From d892fd2b5103aca3c5cd4d51e567c02264fd9376 Mon Sep 17 00:00:00 2001
From: beorn7 <beorn@soundcloud.com>
Date: Fri, 2 Feb 2018 15:50:46 +0100
Subject: [PATCH 1/2] Add test to expose interface upgrade bug

In principle, we needed to iterate through all permutations, mirroring
the same that is happening in the code. For lack of time, I only
picked one of the cases currently buggy.

As said, this really needs code generation, should we ever find
ourselves touching this again.
---
 prometheus/promhttp/instrument_server_test.go | 26 +++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/prometheus/promhttp/instrument_server_test.go b/prometheus/promhttp/instrument_server_test.go
index e9af63e..716c6f4 100644
--- a/prometheus/promhttp/instrument_server_test.go
+++ b/prometheus/promhttp/instrument_server_test.go
@@ -281,6 +281,16 @@ func (t *testResponseWriter) ReadFrom(io.Reader) (int64, error) {
 	return 0, nil
 }
 
+// testFlusher is an http.ResponseWriter that also implements http.Flusher.
+type testFlusher struct {
+	flushCalled bool
+}
+
+func (t *testFlusher) Header() http.Header       { return nil }
+func (t *testFlusher) Write([]byte) (int, error) { return 0, nil }
+func (t *testFlusher) WriteHeader(int)           {}
+func (t *testFlusher) Flush()                    { t.flushCalled = true }
+
 func TestInterfaceUpgrade(t *testing.T) {
 	w := &testResponseWriter{}
 	d := newDelegator(w, nil)
@@ -299,6 +309,22 @@ func TestInterfaceUpgrade(t *testing.T) {
 	if _, ok := d.(http.Hijacker); ok {
 		t.Error("delegator unexpectedly implements http.Hijacker")
 	}
+
+	f := &testFlusher{}
+	d = newDelegator(f, nil)
+	if _, ok := d.(http.CloseNotifier); ok {
+		t.Error("delegator unexpectedly implements http.CloseNotifier")
+	}
+	d.(http.Flusher).Flush()
+	if !w.flushCalled {
+		t.Error("Flush not called")
+	}
+	if _, ok := d.(io.ReaderFrom); ok {
+		t.Error("delegator unexpectedly implements io.ReaderFrom")
+	}
+	if _, ok := d.(http.Hijacker); ok {
+		t.Error("delegator unexpectedly implements http.Hijacker")
+	}
 }
 
 func ExampleInstrumentHandlerDuration() {

From e87046a87e6fa0385d2ff32b11e7ed938664d814 Mon Sep 17 00:00:00 2001
From: beorn7 <beorn@soundcloud.com>
Date: Fri, 2 Feb 2018 15:53:28 +0100
Subject: [PATCH 2/2] Fix more interface upgrade bugs

---
 prometheus/promhttp/delegator.go     | 4 ++--
 prometheus/promhttp/delegator_1_8.go | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/prometheus/promhttp/delegator.go b/prometheus/promhttp/delegator.go
index 659c717..9c1c66d 100644
--- a/prometheus/promhttp/delegator.go
+++ b/prometheus/promhttp/delegator.go
@@ -102,10 +102,10 @@ func init() {
 		return d
 	}
 	pickDelegator[closeNotifier] = func(d *responseWriterDelegator) delegator { // 1
-		return closeNotifierDelegator{d}
+		return &closeNotifierDelegator{d}
 	}
 	pickDelegator[flusher] = func(d *responseWriterDelegator) delegator { // 2
-		return flusherDelegator{d}
+		return &flusherDelegator{d}
 	}
 	pickDelegator[flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 3
 		return struct {
diff --git a/prometheus/promhttp/delegator_1_8.go b/prometheus/promhttp/delegator_1_8.go
index f4d386f..75a905e 100644
--- a/prometheus/promhttp/delegator_1_8.go
+++ b/prometheus/promhttp/delegator_1_8.go
@@ -28,7 +28,7 @@ func (d *pusherDelegator) Push(target string, opts *http.PushOptions) error {
 
 func init() {
 	pickDelegator[pusher] = func(d *responseWriterDelegator) delegator { // 16
-		return pusherDelegator{d}
+		return &pusherDelegator{d}
 	}
 	pickDelegator[pusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 17
 		return struct {