trace: fix race condition

trace/trace.go

@@ -65,6 +65,11 @@ func durationToMilliseconds(timeDuration time.Duration) int64 {
 }
 
 type traceItem interface {
+	// rLock must be called before invoking time or writeItem.
+	rLock()
+	// rUnlock must be called after processing the item is complete.
+	rUnlock()
+
 	// time returns when the trace was recorded as completed.
 	time() time.Time
 	// writeItem outputs the traceItem to the buffer. If stepThreshold is non-nil, only output the
@@ -79,6 +84,10 @@ type traceStep struct {
 	fields   []Field
 }
 
+// rLock doesn't need to do anything because traceStep instances are immutable.
+func (s traceStep) rLock()   {}
+func (s traceStep) rUnlock() {}
+
 func (s traceStep) time() time.Time {
 	return s.stepTime
 }
@@ -106,6 +115,14 @@ type Trace struct {
 	traceItems []traceItem
 }
 
+func (t *Trace) rLock() {
+	t.lock.RLock()
+}
+
+func (t *Trace) rUnlock() {
+	t.lock.RUnlock()
+}
+
 func (t *Trace) time() time.Time {
 	if t.endTime != nil {
 		return *t.endTime
@@ -231,8 +248,10 @@ func (t *Trace) logTrace() {
 func (t *Trace) writeTraceSteps(b *bytes.Buffer, formatter string, stepThreshold *time.Duration) {
 	lastStepTime := t.startTime
 	for _, stepOrTrace := range t.traceItems {
+		stepOrTrace.rLock()
 		stepOrTrace.writeItem(b, formatter, lastStepTime, stepThreshold)
 		lastStepTime = stepOrTrace.time()
+		stepOrTrace.rUnlock()
 	}
 }