proxy: recognize client's cancellation

fix issue #1589

Signed-off-by: Tw <tw19881113@gmail.com>

caddyhttp/httpserver/error.go

@@ -8,6 +8,7 @@ var (
 	_ error = NonHijackerError{}
 	_ error = NonFlusherError{}
 	_ error = NonCloseNotifierError{}
+	_ error = NonPusherError{}
 )
 
 // NonHijackerError is more descriptive error caused by a non hijacker
@@ -42,3 +43,14 @@ type NonCloseNotifierError struct {
 func (c NonCloseNotifierError) Error() string {
 	return fmt.Sprintf("%T is not a closeNotifier", c.Underlying)
 }
+
+// NonPusherError is more descriptive error caused by a non pusher
+type NonPusherError struct {
+	// underlying type which doesn't implement pusher
+	Underlying interface{}
+}
+
+// Implement Error
+func (c NonPusherError) Error() string {
+	return fmt.Sprintf("%T is not a pusher", c.Underlying)
+}

caddyhttp/httpserver/recorder.go

@@ -2,7 +2,6 @@ package httpserver
 
 import (
 	"bufio"
-	"errors"
 	"net"
 	"net/http"
 	"time"
@@ -103,7 +102,7 @@ func (r *ResponseRecorder) Push(target string, opts *http.PushOptions) error {
 		return pusher.Push(target, opts)
 	}
 
-	return errors.New("push is unavailable (probably chained http.ResponseWriter does not implement http.Pusher)")
+	return NonPusherError{Underlying: r.ResponseWriter}
 }
 
 // Interface guards