[MINOR] redirect: in prefix mode a "/" means not to change the URI

If the prefix is set to "/", it means the user does not want to alter
the original URI, so we don't want to insert a new slash before the
original URI.

(cherry-picked from commit 02a35c74942c1bce762e996698add1270e6a5030)

doc/configuration.txt

@@ -2350,7 +2350,10 @@ redirect prefix   <to> [code <code>] <option> {if | unless} <condition>
               the HTTP "Location" header. In case of "redirect prefix", the
               "Location" header is built from the concatenation of <to> and the
               complete URI, including the query string, unless the "drop-query"
-              option is specified (see below).
+              option is specified (see below). As a special case, if <to>
+              equals exactly "/" in prefix mode, then nothing is inserted
+              before the original URI. It allows one to redirect to the same
+              URL.
 
     <code>    The code is optional. It indicates which type of HTTP redirection
               is desired. Only codes 301, 302 and 303 are supported, and 302 is

src/proto_http.c

@@ -1912,9 +1912,14 @@ int http_process_request(struct session *s, struct buffer *req)
 					if (rdr.len + rule->rdr_len + pathlen > sizeof(trash) - 4)
 						goto return_bad_req;
 
-					/* add prefix */
-					memcpy(rdr.str + rdr.len, rule->rdr_str, rule->rdr_len);
-					rdr.len += rule->rdr_len;
+					/* add prefix. Note that if prefix == "/", we don't want to
+					 * add anything, otherwise it makes it hard for the user to
+					 * configure a self-redirection.
+					 */
+					if (rule->rdr_len != 1 || *rule->rdr_str != '/') {
+						memcpy(rdr.str + rdr.len, rule->rdr_str, rule->rdr_len);
+						rdr.len += rule->rdr_len;
+					}
 
 					/* add path */
 					memcpy(rdr.str + rdr.len, path, pathlen);

tests/test-redirect.cfg

@@ -26,8 +26,8 @@ listen  sample1
 	redirect   prefix   /pfx/test code 302 if url_test2
 	redirect   prefix   /pfx/test code 303 drop-query if url_test3
 
-	redirect   location /test4 code 302 set-cookie   SEEN=1 if url_test4 !seen
-	redirect   location /      code 302 clear-cookie SEEN=  if url_test4 seen
+	redirect   prefix   /  code 302 set-cookie   SEEN=1 if url_test4 !seen
+	redirect   location /  code 302 clear-cookie SEEN=  if url_test4 seen
 
 	### unconditional redirection
 	#redirect   location https://example.com/ if TRUE