promhttp: Bite the bullet and implement all 32 possible interface combos

As it turned out, it's not that esay to guess "common" combination of
interface upgrades. So I decided to just implement all 32 possible
combination of interface upgrades. (Only 16 with Go 1.7 and earlier.)

Clearly, this calls for code generation. But right now, we still need
to find out what's the best form of the code. For later additions,
implementing code generation might be useful.

Note that newDelegator is called for each HTTP request. Thus, this
commit aims to make the upgrade selection quick. (After the type
checks, it's just directly accessing an element in a slice.)

prometheus/promhttp/delegator.go

@@ -0,0 +1,199 @@
+// Copyright 2017 The Prometheus 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 promhttp
+
+import (
+	"bufio"
+	"io"
+	"net"
+	"net/http"
+)
+
+const (
+	closeNotifier = 1 << iota
+	flusher
+	hijacker
+	readerFrom
+	pusher
+)
+
+type delegator interface {
+	http.ResponseWriter
+
+	Status() int
+	Written() int64
+}
+
+type responseWriterDelegator struct {
+	http.ResponseWriter
+
+	handler, method    string
+	status             int
+	written            int64
+	wroteHeader        bool
+	observeWriteHeader func(int)
+}
+
+func (r *responseWriterDelegator) Status() int {
+	return r.status
+}
+
+func (r *responseWriterDelegator) Written() int64 {
+	return r.written
+}
+
+func (r *responseWriterDelegator) WriteHeader(code int) {
+	r.status = code
+	r.wroteHeader = true
+	r.ResponseWriter.WriteHeader(code)
+	if r.observeWriteHeader != nil {
+		r.observeWriteHeader(code)
+	}
+}
+
+func (r *responseWriterDelegator) Write(b []byte) (int, error) {
+	if !r.wroteHeader {
+		r.WriteHeader(http.StatusOK)
+	}
+	n, err := r.ResponseWriter.Write(b)
+	r.written += int64(n)
+	return n, err
+}
+
+type closeNotifierDelegator struct{ *responseWriterDelegator }
+type flusherDelegator struct{ *responseWriterDelegator }
+type hijackerDelegator struct{ *responseWriterDelegator }
+type readerFromDelegator struct{ *responseWriterDelegator }
+
+func (d *closeNotifierDelegator) CloseNotify() <-chan bool {
+	return d.ResponseWriter.(http.CloseNotifier).CloseNotify()
+}
+func (d *flusherDelegator) Flush() {
+	d.ResponseWriter.(http.Flusher).Flush()
+}
+func (d *hijackerDelegator) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	return d.ResponseWriter.(http.Hijacker).Hijack()
+}
+func (d *readerFromDelegator) ReadFrom(re io.Reader) (int64, error) {
+	if !d.wroteHeader {
+		d.WriteHeader(http.StatusOK)
+	}
+	n, err := d.ResponseWriter.(io.ReaderFrom).ReadFrom(re)
+	d.written += n
+	return n, err
+}
+
+var pickDelegator = make([]func(*responseWriterDelegator) delegator, 32)
+
+func init() {
+	// TODO(beorn7): Code generation would help here.
+	pickDelegator[0] = func(d *responseWriterDelegator) delegator { // 0
+		return d
+	}
+	pickDelegator[closeNotifier] = func(d *responseWriterDelegator) delegator { // 1
+		return closeNotifierDelegator{d}
+	}
+	pickDelegator[flusher] = func(d *responseWriterDelegator) delegator { // 2
+		return flusherDelegator{d}
+	}
+	pickDelegator[flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 3
+		return struct {
+			*responseWriterDelegator
+			http.Flusher
+			http.CloseNotifier
+		}{d, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[hijacker] = func(d *responseWriterDelegator) delegator { // 4
+		return hijackerDelegator{d}
+	}
+	pickDelegator[hijacker+closeNotifier] = func(d *responseWriterDelegator) delegator { // 5
+		return struct {
+			*responseWriterDelegator
+			http.Hijacker
+			http.CloseNotifier
+		}{d, &hijackerDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[hijacker+flusher] = func(d *responseWriterDelegator) delegator { // 6
+		return struct {
+			*responseWriterDelegator
+			http.Hijacker
+			http.Flusher
+		}{d, &hijackerDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[hijacker+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 7
+		return struct {
+			*responseWriterDelegator
+			http.Hijacker
+			http.Flusher
+			http.CloseNotifier
+		}{d, &hijackerDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[readerFrom] = func(d *responseWriterDelegator) delegator { // 8
+		return readerFromDelegator{d}
+	}
+	pickDelegator[readerFrom+closeNotifier] = func(d *responseWriterDelegator) delegator { // 9
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.CloseNotifier
+		}{d, &readerFromDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[readerFrom+flusher] = func(d *responseWriterDelegator) delegator { // 10
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.Flusher
+		}{d, &readerFromDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[readerFrom+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 11
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.Flusher
+			http.CloseNotifier
+		}{d, &readerFromDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[readerFrom+hijacker] = func(d *responseWriterDelegator) delegator { // 12
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.Hijacker
+		}{d, &readerFromDelegator{d}, &hijackerDelegator{d}}
+	}
+	pickDelegator[readerFrom+hijacker+closeNotifier] = func(d *responseWriterDelegator) delegator { // 13
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.Hijacker
+			http.CloseNotifier
+		}{d, &readerFromDelegator{d}, &hijackerDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[readerFrom+hijacker+flusher] = func(d *responseWriterDelegator) delegator { // 14
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.Hijacker
+			http.Flusher
+		}{d, &readerFromDelegator{d}, &hijackerDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[readerFrom+hijacker+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 15
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.Hijacker
+			http.Flusher
+			http.CloseNotifier
+		}{d, &readerFromDelegator{d}, &hijackerDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+}

prometheus/promhttp/delegator_1_8.go

@@ -20,54 +20,162 @@ import (
 	"net/http"
 )
 
-// newDelegator handles the four different methods of upgrading a
-// http.ResponseWriter to delegator.
+type pusherDelegator struct{ *responseWriterDelegator }
+
+func (d *pusherDelegator) Push(target string, opts *http.PushOptions) error {
+	return d.ResponseWriter.(http.Pusher).Push(target, opts)
+}
+
+func init() {
+	pickDelegator[pusher] = func(d *responseWriterDelegator) delegator { // 16
+		return pusherDelegator{d}
+	}
+	pickDelegator[pusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 17
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+flusher] = func(d *responseWriterDelegator) delegator { // 18
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.Flusher
+		}{d, &pusherDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[pusher+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 19
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.Flusher
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+hijacker] = func(d *responseWriterDelegator) delegator { // 20
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.Hijacker
+		}{d, &pusherDelegator{d}, &hijackerDelegator{d}}
+	}
+	pickDelegator[pusher+hijacker+closeNotifier] = func(d *responseWriterDelegator) delegator { // 21
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.Hijacker
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &hijackerDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+hijacker+flusher] = func(d *responseWriterDelegator) delegator { // 22
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.Hijacker
+			http.Flusher
+		}{d, &pusherDelegator{d}, &hijackerDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[pusher+hijacker+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { //23
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.Hijacker
+			http.Flusher
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &hijackerDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom] = func(d *responseWriterDelegator) delegator { // 24
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+closeNotifier] = func(d *responseWriterDelegator) delegator { // 25
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+flusher] = func(d *responseWriterDelegator) delegator { // 26
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.Flusher
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 27
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.Flusher
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+hijacker] = func(d *responseWriterDelegator) delegator { // 28
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.Hijacker
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &hijackerDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+hijacker+closeNotifier] = func(d *responseWriterDelegator) delegator { // 29
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.Hijacker
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &hijackerDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+hijacker+flusher] = func(d *responseWriterDelegator) delegator { // 30
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.Hijacker
+			http.Flusher
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &hijackerDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+hijacker+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 31
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.Hijacker
+			http.Flusher
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &hijackerDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+}
+
 func newDelegator(w http.ResponseWriter, observeWriteHeaderFunc func(int)) delegator {
 	d := &responseWriterDelegator{
 		ResponseWriter:     w,
 		observeWriteHeader: observeWriteHeaderFunc,
 	}
 
-	_, cn := w.(http.CloseNotifier)
-	_, fl := w.(http.Flusher)
-	_, hj := w.(http.Hijacker)
-	_, ps := w.(http.Pusher)
-	_, rf := w.(io.ReaderFrom)
-
-	// Check for the four most common combination of interfaces a
-	// http.ResponseWriter might implement.
-	switch {
-	case cn && fl && hj && rf && ps:
-		// All interfaces.
-		return &fancyPushDelegator{
-			fancyDelegator: &fancyDelegator{d},
-			p:              &pushDelegator{d},
+	id := 0
+	if _, ok := w.(http.CloseNotifier); ok {
+		id += closeNotifier
 	}
-	case cn && fl && hj && rf:
-		// All interfaces, except http.Pusher.
-		return &fancyDelegator{d}
-	case ps:
-		// Just http.Pusher.
-		return &pushDelegator{d}
+	if _, ok := w.(http.Flusher); ok {
+		id += flusher
+	}
+	if _, ok := w.(http.Hijacker); ok {
+		id += hijacker
+	}
+	if _, ok := w.(io.ReaderFrom); ok {
+		id += readerFrom
+	}
+	if _, ok := w.(http.Pusher); ok {
+		id += pusher
 	}
 
-	return d
-}
-
-type fancyPushDelegator struct {
-	p *pushDelegator
-
-	*fancyDelegator
-}
-
-func (f *fancyPushDelegator) Push(target string, opts *http.PushOptions) error {
-	return f.p.Push(target, opts)
-}
-
-type pushDelegator struct {
-	*responseWriterDelegator
-}
-
-func (f *pushDelegator) Push(target string, opts *http.PushOptions) error {
-	return f.ResponseWriter.(http.Pusher).Push(target, opts)
+	return pickDelegator[id](d)
 }

prometheus/promhttp/delegator_pre_1_8.go

@@ -26,13 +26,19 @@ func newDelegator(w http.ResponseWriter, observeWriteHeaderFunc func(int)) deleg
 		observeWriteHeader: observeWriteHeaderFunc,
 	}
 
-	_, cn := w.(http.CloseNotifier)
-	_, fl := w.(http.Flusher)
-	_, hj := w.(http.Hijacker)
-	_, rf := w.(io.ReaderFrom)
-	if cn && fl && hj && rf {
-		return &fancyDelegator{d}
+	id := 0
+	if _, ok := w.(http.CloseNotifier); ok {
+		id += closeNotifier
+	}
+	if _, ok := w.(http.Flusher); ok {
+		id += flusher
+	}
+	if _, ok := w.(http.Hijacker); ok {
+		id += hijacker
+	}
+	if _, ok := w.(io.ReaderFrom); ok {
+		id += readerFrom
 	}
 
-	return d
+	return pickDelegator[id](d)
 }

prometheus/promhttp/instrument_server.go

@@ -14,9 +14,6 @@
 package promhttp
 
 import (
-	"bufio"
-	"io"
-	"net"
 	"net/http"
 	"strconv"
 	"strings"
@@ -435,71 +432,3 @@ func sanitizeCode(s int) string {
 		return strconv.Itoa(s)
 	}
 }
-
-type delegator interface {
-	Status() int
-	Written() int64
-
-	http.ResponseWriter
-}
-
-type responseWriterDelegator struct {
-	http.ResponseWriter
-
-	handler, method    string
-	status             int
-	written            int64
-	wroteHeader        bool
-	observeWriteHeader func(int)
-}
-
-func (r *responseWriterDelegator) Status() int {
-	return r.status
-}
-
-func (r *responseWriterDelegator) Written() int64 {
-	return r.written
-}
-
-func (r *responseWriterDelegator) WriteHeader(code int) {
-	r.status = code
-	r.wroteHeader = true
-	r.ResponseWriter.WriteHeader(code)
-	if r.observeWriteHeader != nil {
-		r.observeWriteHeader(code)
-	}
-}
-
-func (r *responseWriterDelegator) Write(b []byte) (int, error) {
-	if !r.wroteHeader {
-		r.WriteHeader(http.StatusOK)
-	}
-	n, err := r.ResponseWriter.Write(b)
-	r.written += int64(n)
-	return n, err
-}
-
-type fancyDelegator struct {
-	*responseWriterDelegator
-}
-
-func (r *fancyDelegator) CloseNotify() <-chan bool {
-	return r.ResponseWriter.(http.CloseNotifier).CloseNotify()
-}
-
-func (r *fancyDelegator) Hijack() (net.Conn, *bufio.ReadWriter, error) {
-	return r.ResponseWriter.(http.Hijacker).Hijack()
-}
-
-func (r *fancyDelegator) Flush() {
-	r.ResponseWriter.(http.Flusher).Flush()
-}
-
-func (r *fancyDelegator) ReadFrom(re io.Reader) (int64, error) {
-	if !r.wroteHeader {
-		r.WriteHeader(http.StatusOK)
-	}
-	n, err := r.ResponseWriter.(io.ReaderFrom).ReadFrom(re)
-	r.written += n
-	return n, err
-}

prometheus/promhttp/instrument_server_test.go

@@ -14,6 +14,7 @@
 package promhttp
 
 import (
+	"io"
 	"log"
 	"net/http"
 	"net/http/httptest"
@@ -105,6 +106,45 @@ func TestInstrumentTimeToFirstWrite(t *testing.T) {
 	}
 }
 
+// testResponseWriter is an http.ResponseWriter that also implements
+// http.CloseNotifier, http.Flusher, and io.ReaderFrom.
+type testResponseWriter struct {
+	closeNotifyCalled, flushCalled, readFromCalled bool
+}
+
+func (t *testResponseWriter) Header() http.Header       { return nil }
+func (t *testResponseWriter) Write([]byte) (int, error) { return 0, nil }
+func (t *testResponseWriter) WriteHeader(int)           {}
+func (t *testResponseWriter) CloseNotify() <-chan bool {
+	t.closeNotifyCalled = true
+	return nil
+}
+func (t *testResponseWriter) Flush() { t.flushCalled = true }
+func (t *testResponseWriter) ReadFrom(io.Reader) (int64, error) {
+	t.readFromCalled = true
+	return 0, nil
+}
+
+func TestInterfaceUpgrade(t *testing.T) {
+	w := &testResponseWriter{}
+	d := newDelegator(w, nil)
+	d.(http.CloseNotifier).CloseNotify()
+	if !w.closeNotifyCalled {
+		t.Error("CloseNotify not called")
+	}
+	d.(http.Flusher).Flush()
+	if !w.flushCalled {
+		t.Error("Flush not called")
+	}
+	d.(io.ReaderFrom).ReadFrom(nil)
+	if !w.readFromCalled {
+		t.Error("ReadFrom not called")
+	}
+	if _, ok := d.(http.Hijacker); ok {
+		t.Error("delegator unexpectedly implements http.Hijacker")
+	}
+}
+
 func ExampleInstrumentHandlerDuration() {
 	inFlightGauge := prometheus.NewGauge(prometheus.GaugeOpts{
 		Name: "in_flight_requests",