feat(ddtrace/tracer): introduce ExtractCtx to support propagated context

ddtrace/tracer/context.go

@@ -18,6 +18,11 @@ func ContextWithSpan(ctx context.Context, s *Span) context.Context {
 	return orchestrion.CtxWithValue(ctx, internal.ActiveSpanKey, s)
 }
 
+// contextWithSpanContext returns a copy of the given context which includes the span context sctx.
+func contextWithSpanContext(ctx context.Context, sctx *SpanContext) context.Context {
+	return orchestrion.CtxWithValue(ctx, internal.ActiveSpanContextKey, sctx)
+}
+
 // SpanFromContext returns the span contained in the given context. A second return
 // value indicates if a span was found in the context. If no span is found, a no-op
 // span is returned.
@@ -35,6 +40,19 @@ func SpanFromContext(ctx context.Context) (*Span, bool) {
 	return nil, false
 }
 
+// spanContextFromContext returns the span context contained in the given context. A second return
+// value indicates if a span context was found in the context. If no span context is found, a nil is returned.
+func spanContextFromContext(ctx context.Context) (*SpanContext, bool) {
+	if ctx == nil {
+		return nil, false
+	}
+	v := orchestrion.WrapContext(ctx).Value(internal.ActiveSpanContextKey)
+	if sctx, ok := v.(*SpanContext); ok {
+		return sctx, true
+	}
+	return nil, false
+}
+
 // StartSpanFromContext returns a new span with the given operation name and options. If a span
 // is found in the context, it will be used as the parent of the resulting span. If the ChildOf
 // option is passed, it will only be used as the parent if there is no span found in `ctx`.
@@ -47,6 +65,8 @@ func StartSpanFromContext(ctx context.Context, operationName string, opts ...Sta
 		ctx = context.Background()
 	} else if s, ok := SpanFromContext(ctx); ok {
 		optsLocal = append(optsLocal, ChildOf(s.Context()))
+	} else if sctx, ok := spanContextFromContext(ctx); ok {
+		optsLocal = append(optsLocal, ChildOf(sctx))
 	}
 	optsLocal = append(optsLocal, withContext(ctx))
 	s := StartSpan(operationName, optsLocal...)

ddtrace/tracer/tracer.go

@@ -312,6 +312,19 @@ func Extract(carrier interface{}) (*SpanContext, error) {
 	return getGlobalTracer().Extract(carrier)
 }
 
+// ExtractCtx extracts a span context from the carrier and returns a copy of
+// the given context which includes the span context.
+func ExtractCtx(ctx gocontext.Context, carrier interface{}) (gocontext.Context, error) {
+	sctx, err := Extract(carrier)
+	if err != nil {
+		return nil, err
+	}
+	if sctx == nil {
+		return ctx, nil
+	}
+	return contextWithSpanContext(ctx, sctx), nil
+}
+
 // Inject injects the given SpanContext into the carrier. The carrier is
 // expected to implement TextMapWriter, otherwise an error is returned.
 // If the tracer is not started, calling this function is a no-op.

internal/active_span_key.go

@@ -9,3 +9,6 @@ type contextKey struct{}
 
 // ActiveSpanKey is used to set tracer context on a context.Context objects with a unique key
 var ActiveSpanKey = contextKey{}
+
+// ActiveSpanContextKey is used to set span context on a context.Context objects with a unique key
+var ActiveSpanContextKey = contextKey{}