Core: protection from cycles with named locations and post_action.

Now redirects to named locations are counted against normal uri changes
limit, and post_action respects this limit as well.  As a result at least
the following (bad) configurations no longer trigger infinite cycles:

1. Post action which recursively triggers post action:

    location / {
        post_action /index.html;
    }

2. Post action pointing to nonexistent named location:

    location / {
        post_action @nonexistent;
    }

3. Recursive error page for 500 (Internal Server Error) pointing to
   a nonexistent named location:

    location / {
        recursive_error_pages on;
        error_page 500 @nonexistent;
        return 500;
    }

1 files changed, 4 insertions, 0 deletions

diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
index c35d550a2..a8515f376 100644
--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -2928,6 +2928,10 @@ ngx_http_post_action(ngx_http_request_t *r)
         return NGX_DECLINED;
     }
 
+    if (r->post_action && r->uri_changes == 0) {
+        return NGX_DECLINED;
+    }
+
     ngx_log_debug1(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
                    "post action: \"%V\"", &clcf->post_action);