net/http: fix "http2: no cached connection..." error with x/net/http2

The net/http Transport was testing for a sentinel x/net/http2 error
value with ==, which meant it was only testing the bundled version. If
a user enabled http2 via golang.org/x/net/http2, the error value had a
different name.

This also updates the bundled x/net/http2 to git rev ab555f36 for:

    http2: add internal function isNoCachedConnError to test for ErrNoCachedConn
    https://golang.org/cl/87297

Fixes #22091

Change-Id: I3fb85e2b7ba7d145dd66767e1795a56de633958c
Reviewed-on: https://go-review.googlesource.com/87298
Reviewed-by: Tom Bergan <tombergan@google.com>

1 files changed, 22 insertions, 2 deletions

diff --git a/src/net/http/h2_bundle.go b/src/net/http/h2_bundle.go
index 161a1ed137..7a1564f755 100644
--- a/src/net/http/h2_bundle.go
+++ b/src/net/http/h2_bundle.go
@@ -989,7 +989,7 @@ type http2noDialH2RoundTripper struct{ t *http2Transport }
 
 func (rt http2noDialH2RoundTripper) RoundTrip(req *Request) (*Response, error) {
 	res, err := rt.t.RoundTrip(req)
-	if err == http2ErrNoCachedConn {
+	if http2isNoCachedConnError(err) {
 		return nil, ErrSkipAltProtocol
 	}
 	return res, err
@@ -6856,7 +6856,27 @@ func (sew http2stickyErrWriter) Write(p []byte) (n int, err error) {
 	return
 }
 
-var http2ErrNoCachedConn = errors.New("http2: no cached connection was available")
+// noCachedConnError is the concrete type of ErrNoCachedConn, needs to be detected
+// by net/http regardless of whether it's its bundled version (in h2_bundle.go with a rewritten type name)
+// or from a user's x/net/http2. As such, as it has a unique method name (IsHTTP2NoCachedConnError) that
+// net/http sniffs for via func isNoCachedConnError.
+type http2noCachedConnError struct{}
+
+func (http2noCachedConnError) IsHTTP2NoCachedConnError() {}
+
+func (http2noCachedConnError) Error() string { return "http2: no cached connection was available" }
+
+// isNoCachedConnError reports whether err is of type noCachedConnError
+// or its equivalent renamed type in net/http2's h2_bundle.go. Both types
+// may coexist in the same running program.
+func http2isNoCachedConnError(err error) bool {
+	_, ok := err.(interface {
+		IsHTTP2NoCachedConnError()
+	})
+	return ok
+}
+
+var http2ErrNoCachedConn error = http2noCachedConnError{}
 
 // RoundTripOpt are options for the Transport.RoundTripOpt method.
 type http2RoundTripOpt struct {