Allow for setting an explicit SpanContext on Span creation

api/trace/api.go

@@ -97,12 +97,13 @@ type StartOption func(*StartConfig)
 // StartConfig provides options to set properties of span at the time of starting
 // a new span.
 type StartConfig struct {
-	Attributes []core.KeyValue
-	StartTime  time.Time
-	Links      []Link
-	Relation   Relation
-	Record     bool
-	SpanKind   SpanKind
+	Attributes  []core.KeyValue
+	StartTime   time.Time
+	Links       []Link
+	Relation    Relation
+	Record      bool
+	SpanKind    SpanKind
+	SpanContext core.SpanContext
 }
 
 // Relation is used to establish relationship between newly created span and the
@@ -249,3 +250,10 @@ func WithSpanKind(sk SpanKind) StartOption {
 		c.SpanKind = sk
 	}
 }
+
+// WithSpanContext specifies the SpanContext to use for a Trace.
+func WithSpanContext(ctx core.SpanContext) StartOption {
+	return func(c *StartConfig) {
+		c.SpanContext = ctx
+	}
+}

sdk/trace/span.go

@@ -248,15 +248,25 @@ func (s *span) addChild() {
 func startSpanInternal(tr *tracer, name string, parent core.SpanContext, remoteParent bool, o apitrace.StartConfig) *span {
 	var noParent bool
 	span := &span{}
-	span.spanContext = parent
-
 	cfg := tr.provider.config.Load().(*Config)
 
-	if parent == core.EmptySpanContext() {
-		span.spanContext.TraceID = cfg.IDGenerator.NewTraceID()
-		noParent = true
+	if o.SpanContext.IsValid() {
+		// if an explicit span context was provided, use it ...
+		span.spanContext = o.SpanContext
+	} else {
+		// ... otherwise, use the parent ...
+		span.spanContext = parent
+
+		// ... unless there was no parent, in which case generate a new TraceID ...
+		if parent == core.EmptySpanContext() {
+			noParent = true
+			span.spanContext.TraceID = cfg.IDGenerator.NewTraceID()
+		}
+
+		// ... always generate a new SpanID
+		span.spanContext.SpanID = cfg.IDGenerator.NewSpanID()
 	}
-	span.spanContext.SpanID = cfg.IDGenerator.NewSpanID()
+
 	data := samplingData{
 		noParent:     noParent,
 		remoteParent: remoteParent,

sdk/trace/trace_test.go

@@ -208,6 +208,28 @@ func TestSampling(t *testing.T) {
 	}
 }
 
+func TestStartSpanWithExplicitSpanContext(t *testing.T) {
+	tp, _ := NewProvider()
+	tr := tp.Tracer("WithSpanContext")
+
+	sc1 := core.SpanContext{
+		TraceID:    tid,
+		SpanID:     sid,
+		TraceFlags: 0x0,
+	}
+
+	_, s := tr.Start(context.Background(), "span1", apitrace.WithSpanContext(sc1))
+	if s.SpanContext() != sc1 {
+		t.Errorf("got %+v, want %+v", s.SpanContext(), sc1)
+	}
+
+	sc2 := core.EmptySpanContext()
+	_, s = tr.Start(context.Background(), "span2", apitrace.WithSpanContext(sc2))
+	if s.SpanContext() == sc2 {
+		t.Errorf("got %+v, wanted a non-EmptySpanContext", s.SpanContext())
+	}
+}
+
 func TestStartSpanWithChildOf(t *testing.T) {
 	tp, _ := NewProvider()
 	tr := tp.Tracer("SpanWith ChildOf")