net/http: allow upgrading non keepalive connections

If one was using http.Transport with DisableKeepAlives and trying
to upgrade a connection against net/http's Server, the Server
would not allow a "Connection: Upgrade" header to be written
and instead override it to "Connection: Close" which would
break the handshake.

This change ensures net/http's Server does not override the
connection header for successful protocol switch responses.

Fixes #36381.

Change-Id: I882aad8539e6c87ff5f37c20e20b3a7fa1a30357
GitHub-Last-Rev: dc0de83201dc26236527b68bd49dffc53dd0389b
GitHub-Pull-Request: golang/go#36382
Reviewed-on: https://go-review.googlesource.com/c/go/+/213277
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 11 insertions, 5 deletions

diff --git a/src/net/http/response.go b/src/net/http/response.go
index 72812f0642..b95abae646 100644
--- a/src/net/http/response.go
+++ b/src/net/http/response.go
@@ -352,10 +352,16 @@ func (r *Response) bodyIsWritable() bool {
 	return ok
 }
 
-// isProtocolSwitch reports whether r is a response to a successful
-// protocol upgrade.
+// isProtocolSwitch reports whether the response code and header
+// indicate a successful protocol upgrade response.
 func (r *Response) isProtocolSwitch() bool {
-	return r.StatusCode == StatusSwitchingProtocols &&
-		r.Header.Get("Upgrade") != "" &&
-		httpguts.HeaderValuesContainsToken(r.Header["Connection"], "Upgrade")
+	return isProtocolSwitchResponse(r.StatusCode, r.Header)
+}
+
+// isProtocolSwitchResponse reports whether the response code and
+// response header indicate a successful protocol upgrade response.
+func isProtocolSwitchResponse(code int, h Header) bool {
+	return code == StatusSwitchingProtocols &&
+		h.Get("Upgrade") != "" &&
+		httpguts.HeaderValuesContainsToken(h["Connection"], "Upgrade")
 }