net/http/httputil: allow MIME parameters when detecting SSE in ReverseProxy

This change allows httputil.ReverseProxy to detect SSE (server-sent events)
content when the response's Content-Type header includes MIME parameters,
such as "text/event-stream;charset=utf-8".

Prior to this change the value of the Content-Type header was compared
directly to the literal "text/event-stream". This caused a false-negative
which failed to set the FlushInterval correctly when MIME parameters were
present.

Change-Id: If8bb43efb78787b6519d7fe7599ca018a0da0023
GitHub-Last-Rev: 224518c
GitHub-Pull-Request: #48427
Reviewed-on: https://go-review.googlesource.com/c/go/+/350509
Trust: Alexander Rakoczy <[email protected]>
Trust: Damien Neil <[email protected]>
Run-TryBot: Alexander Rakoczy <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Damien Neil <[email protected]>

Author: jmalloc

src/net/http/httputil/reverseproxy.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"io"
 	"log"
+	"mime"
 	"net"
 	"net/http"
 	"net/http/internal/ascii"
@@ -412,7 +413,7 @@ func (p *ReverseProxy) flushInterval(res *http.Response) time.Duration {
 
 	// For Server-Sent Events responses, flush immediately.
 	// The MIME type is defined in https://www.w3.org/TR/eventsource/#text-event-stream
-	if resCT == "text/event-stream" {
+	if baseCT, _, _ := mime.ParseMediaType(resCT); baseCT == "text/event-stream" {
 		return -1 // negative means immediately
 	}
 

src/net/http/httputil/reverseproxy_test.go

@@ -1194,6 +1194,26 @@ func TestSelectFlushInterval(t *testing.T) {
 			p:    &ReverseProxy{FlushInterval: 0},
 			want: -1,
 		},
+		{
+			name: "server-sent events with media-type parameters overrides non-zero",
+			res: &http.Response{
+				Header: http.Header{
+					"Content-Type": {"text/event-stream;charset=utf-8"},
+				},
+			},
+			p:    &ReverseProxy{FlushInterval: 123},
+			want: -1,
+		},
+		{
+			name: "server-sent events with media-type parameters overrides zero",
+			res: &http.Response{
+				Header: http.Header{
+					"Content-Type": {"text/event-stream;charset=utf-8"},
+				},
+			},
+			p:    &ReverseProxy{FlushInterval: 0},
+			want: -1,
+		},
 		{
 			name: "Content-Length: -1, overrides non-zero",
 			res: &http.Response{