fix(tracing): Make Span.Finish a no-op when the span is already finished

tracing.go

@@ -60,6 +60,8 @@ type Span struct { //nolint: maligned // prefer readability over optimal memory
 	contexts map[string]Context
 	// profiler instance if attached, nil otherwise.
 	profiler transactionProfiler
+	// a Once instance to make sure that Finish() is only called once.
+	finishOnce sync.Once
 }
 
 // TraceParentContext describes the context of a (remote) parent span.
@@ -196,35 +198,11 @@ func StartSpan(ctx context.Context, operation string, options ...SpanOption) *Sp
 
 // Finish sets the span's end time, unless already set. If the span is the root
 // of a span tree, Finish sends the span tree to Sentry as a transaction.
+//
+// The logic is executed at most once per span, so that (incorrectly) calling it twice
+// never double sends to Sentry.
 func (s *Span) Finish() {
-	// TODO(tracing): maybe make Finish run at most once, such that
-	// (incorrectly) calling it twice never double sends to Sentry.
-
-	// For the timing to be correct, the profiler must be stopped before s.EndTime.
-	var profile *profileInfo
-	if s.profiler != nil {
-		profile = s.profiler.Finish(s)
-	}
-
-	if s.EndTime.IsZero() {
-		s.EndTime = monotonicTimeSince(s.StartTime)
-	}
-
-	if !s.Sampled.Bool() {
-		return
-	}
-	event := s.toEvent()
-	if event == nil {
-		return
-	}
-
-	event.sdkMetaData.transactionProfile = profile
-
-	// TODO(tracing): add breadcrumbs
-	// (see https://github.com/getsentry/sentry-python/blob/f6f3525f8812f609/sentry_sdk/tracing.py#L372)
-
-	hub := hubFromContext(s.ctx)
-	hub.CaptureEvent(event)
+	s.finishOnce.Do(s.doFinish)
 }
 
 // Context returns the context containing the span.
@@ -353,6 +331,35 @@ func (s *Span) SetDynamicSamplingContext(dsc DynamicSamplingContext) {
 	}
 }
 
+// doFinish runs the actual Span.Finish() logic.
+func (s *Span) doFinish() {
+	// For the timing to be correct, the profiler must be stopped before s.EndTime.
+	var profile *profileInfo
+	if s.profiler != nil {
+		profile = s.profiler.Finish(s)
+	}
+
+	if s.EndTime.IsZero() {
+		s.EndTime = monotonicTimeSince(s.StartTime)
+	}
+
+	if !s.Sampled.Bool() {
+		return
+	}
+	event := s.toEvent()
+	if event == nil {
+		return
+	}
+
+	event.sdkMetaData.transactionProfile = profile
+
+	// TODO(tracing): add breadcrumbs
+	// (see https://github.com/getsentry/sentry-python/blob/f6f3525f8812f609/sentry_sdk/tracing.py#L372)
+
+	hub := hubFromContext(s.ctx)
+	hub.CaptureEvent(event)
+}
+
 // sentryTracePattern matches either
 //
 //	TRACE_ID - SPAN_ID

tracing_test.go

@@ -980,3 +980,28 @@ func TestAdjustingTransactionSourceBeforeSending(t *testing.T) {
 		})
 	}
 }
+
+// This is a regression test for https://github.com/getsentry/sentry-go/issues/587
+// Without the "spans can be finished only once" fix, this test will fail
+// when run with race detection ("-race").
+func TestSpanFinishConcurrentlyWithoutRaces(t *testing.T) {
+	ctx := NewTestContext(ClientOptions{
+		EnableTracing:    true,
+		TracesSampleRate: 1,
+	})
+	transaction := StartTransaction(ctx, "op")
+
+	go func() {
+		for {
+			transaction.Finish()
+		}
+	}()
+
+	go func() {
+		for {
+			transaction.Finish()
+		}
+	}()
+
+	time.Sleep(50 * time.Millisecond)
+}