ddtrace/tracer: fix concurrent map writes when applying trace samplin…

…g rules and setting tags concurrently

ddtrace/tracer/rules_sampler.go

@@ -498,20 +498,23 @@ func (rs *traceRulesSampler) sampleRules(span *span) bool {
 }
 
 func (rs *traceRulesSampler) applyRate(span *span, rate float64, now time.Time, sampler samplernames.SamplerName) {
+	span.Lock()
+	defer span.Unlock()
+
 	span.setMetric(keyRulesSamplerAppliedRate, rate)
 	delete(span.Metrics, keySamplingPriorityRate)
 	if !sampledByRate(span.TraceID, rate) {
-		span.setSamplingPriority(ext.PriorityUserReject, sampler)
+		span.setSamplingPriorityLocked(ext.PriorityUserReject, sampler)
 		return
 	}
 
 	sampled, rate := rs.limiter.allowOne(now)
 	if sampled {
-		span.setSamplingPriority(ext.PriorityUserKeep, sampler)
+		span.setSamplingPriorityLocked(ext.PriorityUserKeep, sampler)
 	} else {
-		span.setSamplingPriority(ext.PriorityUserReject, sampler)
+		span.setSamplingPriorityLocked(ext.PriorityUserReject, sampler)
 	}
-	span.SetTag(keyRulesSamplerLimiterRate, rate)
+	span.setMetric(keyRulesSamplerLimiterRate, rate)
 }
 
 // limit returns the rate limit set in the rules sampler, controlled by DD_TRACE_RATE_LIMIT, and

ddtrace/tracer/span_test.go

@@ -1076,3 +1076,30 @@ type stringer struct{}
 func (s *stringer) String() string {
 	return "string"
 }
+
+// TestConcurrentSpanSetTag tests that setting tags concurrently on a span directly or
+// not (through tracer.Inject when trace sampling rules are in place) does not cause
+// concurrent map writes. It seems to only be consistently reproduced with the -count=100
+// flag when running go test, but it's a good test to have.
+func TestConcurrentSpanSetTag(t *testing.T) {
+	tracer, _, _, stop := startTestTracer(t, WithSamplingRules([]SamplingRule{NameRule("root", 1.0)}))
+	defer stop()
+
+	span := tracer.StartSpan("root")
+	defer span.Finish()
+
+	const n = 100
+	wg := sync.WaitGroup{}
+	wg.Add(n * 2)
+	for i := 0; i < n; i++ {
+		go func() {
+			span.SetTag("key", "value")
+			wg.Done()
+		}()
+		go func() {
+			tracer.Inject(span.Context(), TextMapCarrier(map[string]string{}))
+			wg.Done()
+		}()
+	}
+	wg.Wait()
+}