Do not lower header tokens in headerTokens() (coder#273)

HTTP header values, as opposed to header keys,
are case sensitive, but implementation of headerTokens()
before this patch would return lowered values always.

This old behavior could lead to chromium (v87) WebSocket
rejecting connnection because negotiated subprotocol,
returned in Sec-WebSocket-Protocol header (lowered
be headerToken() function) would not match one sent
by client, in case client specified value with capital
letters.

Author: Egor Gorbunov

accept.go

@@ -159,13 +159,13 @@ func verifyClientRequest(w http.ResponseWriter, r *http.Request) (errCode int, _
 		return http.StatusUpgradeRequired, fmt.Errorf("WebSocket protocol violation: handshake request must be at least HTTP/1.1: %q", r.Proto)
 	}
 
-	if !headerContainsToken(r.Header, "Connection", "Upgrade") {
+	if !headerContainsTokenIgnoreCase(r.Header, "Connection", "Upgrade") {
 		w.Header().Set("Connection", "Upgrade")
 		w.Header().Set("Upgrade", "websocket")
 		return http.StatusUpgradeRequired, fmt.Errorf("WebSocket protocol violation: Connection header %q does not contain Upgrade", r.Header.Get("Connection"))
 	}
 
-	if !headerContainsToken(r.Header, "Upgrade", "websocket") {
+	if !headerContainsTokenIgnoreCase(r.Header, "Upgrade", "websocket") {
 		w.Header().Set("Connection", "Upgrade")
 		w.Header().Set("Upgrade", "websocket")
 		return http.StatusUpgradeRequired, fmt.Errorf("WebSocket protocol violation: Upgrade header %q does not contain websocket", r.Header.Get("Upgrade"))
@@ -309,11 +309,9 @@ func acceptWebkitDeflate(w http.ResponseWriter, ext websocketExtension, mode Com
 	return copts, nil
 }
 
-func headerContainsToken(h http.Header, key, token string) bool {
-	token = strings.ToLower(token)
-
+func headerContainsTokenIgnoreCase(h http.Header, key, token string) bool {
 	for _, t := range headerTokens(h, key) {
-		if t == token {
+		if strings.EqualFold(t, token) {
 			return true
 		}
 	}
@@ -354,7 +352,6 @@ func headerTokens(h http.Header, key string) []string {
 	for _, v := range h[key] {
 		v = strings.TrimSpace(v)
 		for _, t := range strings.Split(v, ",") {
-			t = strings.ToLower(t)
 			t = strings.TrimSpace(t)
 			tokens = append(tokens, t)
 		}

accept_test.go

@@ -224,6 +224,12 @@ func Test_selectSubprotocol(t *testing.T) {
 			serverProtocols: []string{"echo2", "echo3"},
 			negotiated:      "echo3",
 		},
+		{
+			name:            "clientCasePresered",
+			clientProtocols: []string{"Echo1"},
+			serverProtocols: []string{"echo1"},
+			negotiated:      "Echo1",
+		},
 	}
 
 	for _, tc := range testCases {

dial.go

@@ -194,11 +194,11 @@ func verifyServerResponse(opts *DialOptions, copts *compressionOptions, secWebSo
 		return nil, fmt.Errorf("expected handshake response status code %v but got %v", http.StatusSwitchingProtocols, resp.StatusCode)
 	}
 
-	if !headerContainsToken(resp.Header, "Connection", "Upgrade") {
+	if !headerContainsTokenIgnoreCase(resp.Header, "Connection", "Upgrade") {
 		return nil, fmt.Errorf("WebSocket protocol violation: Connection header %q does not contain Upgrade", resp.Header.Get("Connection"))
 	}
 
-	if !headerContainsToken(resp.Header, "Upgrade", "WebSocket") {
+	if !headerContainsTokenIgnoreCase(resp.Header, "Upgrade", "WebSocket") {
 		return nil, fmt.Errorf("WebSocket protocol violation: Upgrade header %q does not contain websocket", resp.Header.Get("Upgrade"))
 	}