[client] Fix semaphore slot leaks (#5018)

- Remove WaitGroup, make SemaphoreGroup a pure semaphore
- Make Add() return error instead of silently failing on context cancel
- Remove context parameter from Done() to prevent slot leaks
- Fix missing Done() call in conn.go error path

client/internal/peer/conn.go

@@ -148,13 +148,15 @@ func NewConn(config ConnConfig, services ServiceDependencies) (*Conn, error) {
 // It will try to establish a connection using ICE and in parallel with relay. The higher priority connection type will
 // be used.
 func (conn *Conn) Open(engineCtx context.Context) error {
-	conn.semaphore.Add(engineCtx)
+	if err := conn.semaphore.Add(engineCtx); err != nil {
+		return err
+	}
 
 	conn.mu.Lock()
 	defer conn.mu.Unlock()
 
 	if conn.opened {
-		conn.semaphore.Done(engineCtx)
+		conn.semaphore.Done()
 		return nil
 	}
 
@@ -165,6 +167,7 @@ func (conn *Conn) Open(engineCtx context.Context) error {
 	relayIsSupportedLocally := conn.workerRelay.RelayIsSupportedLocally()
 	workerICE, err := NewWorkerICE(conn.ctx, conn.Log, conn.config, conn, conn.signaler, conn.iFaceDiscover, conn.statusRecorder, relayIsSupportedLocally)
 	if err != nil {
+		conn.semaphore.Done()
 		return err
 	}
 	conn.workerICE = workerICE
@@ -200,7 +203,7 @@ func (conn *Conn) Open(engineCtx context.Context) error {
 		defer conn.wg.Done()
 
 		conn.waitInitialRandomSleepTime(conn.ctx)
-		conn.semaphore.Done(conn.ctx)
+		conn.semaphore.Done()
 
 		conn.guard.Start(conn.ctx, conn.onGuardEvent)
 	}()

util/semaphore-group/semaphore_group.go

@@ -2,12 +2,10 @@ package semaphoregroup
 
 import (
 	"context"
-	"sync"
 )
 
 // SemaphoreGroup is a custom type that combines sync.WaitGroup and a semaphore.
 type SemaphoreGroup struct {
-	waitGroup sync.WaitGroup
 	semaphore chan struct{}
 }
 
@@ -18,31 +16,18 @@ func NewSemaphoreGroup(limit int) *SemaphoreGroup {
 	}
 }
 
-// Add increments the internal WaitGroup counter and acquires a semaphore slot.
-func (sg *SemaphoreGroup) Add(ctx context.Context) {
-	sg.waitGroup.Add(1)
-
+// Add acquire a slot
+func (sg *SemaphoreGroup) Add(ctx context.Context) error {
 	// Acquire semaphore slot
 	select {
 	case <-ctx.Done():
-		return
+		return ctx.Err()
 	case sg.semaphore <- struct{}{}:
+		return nil
 	}
 }
 
-// Done decrements the internal WaitGroup counter and releases a semaphore slot.
-func (sg *SemaphoreGroup) Done(ctx context.Context) {
-	sg.waitGroup.Done()
-
-	// Release semaphore slot
-	select {
-	case <-ctx.Done():
-		return
-	case <-sg.semaphore:
-	}
-}
-
-// Wait waits until the internal WaitGroup counter is zero.
-func (sg *SemaphoreGroup) Wait() {
-	sg.waitGroup.Wait()
+// Done releases a slot. Must be called after a successful Add.
+func (sg *SemaphoreGroup) Done() {
+	<-sg.semaphore
 }

util/semaphore-group/semaphore_group_test.go

@@ -2,65 +2,89 @@ package semaphoregroup
 
 import (
 	"context"
+	"sync"
 	"testing"
 	"time"
 )
 
 func TestSemaphoreGroup(t *testing.T) {
-	semGroup := NewSemaphoreGroup(2)
-
-	for i := 0; i < 5; i++ {
-		semGroup.Add(context.Background())
-		go func(id int) {
-			defer semGroup.Done(context.Background())
-
-			got := len(semGroup.semaphore)
-			if got == 0 {
-				t.Errorf("Expected semaphore length > 0 , got 0")
-			}
-
-			time.Sleep(time.Millisecond)
-			t.Logf("Goroutine %d is running\n", id)
-		}(i)
-	}
-
-	semGroup.Wait()
-
-	want := 0
-	got := len(semGroup.semaphore)
-	if got != want {
-		t.Errorf("Expected semaphore length %d, got %d", want, got)
-	}
-}
-
-func TestSemaphoreGroupContext(t *testing.T) {
 	semGroup := NewSemaphoreGroup(1)
-	semGroup.Add(context.Background())
-	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
+	_ = semGroup.Add(context.Background())
+
+	ctxTimeout, cancel := context.WithTimeout(context.Background(), 10*time.Millisecond)
 	t.Cleanup(cancel)
-	rChan := make(chan struct{})
 
-	go func() {
-		semGroup.Add(ctx)
-		rChan <- struct{}{}
-	}()
-	select {
-	case <-rChan:
-	case <-time.NewTimer(2 * time.Second).C:
-		t.Error("Adding to semaphore group should not block when context is not done")
-	}
-
-	semGroup.Done(context.Background())
-
-	ctxDone, cancelDone := context.WithTimeout(context.Background(), 1*time.Second)
-	t.Cleanup(cancelDone)
-	go func() {
-		semGroup.Done(ctxDone)
-		rChan <- struct{}{}
-	}()
-	select {
-	case <-rChan:
-	case <-time.NewTimer(2 * time.Second).C:
-		t.Error("Releasing from semaphore group should not block when context is not done")
+	if err := semGroup.Add(ctxTimeout); err == nil {
+		t.Error("Adding to semaphore group should not block")
+	}
+}
+
+func TestSemaphoreGroupFreeUp(t *testing.T) {
+	semGroup := NewSemaphoreGroup(1)
+	_ = semGroup.Add(context.Background())
+	semGroup.Done()
+
+	ctxTimeout, cancel := context.WithTimeout(context.Background(), 10*time.Millisecond)
+	t.Cleanup(cancel)
+	if err := semGroup.Add(ctxTimeout); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestSemaphoreGroupCanceledContext(t *testing.T) {
+	semGroup := NewSemaphoreGroup(1)
+	_ = semGroup.Add(context.Background())
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // Cancel immediately
+
+	if err := semGroup.Add(ctx); err == nil {
+		t.Error("Add should return error when context is already canceled")
+	}
+}
+
+func TestSemaphoreGroupCancelWhileWaiting(t *testing.T) {
+	semGroup := NewSemaphoreGroup(1)
+	_ = semGroup.Add(context.Background())
+
+	ctx, cancel := context.WithCancel(context.Background())
+	errChan := make(chan error, 1)
+
+	go func() {
+		errChan <- semGroup.Add(ctx)
+	}()
+
+	time.Sleep(10 * time.Millisecond)
+	cancel()
+
+	if err := <-errChan; err == nil {
+		t.Error("Add should return error when context is canceled while waiting")
+	}
+}
+
+func TestSemaphoreGroupHighConcurrency(t *testing.T) {
+	const limit = 10
+	const numGoroutines = 100
+
+	semGroup := NewSemaphoreGroup(limit)
+	var wg sync.WaitGroup
+
+	for i := 0; i < numGoroutines; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if err := semGroup.Add(context.Background()); err != nil {
+				t.Errorf("Unexpected error: %v", err)
+				return
+			}
+			time.Sleep(time.Millisecond)
+			semGroup.Done()
+		}()
+	}
+
+	wg.Wait()
+
+	// Verify all slots were released
+	if got := len(semGroup.semaphore); got != 0 {
+		t.Errorf("Expected semaphore to be empty, got %d slots occupied", got)
 	}
 }