add noop propagator.

api/propagation/noop_propagator.go

@@ -0,0 +1,53 @@
+// Copyright 2019, OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package propagation
+
+import (
+	"go.opentelemetry.io/api/core"
+	"go.opentelemetry.io/api/tag"
+)
+
+// NoopTextFormatPropagator implements TextFormatPropagator that does nothing.
+type NoopTextFormatPropagator struct{}
+
+var _ TextFormatPropagator = NoopTextFormatPropagator{}
+
+// CarrierExtractor returns NoopExtractor
+func (ntp NoopTextFormatPropagator) CarrierExtractor(carrier interface{}) Extractor {
+	return NoopExtractor{}
+}
+
+// CarrierInjector returns NoopInjector
+func (ntp NoopTextFormatPropagator) CarrierInjector(carrier interface{}) Injector {
+	return NoopInjector{}
+}
+
+// NoopInjector implements Injector interface that does nothing.
+type NoopInjector struct{}
+
+var _ Injector = NoopInjector{}
+
+func (ni NoopInjector) Inject(sc core.SpanContext, tm tag.Map) {
+}
+
+// NoopExtractor implements Extractor interface that does nothing.
+type NoopExtractor struct{}
+
+var _ Extractor = NoopExtractor{}
+
+// Extract method always returns Invalid SpanContext and empty tag.Map
+func (ne NoopExtractor) Extract() (core.SpanContext, tag.Map) {
+	return core.EmptySpanContext(), tag.NewEmptyMap()
+}

propagation/http_trace_context_propagator.go

@@ -40,27 +40,27 @@ var _ apipropagation.TextFormatPropagator = httpTraceContextPropagator{}
 // CarrierExtractor implements TextFormatPropagator interface.
 //
 // It creates CarrierExtractor object and binds carrier to the object. The carrier
-// is expected to be *http.Request. If the carrier type is not *http.Request
-// then an empty extractor. Extract method on empty extractor does nothing.
+// is expected to be *http.Request. If the carrier is nil or its type is not *http.Request
+// then a NoopExtractor is returned.
 func (hp httpTraceContextPropagator) CarrierExtractor(carrier interface{}) apipropagation.Extractor {
-	req, ok := carrier.(*http.Request)
-	if ok {
+	req, _ := carrier.(*http.Request)
+	if req != nil {
 		return traceContextExtractor{req: req}
 	}
-	return traceContextExtractor{}
+	return apipropagation.NoopExtractor{}
 }
 
 // CarrierInjector implements TextFormatPropagator interface.
 //
 // It creates CarrierInjector object and binds carrier to the object. The carrier
-// is expected to be of type *http.Request. If the carrier type is not *http.Request
-// then an empty injector is returned. Inject method on empty injector does nothing.
+// is expected to be of type *http.Request. If the carrier is nil or its type is not *http.Request
+// then a NoopInjector is returned.
 func (hp httpTraceContextPropagator) CarrierInjector(carrier interface{}) apipropagation.Injector {
-	req, ok := carrier.(*http.Request)
-	if ok {
+	req, _ := carrier.(*http.Request)
+	if req != nil {
 		return traceContextInjector{req: req}
 	}
-	return traceContextInjector{}
+	return apipropagation.NoopInjector{}
 }
 
 // HttpTraceContextPropagator creates a new text format propagator that propagates SpanContext