RFC: Add an extension to Tracer interface for custom go context creation

ext.go

@@ -0,0 +1,24 @@
+package opentracing
+
+import (
+	"context"
+)
+
+// TracerContextWithSpanExtension is an extension interface that the
+// implementation of the Tracer interface may want to implement. It
+// allows to have some control over the go context when the
+// ContextWithSpan is invoked.
+//
+// The primary purpose of this extension are adapters from opentracing
+// API to some other tracing API.
+type TracerContextWithSpanExtension interface {
+	// ContextWithSpanHook gets called by the ContextWithSpan
+	// function, when the Tracer implementation also implements
+	// this interface. It allows to put extra information into the
+	// context and make it available to the callers of the
+	// ContextWithSpan.
+	//
+	// This hook is invoked before the ContextWithSpan function
+	// actually puts the span into the context.
+	ContextWithSpanHook(ctx context.Context, span Span) context.Context
+}

gocontext.go

@@ -9,6 +9,9 @@ var activeSpanKey = contextKey{}
 // ContextWithSpan returns a new `context.Context` that holds a reference to
 // `span`'s SpanContext.
 func ContextWithSpan(ctx context.Context, span Span) context.Context {
+	if tracerWithHook, ok := span.Tracer().(TracerContextWithSpanExtension); ok {
+		ctx = tracerWithHook.ContextWithSpanHook(ctx, span)
+	}
 	return context.WithValue(ctx, activeSpanKey, span)
 }
 

gocontext_test.go

@@ -29,6 +29,41 @@ func TestContextWithSpan(t *testing.T) {
 	}
 }
 
+type noopExtTracer struct {
+	NoopTracer
+}
+
+type noopExtTracerCtxType struct{}
+
+func (noopExtTracer) ContextWithSpanHook(ctx context.Context, span Span) context.Context {
+	return context.WithValue(ctx, noopExtTracerCtxType{}, noopExtTracerCtxType{})
+}
+
+var _ Tracer = noopExtTracer{}
+var _ TracerContextWithSpanExtension = noopExtTracer{}
+
+type noopExtSpan struct {
+	noopSpan
+}
+
+func (noopExtSpan) Tracer() Tracer {
+	return noopExtTracer{}
+}
+
+var _ Span = noopExtSpan{}
+
+func TestContextWithSpanWithExtension(t *testing.T) {
+	span := &noopExtSpan{}
+	ctx := ContextWithSpan(context.Background(), span)
+	span2 := SpanFromContext(ctx)
+	if span != span2 {
+		t.Errorf("Not the same span returned from context, expected=%+v, actual=%+v", span, span2)
+	}
+	if _, ok := ctx.Value(noopExtTracerCtxType{}).(noopExtTracerCtxType); !ok {
+		t.Error("ContextWithSpanHook was not called")
+	}
+}
+
 func TestStartSpanFromContext(t *testing.T) {
 	testTracer := testTracer{}