fix: lock accesses to encoder in json stdout exporter to prevent crash (#2265)

* lock accesses to encoder

fixes #2264

* move locking outside loop to avoid deadlock

* Update CHANGELOG.md

CHANGELOG.md

@@ -8,6 +8,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
+### Fixed
+
+- json stdout exporter no longer crashes due to concurrency bug. (#2265)
+
 ### Changed
 
 - NoopMeterProvider is now private and NewNoopMeterProvider must be used to obtain a noopMeterProvider. (#2237)

exporters/stdout/stdouttrace/trace.go

@@ -49,6 +49,7 @@ func New(options ...Option) (*Exporter, error) {
 // Exporter is an implementation of trace.SpanSyncer that writes spans to stdout.
 type Exporter struct {
 	encoder    *json.Encoder
+	encoderMu  sync.Mutex
 	timestamps bool
 
 	stoppedMu sync.RWMutex
@@ -70,6 +71,8 @@ func (e *Exporter) ExportSpans(ctx context.Context, spans []trace.ReadOnlySpan)
 
 	stubs := tracetest.SpanStubsFromReadOnlySpans(spans)
 
+	e.encoderMu.Lock()
+	defer e.encoderMu.Unlock()
 	for i := range stubs {
 		stub := &stubs[i]
 		// Remove timestamps