Falls back to SPDY for gorilla/websocket https proxy error

Kubernetes-commit: 9d560540c5268e0e2aebf5306907494cf522c260

pkg/util/httpstream/httpstream.go

@@ -116,6 +116,15 @@ func IsUpgradeFailure(err error) bool {
 	return errors.As(err, &upgradeErr)
 }
 
+// isHTTPSProxyError returns true if error is Gorilla/Websockets HTTPS Proxy dial error;
+// false otherwise (see https://github.com/kubernetes/kubernetes/issues/126134).
+func IsHTTPSProxyError(err error) bool {
+	if err == nil {
+		return false
+	}
+	return strings.Contains(err.Error(), "proxy: unknown scheme: https")
+}
+
 // IsUpgradeRequest returns true if the given request is a connection upgrade request
 func IsUpgradeRequest(req *http.Request) bool {
 	for _, h := range req.Header[http.CanonicalHeaderKey(HeaderConnection)] {

pkg/util/httpstream/httpstream_test.go

@@ -168,3 +168,32 @@ func TestIsUpgradeFailureError(t *testing.T) {
 		})
 	}
 }
+
+func TestIsHTTPSProxyError(t *testing.T) {
+	testCases := map[string]struct {
+		err      error
+		expected bool
+	}{
+		"nil error should return false": {
+			err:      nil,
+			expected: false,
+		},
+		"Not HTTPS proxy error should return false": {
+			err:      errors.New("this is not an upgrade error"),
+			expected: false,
+		},
+		"HTTPS proxy error should return true": {
+			err:      errors.New("proxy: unknown scheme: https"),
+			expected: true,
+		},
+	}
+
+	for name, test := range testCases {
+		t.Run(name, func(t *testing.T) {
+			actual := IsHTTPSProxyError(test.err)
+			if test.expected != actual {
+				t.Errorf("expected HTTPS proxy error %t, got %t", test.expected, actual)
+			}
+		})
+	}
+}