when using WithNewRoot, don't use the parent context for sampling (#2032)

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

CHANGELOG.md

@@ -26,6 +26,8 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ### Fixed
 
+- When using WithNewRoot, don't use the parent context for making sampling decisions. (#2032)
+
 ### Security
 
 ## [1.0.0-RC1] / 0.21.0 - 2021-06-18

sdk/trace/span.go

@@ -559,7 +559,9 @@ func startSpanInternal(ctx context.Context, tr *tracer, name string, o *trace.Sp
 	// If told explicitly to make this a new root use a zero value SpanContext
 	// as a parent which contains an invalid trace ID and is not remote.
 	var psc trace.SpanContext
-	if !o.NewRoot() {
+	if o.NewRoot() {
+		ctx = trace.ContextWithSpanContext(ctx, psc)
+	} else {
 		psc = trace.SpanContextFromContext(ctx)
 	}
 

sdk/trace/trace_test.go

@@ -376,6 +376,30 @@ func TestStartSpanWithParent(t *testing.T) {
 	}
 }
 
+func TestStartSpanNewRootNotSampled(t *testing.T) {
+	alwaysSampleTp := NewTracerProvider()
+	sampledTr := alwaysSampleTp.Tracer("AlwaysSampled")
+	neverSampleTp := NewTracerProvider(WithSampler(ParentBased(NeverSample())))
+	neverSampledTr := neverSampleTp.Tracer("ParentBasedNeverSample")
+	ctx := context.Background()
+
+	ctx, s1 := sampledTr.Start(trace.ContextWithRemoteSpanContext(ctx, sc), "span1-sampled")
+	if err := checkChild(t, sc, s1); err != nil {
+		t.Error(err)
+	}
+
+	_, s2 := neverSampledTr.Start(ctx, "span2-no-newroot")
+	if !s2.SpanContext().IsSampled() {
+		t.Error(fmt.Errorf("got child span is not sampled, want child span with sampler: ParentBased(NeverSample()) to be sampled"))
+	}
+
+	// Adding WithNewRoot causes child spans to not sample based on parent context
+	_, s3 := neverSampledTr.Start(ctx, "span3-newroot", trace.WithNewRoot())
+	if s3.SpanContext().IsSampled() {
+		t.Error(fmt.Errorf("got child span is sampled, want child span WithNewRoot() and with sampler: ParentBased(NeverSample()) to not be sampled"))
+	}
+}
+
 func TestSetSpanAttributesOnStart(t *testing.T) {
 	te := NewTestExporter()
 	tp := NewTracerProvider(WithSyncer(te), WithResource(resource.Empty()))