fix: UpdateContext skips Nop and zero-value loggers (#754)

UpdateContext previously used pointer equality (l == disabledLogger) to
detect disabled loggers. This only caught the package-level singleton
returned by Ctx() but missed loggers created via Nop() or zero-value
Logger{}, which could lead to data races when a shared Nop logger was
used from multiple goroutines.

Replace the pointer comparison with a check for nil writer or Disabled
level, consistent with how Logger.should() determines if logging is
active. This prevents unnecessary context mutations on loggers that
will never produce output.

Fixes #643

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

log.go

@@ -294,7 +294,7 @@ func (l Logger) With() Context {
 // Caution: This method is not concurrency safe.
 // Use the With method to create a child logger before modifying the context from concurrent goroutines.
 func (l *Logger) UpdateContext(update func(c Context) Context) {
-	if l == disabledLogger {
+	if l.disabled() {
 		return
 	}
 	if cap(l.context) == 0 {
@@ -508,9 +508,14 @@ func (l *Logger) scratchEvent() *Event {
 	return newEvent(LevelWriterAdapter{io.Discard}, DebugLevel, l.stack, l.ctx, l.hooks)
 }
 
+// disabled returns true if the logger is a disabled or nop logger.
+func (l *Logger) disabled() bool {
+	return l.w == nil || l.level == Disabled
+}
+
 // should returns true if the log event should be logged.
 func (l *Logger) should(lvl Level) bool {
-	if l.w == nil {
+	if l.disabled() {
 		return false
 	}
 	if lvl < l.level || lvl < GlobalLevel() {

log_test.go

@@ -1214,6 +1214,39 @@ func TestUpdateContextOnDisabledLogger(t *testing.T) {
 	}
 }
 
+func TestUpdateContextOnNopLogger(t *testing.T) {
+	// Nop() creates a new disabled logger that is a different pointer than
+	// the package-level disabledLogger. UpdateContext should still treat it
+	// as disabled and skip the update function. See issue #643.
+	log := Nop()
+
+	called := false
+	log.UpdateContext(func(c Context) Context {
+		called = true
+		return c.Str("foo", "bar")
+	})
+
+	if called {
+		t.Error("UpdateContext should not call update func on a Nop logger")
+	}
+}
+
+func TestUpdateContextOnZeroValueLogger(t *testing.T) {
+	// A zero-value Logger has a nil writer and should be treated as
+	// disabled by UpdateContext. See issue #643.
+	var log Logger
+
+	called := false
+	log.UpdateContext(func(c Context) Context {
+		called = true
+		return c.Str("foo", "bar")
+	})
+
+	if called {
+		t.Error("UpdateContext should not call update func on a zero-value logger")
+	}
+}
+
 func TestLevel_String(t *testing.T) {
 	tests := []struct {
 		name string