MINOR: http: add a small helper to compute how far to rewind to find URI

http_uri_rewind() returns the number of bytes to rewind before buf->p to
find the URI. It relies on http_hdr_rewind() to find the beginning and
is just here to simplify operations.

The purpose is to centralize further ->sov changes aiming at avoiding
to rely on buf->o.

include/proto/proto_http.h

@@ -140,6 +140,15 @@ static inline int http_hdr_rewind(const struct http_msg *msg)
 	return msg->chn->buf->o;
 }
 
+/* Return the amount of bytes that need to be rewound before buf->p to access
+ * the current message's URI. The purpose is to be able to easily fetch
+ * the message's beginning before headers are forwarded, as well as after.
+ */
+static inline int http_uri_rewind(const struct http_msg *msg)
+{
+	return http_hdr_rewind(msg) - msg->sl.rq.u;
+}
+
 /* Return the maximum amount of bytes that may be read after the beginning of
  * the message body, according to the advertised length. The function is safe
  * for use between HTTP_MSG_BODY and HTTP_MSG_DATA regardless of whether the

src/backend.c

@@ -615,7 +615,7 @@ int assign_server(struct session *s)
 				if (s->txn.req.msg_state < HTTP_MSG_BODY)
 					break;
 				srv = get_server_uh(s->be,
-						    b_ptr(s->req->buf, (int)(s->txn.req.sl.rq.u - http_hdr_rewind(&s->txn.req))),
+						    b_ptr(s->req->buf, -http_uri_rewind(&s->txn.req)),
 						    s->txn.req.sl.rq.u_l);
 				break;
 
@@ -625,7 +625,7 @@ int assign_server(struct session *s)
 					break;
 
 				srv = get_server_ph(s->be,
-						    b_ptr(s->req->buf, (int)(s->txn.req.sl.rq.u - http_hdr_rewind(&s->txn.req))),
+						    b_ptr(s->req->buf, -http_uri_rewind(&s->txn.req)),
 						    s->txn.req.sl.rq.u_l);
 
 				if (!srv && s->txn.meth == HTTP_METH_POST)