[PATCH 2 of 4] Upstream: fixed usage of closed sockets with filter finalization

[Maxim Dounin]

# HG changeset patch
# User Maxim Dounin <mdounin at mdounin.ru>
# Date 1701049758 -10800
#      Mon Nov 27 04:49:18 2023 +0300
# Node ID faf0b9defc76b8683af466f8a950c2c241382970
# Parent  a5e39e9d1f4c84dcbe6a2f9e079372a3d63aef0b
Upstream: fixed usage of closed sockets with filter finalization.

When filter finalization is triggered when working with an upstream server,
and error_page redirects request processing to some simple handler,
ngx_http_request_finalize() triggers request termination when the response
is sent.  In particular, via the upstream cleanup handler, nginx will close
the upstream connection and the corresponding socket.

Still, this can happen to be with ngx_event_pipe() on stack.  While
the code will set p->downstream_error due to NGX_ERROR returned from the
output filter chain by filter finalization, otherwise the error will be
ignored till control returns to ngx_http_upstream_process_request().
And event pipe might try reading from the (already closed) socket, resulting
in "readv() failed (9: Bad file descriptor) while reading upstream" errors
(or even segfaults with SSL).

Such errors were seen with the following configuration:

    location /t2 {
        proxy_pass http://127.0.0.1:8080/big;

        image_filter_buffer 10m;
        image_filter   resize  150 100;
        error_page     415   = /empty;
    }

    location /empty {
        return 204;
    }

    location /big {
        # big enough static file
    }

Fix is to set p->upstream_error in ngx_http_upstream_finalize_request(),
so the existing checks in ngx_event_pipe_read_upstream() will prevent
further reading from the closed upstream connection.

Similarly, p->upstream_error is now checked when handling events at
ngx_event_pipe() exit, as checking p->upstream->fd is not enough if
keepalive upstream connections are being used and the connection was
saved to cache during request termination.

diff --git a/src/event/ngx_event_pipe.c b/src/event/ngx_event_pipe.c
--- a/src/event/ngx_event_pipe.c
+++ b/src/event/ngx_event_pipe.c
@@ -57,7 +57,9 @@ ngx_event_pipe(ngx_event_pipe_t *p, ngx_
         do_write = 1;
     }
 
-    if (p->upstream->fd != (ngx_socket_t) -1) {
+    if (p->upstream->fd != (ngx_socket_t) -1
+        && !p->upstream_error)
+    {
         rev = p->upstream->read;
 
         flags = (rev->eof || rev->error) ? NGX_CLOSE_EVENT : 0;
diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -4561,6 +4561,10 @@ ngx_http_upstream_finalize_request(ngx_h
 
     u->peer.connection = NULL;
 
+    if (u->pipe) {
+        u->pipe->upstream_error = 1;
+    }
+
     if (u->pipe && u->pipe->temp_file) {
         ngx_log_debug1(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
                        "http upstream temp fd: %d",