syncs: add Semaphore.Len (#16981)

The Len reports the number of acquired tokens for metrics.

Updates tailscale/corp#31252

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

syncs/syncs.go

@@ -201,6 +201,13 @@ func NewSemaphore(n int) Semaphore {
 	return Semaphore{c: make(chan struct{}, n)}
 }
 
+// Len reports the number of in-flight acquisitions.
+// It is incremented whenever the semaphore is acquired.
+// It is decremented whenever the semaphore is released.
+func (s Semaphore) Len() int {
+	return len(s.c)
+}
+
 // Acquire blocks until a resource is acquired.
 func (s Semaphore) Acquire() {
 	s.c <- struct{}{}

syncs/syncs_test.go

@@ -162,10 +162,20 @@ func TestClosedChan(t *testing.T) {
 
 func TestSemaphore(t *testing.T) {
 	s := NewSemaphore(2)
+	assertLen := func(want int) {
+		t.Helper()
+		if got := s.Len(); got != want {
+			t.Fatalf("Len = %d, want %d", got, want)
+		}
+	}
+
+	assertLen(0)
 	s.Acquire()
+	assertLen(1)
 	if !s.TryAcquire() {
 		t.Fatal("want true")
 	}
+	assertLen(2)
 	if s.TryAcquire() {
 		t.Fatal("want false")
 	}
@@ -175,11 +185,15 @@ func TestSemaphore(t *testing.T) {
 		t.Fatal("want false")
 	}
 	s.Release()
+	assertLen(1)
 	if !s.AcquireContext(context.Background()) {
 		t.Fatal("want true")
 	}
+	assertLen(2)
 	s.Release()
+	assertLen(1)
 	s.Release()
+	assertLen(0)
 }
 
 func TestMap(t *testing.T) {