log: cache default level when no vmodule patterns configured and add downgrade test

log/handler_glog.go

@@ -213,9 +213,20 @@ func (h *GlogHandler) Handle(ctx context.Context, r slog.Record) error {
 		lvl = slog.Level(h.level.Load())
 	}
 
-	// Store only if patterns are still the same slice (avoid caching stale results).
+	// Check if we should cache this result
 	h.lock.Lock()
-	if len(h.patterns) > 0 && len(curPatterns) > 0 && &h.patterns[0] == &curPatterns[0] {
+	shouldCache := false
+	switch {
+	case len(curPatterns) == 0 && len(h.patterns) == 0:
+		// Cache the default/global level to avoid re-evaluating the callsite each time.
+		shouldCache = true
+	case len(h.patterns) > 0 && len(curPatterns) > 0:
+		// Only cache the result if the vmodule patterns have not changed since we
+		// snapshotted them. This avoids inserting stale entries if Vmodule() updates
+		// the pattern list concurrently with Handle().
+		shouldCache = (&h.patterns[0] == &curPatterns[0])
+	}
+	if shouldCache {
 		cache.Store(r.PC, lvl)
 	}
 	h.lock.Unlock()

log/logger_test.go

@@ -33,6 +33,35 @@ func TestLoggingWithVmodule(t *testing.T) {
 	}
 }
 
+func TestLoggingWithVmoduleDowngrade(t *testing.T) {
+	out := new(bytes.Buffer)
+	glog := NewGlogHandler(NewTerminalHandlerWithLevel(out, LevelTrace, false))
+	glog.Verbosity(LevelTrace) // Allow all logs globally
+	logger := NewLogger(glog)
+
+	// This should appear (global level allows it)
+	logger.Info("before vmodule downgrade, this should be logged")
+	if !bytes.Contains(out.Bytes(), []byte("before vmodule downgrade")) {
+		t.Fatal("expected 'before vmodule downgrade' to be logged")
+	}
+	out.Reset()
+
+	// Downgrade this file to only allow Warn and above
+	glog.Vmodule("logger_test.go=2")
+
+	// Info should now be filtered out
+	logger.Info("after vmodule downgrade, this should be filtered")
+	if bytes.Contains(out.Bytes(), []byte("after vmodule downgrade, this should be filtered")) {
+		t.Fatal("expected 'after vmodule downgrade, this should be filtered' to NOT be logged after vmodule downgrade")
+	}
+
+	// Warn should still appear
+	logger.Warn("after vmodule downgrade, this should be logged")
+	if !bytes.Contains(out.Bytes(), []byte("after vmodule downgrade, this should be logged")) {
+		t.Fatal("expected 'should appear' to be logged")
+	}
+}
+
 func TestTerminalHandlerWithAttrs(t *testing.T) {
 	out := new(bytes.Buffer)
 	glog := NewGlogHandler(NewTerminalHandlerWithLevel(out, LevelTrace, false).WithAttrs([]slog.Attr{slog.String("baz", "bat")}))