ddtrace/tracer: Always count dropped p0 traces and spans (#1461)

ddtrace/tracer/tracer.go

@@ -334,25 +334,24 @@ func (t *tracer) sampleFinishedTrace(info *finishedTrace) {
 	if info.decision == decisionKeep {
 		return
 	}
-	if !t.rulesSampling.HasSpanRules() {
-		info.spans = nil
-		return
-	}
-	// if trace sampling decision is drop, we still want to send single spans
-	// unless there are no single span sampling rules defined
 	var kept []*span
-	for _, span := range info.spans {
-		if t.rulesSampling.SampleSpan(span) {
-			kept = append(kept, span)
+	if t.rulesSampling.HasSpanRules() {
+		// Apply sampling rules to individual spans in the trace.
+		for _, span := range info.spans {
+			if t.rulesSampling.SampleSpan(span) {
+				kept = append(kept, span)
+			}
+		}
+		if len(kept) > 0 && len(kept) < len(info.spans) {
+			// Some spans in the trace were kept, so a partial trace will be sent.
+			atomic.AddUint64(&t.partialTraces, 1)
 		}
 	}
-	atomic.AddUint64(&t.droppedP0Spans, uint64(len(info.spans)-len(kept)))
-	info.spans = kept
 	if len(kept) == 0 {
 		atomic.AddUint64(&t.droppedP0Traces, 1)
-		return // no spans matched the rules and were sampled
 	}
-	atomic.AddUint64(&t.partialTraces, 1)
+	atomic.AddUint64(&t.droppedP0Spans, uint64(len(info.spans)-len(kept)))
+	info.spans = kept
 }
 
 func (t *tracer) pushTrace(trace *finishedTrace) {

ddtrace/tracer/tracer_test.go

@@ -351,6 +351,11 @@ func TestSamplingDecision(t *testing.T) {
 
 	t.Run("client_dropped", func(t *testing.T) {
 		tracer, _, _, stop := startTestTracer(t)
+		defer func() {
+			// Must check these after tracer is stopped to avoid flakiness
+			assert.Equal(t, uint64(1), tracer.droppedP0Traces)
+			assert.Equal(t, uint64(2), tracer.droppedP0Spans)
+		}()
 		defer stop()
 		tracer.config.agent.DropP0s = true
 		tracer.config.sampler = NewRateSampler(0)

internal/version/version.go

@@ -8,4 +8,4 @@ package version
 // Tag specifies the current release tag. It needs to be manually
 // updated. A test checks that the value of Tag never points to a
 // git tag that is older than HEAD.
-const Tag = "v1.41.0"
+const Tag = "v1.41.1"