add SpanOrNoopFromContext convenience function

Author: basvanbeek

context.go

@@ -4,6 +4,8 @@ import (
 	"context"
 )
 
+var defaultNoopSpan = &noopSpan{}
+
 // SpanFromContext retrieves a Zipkin Span from Go's context propagation
 // mechanism if found. If not found, returns nil.
 func SpanFromContext(ctx context.Context) Span {
@@ -13,6 +15,19 @@ func SpanFromContext(ctx context.Context) Span {
 	return nil
 }
 
+// SpanOrNoopFromContext retrieves a Zipkin Span from Go's context propagation
+// mechanism if found. If not found, returns a noopSpan.
+// This function typically is used for modules that want to provide existing
+// Zipkin spans with additional data, but can't guarantee that spans are
+// properly propagated. It is preferred to use SpanFromContext() and test for
+// Nil instead of using this function.
+func SpanOrNoopFromContext(ctx context.Context) Span {
+	if s, ok := ctx.Value(spanKey).(Span); ok {
+		return s
+	}
+	return defaultNoopSpan
+}
+
 // NewContext stores a Zipkin Span into Go's context propagation mechanism.
 func NewContext(ctx context.Context, s Span) context.Context {
 	return context.WithValue(ctx, spanKey, s)

context_test.go

@@ -0,0 +1,25 @@
+package zipkin
+
+import (
+	"context"
+	"testing"
+)
+
+func TestSpanOrNoopFromContext(t *testing.T) {
+	var (
+		ctx   = context.Background()
+		tr, _ = NewTracer(nil, WithLocalEndpoint(nil))
+		span  = tr.StartSpan("test")
+	)
+
+	if want, have := defaultNoopSpan, SpanOrNoopFromContext(ctx); want != have {
+		t.Errorf("Invalid response want %+v, have %+v", want, have)
+	}
+
+	ctx = NewContext(ctx, span)
+
+	if want, have := span, SpanOrNoopFromContext(ctx); want != have {
+		t.Errorf("Invalid response want %+v, have %+v", want, have)
+	}
+
+}