Expose optional ResponseWriter interfaces.

http.ResponseWriters may implement additional interfaces
(http.CloseNotifier, http.Flusher, http.Hijacker, http.Pusher,
io.ReaderFrom) that get lost when the ResponseWriter is wrapped in
another object. This change uses the httpsnoop package to wrap the
ResponseWriter so that the resulting object implements any of the
optional interfaces that the original ResponseWriter implements as
well as using the replacement ResponseWriter methods that gather
information for tracing.

go.mod

@@ -6,6 +6,7 @@ require (
 	github.com/DataDog/sketches-go v0.0.0-20190923095040-43f19ad77ff7
 	github.com/benbjohnson/clock v1.0.3
 	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/felixge/httpsnoop v1.0.1
 	github.com/golang/protobuf v1.4.2
 	github.com/google/go-cmp v0.5.1
 	github.com/google/gofuzz v1.0.0 // indirect

go.sum

@@ -15,6 +15,8 @@ github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSs
 github.com/envoyproxy/go-control-plane v0.9.0/go.mod h1:YTl/9mNaCwkRvm6d1a2C3ymFceY/DCBVvsKhRF0iEA4=
 github.com/envoyproxy/go-control-plane v0.9.4/go.mod h1:6rpuAdCZL397s3pYoYcLgu1mIlRU8Am5FuJP05cCM98=
 github.com/envoyproxy/protoc-gen-validate v0.1.0/go.mod h1:iSmxcyjqTsJpI2R4NaDN7+kN2VEUnK/pcBlmesArF7c=
+github.com/felixge/httpsnoop v1.0.1 h1:lvB5Jl89CsZtGIWuTcDM1E/vkVs49/Ml7JJe07l8SPQ=
+github.com/felixge/httpsnoop v1.0.1/go.mod h1:m8KPJKqk1gH5J9DgRY2ASl2lWCfGKXixSwevea8zH2U=
 github.com/golang/glog v0.0.0-20160126235308-23def4e6c14b h1:VKtxabqXZkF25pY9ekfRL6a582T4P37/31XEstQ5p58=
 github.com/golang/glog v0.0.0-20160126235308-23def4e6c14b/go.mod h1:SBH7ygxi8pfUlaOkMMuAQtPIUF8ecWP5IEl/CR7VP2Q=
 github.com/golang/mock v1.1.1/go.mod h1:oTYuIxOrZwtPieC+H1uAHpcLFnEyAGVDL/k47Jfbm0A=

instrumentation/othttp/handler.go

@@ -19,6 +19,7 @@ import (
 	"net/http"
 	"time"
 
+	"github.com/felixge/httpsnoop"
 	"go.opentelemetry.io/otel/api/global"
 	"go.opentelemetry.io/otel/api/kv"
 	"go.opentelemetry.io/otel/api/metric"
@@ -152,7 +153,23 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	rww := &respWriterWrapper{ResponseWriter: w, record: writeRecordFunc, ctx: ctx, props: h.propagators}
 
-	h.handler.ServeHTTP(rww, r.WithContext(ctx))
+	// Wrap w to use our ResponseWriter methods while also exposing
+	// other interfaces that w may implement (http.CloseNotifier,
+	// http.Flusher, http.Hijacker, http.Pusher, io.ReaderFrom).
+
+	w = httpsnoop.Wrap(w, httpsnoop.Hooks{
+		Header: func(httpsnoop.HeaderFunc) httpsnoop.HeaderFunc {
+			return rww.Header
+		},
+		Write: func(httpsnoop.WriteFunc) httpsnoop.WriteFunc {
+			return rww.Write
+		},
+		WriteHeader: func(httpsnoop.WriteHeaderFunc) httpsnoop.WriteHeaderFunc {
+			return rww.WriteHeader
+		},
+	})
+
+	h.handler.ServeHTTP(w, r.WithContext(ctx))
 
 	setAfterServeAttributes(span, bw.read, rww.written, rww.statusCode, bw.err, rww.err)
 

instrumentation/othttp/handler_test.go

@@ -134,3 +134,33 @@ func TestHandlerNoWrite(t *testing.T) {
 		t.Fatalf("Expected *moctrace.MockSpan, got %T", span)
 	}
 }
+
+func TestResponseWriterOptionalInterfaces(t *testing.T) {
+	rr := httptest.NewRecorder()
+
+	var id uint64
+	tracer := mocktrace.MockTracer{StartSpanID: &id}
+
+	// ResponseRecorder implements the Flusher interface. Make sure the
+	// wrapped ResponseWriter passed to the handler still implements
+	// Flusher.
+
+	var isFlusher bool
+	h := NewHandler(
+		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			_, isFlusher = w.(http.Flusher)
+			if _, err := io.WriteString(w, "hello world"); err != nil {
+				t.Fatal(err)
+			}
+		}), "test_handler",
+		WithTracer(&tracer))
+
+	r, err := http.NewRequest(http.MethodGet, "http://localhost/", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	h.ServeHTTP(rr, r)
+	if !isFlusher {
+		t.Fatal("http.Flusher interface not exposed")
+	}
+}