log: fix race when adding a log output while logging

Handle read the list of extra outputs without holding the mutex, so
calling AddOutput while logging caused a data race.

This reads the extra outputs once under the mutex.

fs/log/slog.go

@@ -360,17 +360,19 @@ func (h *OutputHandler) Handle(ctx context.Context, r slog.Record) (err error) {
 		buf     *bytes.Buffer
 	)
 
-	// Read the format under the mutex once so it is consistent
-	// throughout this call. The mutex is released before calling
-	// jsonLog/textLog (which may re-acquire it) to avoid deadlock.
+	// Read the format and the extra outputs under the mutex once
+	// so they are consistent throughout this call. The mutex is
+	// released before calling jsonLog/textLog (which may re-acquire
+	// it) to avoid deadlock.
 	h.mu.Lock()
 	format := h.format
+	outputExtra := h.outputExtra
 	h.mu.Unlock()
 
 	// Check whether we need to build Text or JSON logs or both
 	needJSON := format&logFormatJSON != 0
 	needText := !needJSON
-	for _, out := range h.outputExtra {
+	for _, out := range outputExtra {
 		if out.json {
 			needJSON = true
 		} else {
@@ -413,7 +415,7 @@ func (h *OutputHandler) Handle(ctx context.Context, r slog.Record) (err error) {
 	}
 
 	// Log to any additional destinations required
-	for _, out := range h.outputExtra {
+	for _, out := range outputExtra {
 		if out.json {
 			out.output(r.Level, bufJSON.String())
 		} else {

fs/log/slog_test.go

@@ -346,6 +346,15 @@ func TestOutputHandlerConcurrency(t *testing.T) {
 		}
 	})
 
+	// Goroutine calling AddOutput alternating JSON and text
+	wg.Go(func() {
+		noop := func(_ slog.Level, _ string) {}
+		for j := range 20 {
+			h.AddOutput(j%2 == 0, noop)
+			time.Sleep(time.Millisecond)
+		}
+	})
+
 	// Use a channel with a timeout to detect deadlocks
 	done := make(chan struct{})
 	go func() {