proxy: take original URL path into account when remove prefix

fix issue #1604
Signed-off-by: Tw <tw19881113@gmail.com>

caddyhttp/proxy/proxy_test.go

@@ -977,6 +977,13 @@ func TestProxyDirectorURL(t *testing.T) {
 			targetURL:    `https://localhost:2021/`,
 			expectURL:    `https://localhost:2021/%2F/test`,
 		},
+		{
+			originalPath: `/test///mypath`,
+			requestURL:   `http://localhost:2020/test/%2F/mypath`,
+			targetURL:    `https://localhost:2021/t/`,
+			expectURL:    `https://localhost:2021/t/%2F/mypath`,
+			without:      "/test",
+		},
 	} {
 		targetURL, err := url.Parse(c.targetURL)
 		if err != nil {

caddyhttp/proxy/reverseproxy.go

@@ -121,6 +121,7 @@ func NewSingleHostReverseProxy(target *url.URL, without string, keepalive int) *
 		}
 
 		// We should remove the `without` prefix at first.
+		untouchedPath, _ := req.Context().Value(staticfiles.URLPathCtxKey).(string)
 		if without != "" {
 			req.URL.Path = strings.TrimPrefix(req.URL.Path, without)
 			if req.URL.Opaque != "" {
@@ -129,6 +130,9 @@ func NewSingleHostReverseProxy(target *url.URL, without string, keepalive int) *
 			if req.URL.RawPath != "" {
 				req.URL.RawPath = strings.TrimPrefix(req.URL.RawPath, without)
 			}
+			if untouchedPath != "" {
+				untouchedPath = strings.TrimPrefix(untouchedPath, without)
+			}
 		}
 
 		// prefer returns val if it isn't empty, otherwise def
@@ -155,7 +159,6 @@ func NewSingleHostReverseProxy(target *url.URL, without string, keepalive int) *
 				prefer(target.RawPath, target.Path),
 				prefer(req.URL.RawPath, req.URL.Path))
 		}
-		untouchedPath, _ := req.Context().Value(staticfiles.URLPathCtxKey).(string)
 		req.URL.Path = singleJoiningSlash(target.Path,
 			prefer(untouchedPath, req.URL.Path))
 		// req.URL.Path must be consistent with decoded form of req.URL.RawPath if any