log: make write timeout per waiter instead of shared

log.go

@@ -136,7 +136,6 @@ func LockWithTimeoutAction(ws zapcore.WriteSyncer, timeout int, action string) z
 	r := &lockWithTimeoutWrapper{
 		ws:            ws,
 		lock:          make(chan struct{}, 1),
-		t:             time.NewTicker(time.Second),
 		timeout:       timeout,
 		timeoutAction: normalizeTimeoutAction(action),
 	}
@@ -146,7 +145,6 @@ func LockWithTimeoutAction(ws zapcore.WriteSyncer, timeout int, action string) z
 type lockWithTimeoutWrapper struct {
 	ws            zapcore.WriteSyncer
 	lock          chan struct{}
-	t             *time.Ticker
 	timeout       int
 	timeoutAction string
 	timedOut      atomic.Bool
@@ -173,16 +171,25 @@ func (s *lockWithTimeoutWrapper) getLockOrBlock() bool {
 		}
 	}
 
-	for i := 0; i < s.timeout; {
-		select {
-		case s.lock <- struct{}{}:
-			s.timedOut.Store(false)
-			return true
-		case <-s.t.C:
-			i++
-		}
+	select {
+	case s.lock <- struct{}{}:
+		s.timedOut.Store(false)
+		return true
+	default:
+	}
+
+	// Each caller waits on its own timer. A ticker shared by all waiters would
+	// deliver each tick to only one of them, stretching the wait to roughly
+	// timeout * waiters.
+	timer := time.NewTimer(time.Duration(s.timeout) * time.Second)
+	defer timer.Stop()
+	select {
+	case s.lock <- struct{}{}:
+		s.timedOut.Store(false)
+		return true
+	case <-timer.C:
+		return false
 	}
-	return false
 }
 
 func (s *lockWithTimeoutWrapper) unlock() {

log_test.go

@@ -18,6 +18,7 @@ import (
 	"bufio"
 	"bytes"
 	"net/url"
+	"sync"
 	"testing"
 	"time"
 
@@ -178,6 +179,35 @@ func TestTimeoutActionDiscard(t *testing.T) {
 	require.Contains(t, blocking.String(), "written")
 }
 
+func TestTimeoutNotScaledByWaiters(t *testing.T) {
+	const waiters = 8
+	for _, action := range []string{LogTimeoutActionDiscard, LogTimeoutActionPanic} {
+		t.Run(action, func(t *testing.T) {
+			blocking := newBlockingOnceWriteSyncer()
+			ws := LockWithTimeoutAction(blocking, 1, action)
+			go func() { _, _ = ws.Write([]byte("first")) }()
+			<-blocking.started
+
+			// All waiters queue up behind the stuck write at the same time. Each
+			// of them must give up after its own timeout, not after
+			// timeout * waiters.
+			start := time.Now()
+			var wg sync.WaitGroup
+			for range waiters {
+				wg.Add(1)
+				go func() {
+					defer wg.Done()
+					defer func() { _ = recover() }()
+					_, _ = ws.Write([]byte("blocked"))
+				}()
+			}
+			wg.Wait()
+			require.Less(t, time.Since(start), 3*time.Second)
+			close(blocking.release)
+		})
+	}
+}
+
 type hang struct{}
 
 func (_ hang) Write(_ []byte) (int, error) {