	defer semTimers.Put(timer)
 	timer.Reset(timeout)
 
-	start := time.Now()
-
 	for {
 		select {
 		case <-ctx.Done():
@@ -100,23 +98,15 @@ func (s *FastSemaphore) Acquire(ctx context.Context, timeout time.Duration, time
 			return ctx.Err()
 
 		case <-s.waitCh:
-			// Someone released a token, try to acquire it
-			if s.TryAcquire() {
-				if !timer.Stop() {
-					<-timer.C
-				}
-				return nil
+			// Someone released a token, we got the spot
+			// no need to touch the counter
+			if !timer.Stop() {
+				<-timer.C
 			}
-			// Failed to acquire (race with another goroutine), continue waiting
-
+			return nil
 		case <-timer.C:
 			return timeoutErr
 		}
-
-		// Periodically check if we can acquire (handles race conditions)
-		if time.Since(start) > timeout {
-			return timeoutErr
-		}
 	}
 }
 
@@ -132,25 +122,26 @@ func (s *FastSemaphore) AcquireBlocking() {
 	// Slow path: wait for a token
 	for {
 		<-s.waitCh
-		if s.TryAcquire() {
-			return
-		}
-		// Failed to acquire (race with another goroutine), continue waiting
+		// Someone released a token, we got the spot
+		// no need to touch the counter
+		return
 	}
 }
 
 // Release releases a token back to the semaphore.
 // This wakes up one waiting goroutine if any are blocked.
 func (s *FastSemaphore) Release() {
-	s.count.Add(-1)
 
 	// Try to wake up a waiter (non-blocking)
 	// If no one is waiting, this is a no-op
 	select {
 	case s.waitCh <- struct{}{}:
 		// Successfully notified a waiter
+		// no need to decrement the counter, the waiter will use this spot
 	default:
 		// No waiters, that's fine
+		// decrement the counter
+		s.count.Add(-1)
 	}
 }