Hello! On Mon, Feb 05, 2024 at 06:01:54PM +0100, Jan Prachař wrote:
> Hello, > > thank you for your responses. > > On Sat, 2024-02-03 at 04:25 +0300, Maxim Dounin wrote: > > Hello! > > > > On Fri, Feb 02, 2024 at 01:47:51PM +0100, Jan Prachař wrote: > > > > > On Fri, 2024-02-02 at 12:48 +0100, Jiří Setnička via nginx-devel wrote: > > > > Hello, > > > > > > > > Also, I believe that the core of the problem is because of the > > > > ngx_http_finalize_request(r, NGX_DONE); call in the > > > > ngx_http_upstream_process_headers function. This call is needed when > > > > doing an internal redirect after the real upstream request (to close > > > > the > > > > upstream request), but when serving from the cache, there is no > > > > upstream > > > > request to close and this call causes ngx_http_set_lingering_close to > > > > be > > > > called from the ngx_http_finalize_connection with no active request on > > > > the connection yielding to the segfault. > > > > > > Hello, > > > > > > I am Jiří's colleague, and so I have taken a closer look at the problem. > > > Another > > > indication of the issue is the alert in the error log for non-keepalive > > > connections, > > > stating "http request count is zero while closing request." > > > > > > Upon reviewing the nginx source code, I discovered that the function > > > ngx_http_upstream_finalize_request(), when called with rc = NGX_DECLINED, > > > does not invoke > > > ngx_http_finalize_request(). However, when there is nothing to clean up > > > (u->cleanup == > > > NULL), it does. Therefore, I believe the appropriate fix is to follow the > > > patch below. > > > > > > Best, Jan Prachař > > > > > > # User Jan Prachař <jan.prac...@gmail.com> > > > # Date 1706877176 -3600 > > > # Fri Feb 02 13:32:56 2024 +0100 > > > # Node ID 851c994b48c48c9cd3d32b9aa402f4821aeb8bb2 > > > # Parent cf3d537ec6706f8713a757df256f2cfccb8f9b01 > > > Upstream: Fix "request count is zero" when procesing X-Accel-Redirect > > > > > > ngx_http_upstream_finalize_request(r, u, NGX_DECLINED) should not call > > > ngx_http_finalize_request(). > > > > > > diff -r cf3d537ec670 -r 851c994b48c4 src/http/ngx_http_upstream.c > > > --- a/src/http/ngx_http_upstream.c Thu Nov 26 21:00:25 2020 +0100 > > > +++ b/src/http/ngx_http_upstream.c Fri Feb 02 13:32:56 2024 +0100 > > > @@ -4340,6 +4340,11 @@ > > > > > > if (u->cleanup == NULL) { > > > /* the request was already finalized */ > > > + > > > + if (rc == NGX_DECLINED) { > > > + return; > > > + } > > > + > > > ngx_http_finalize_request(r, NGX_DONE); > > > return; > > > } > > > > I somewhat agree: the approach suggested by Jiří certainly looks > > incorrect. The ngx_http_upstream_cache_send() function, which > > calls ngx_http_upstream_process_headers() with r->cached set, can > > be used in two contexts: before the cleanup handler is installed > > (i.e., when sending a cached response during upstream request > > initialization) and after it is installed (i.e., when sending a > > stale cached response on upstream errors). In the latter case > > skipping finalization would mean a socket leak. > > > > Still, checking for NGX_DECLINED explicitly also looks wrong, for > > a number of reasons. > > > > First, the specific code path isn't just for "nothing to clean > > up", it's for the very specific case when the request was already > > finalized due to filter finalization, see 5994:5abf5af257a7. This > > code path is not expected to be triggered when the cleanup handler > > isn't installed yet - before the cleanup handler is installed, > > upstream code is expected to call ngx_http_finalize_request() > > directly instead. And it would be semantically wrong to check for > > NGX_DECLINED: if it's here, it means something already gone wrong. > > > > I think the generic issue here is that > > ngx_http_upstream_process_headers(), which is normally used for > > upstream responses and calls ngx_http_upstream_finalize_request(), > > is also used for cached responses. Still, it assumes it is used > > for an upstream response, and calls > > ngx_http_upstream_finalize_request(). > > > > As can be seen from the rest of the > > ngx_http_upstream_process_headers() code, apart from the issue > > with X-Accel-Redirect, it can also call > > ngx_http_upstream_finalize_request(NGX_HTTP_INTERNAL_SERVER_ERROR) > > when hh->copy_handler() or ngx_http_upstream_copy_header_line() > > fails. This will similarly end up in > > ngx_http_finalize_request(NGX_DONE) since there is no u->cleanup, > > leading to a request hang. And it would be certainly wrong to > > check for NGX_HTTP_INTERNAL_SERVER_ERROR similarly to NGX_DECLINED > > in your patch, because it can theoretically happen after filter > > finalization. > > > > Proper solution would probably require re-thinking > > ngx_http_upstream_process_headers() interface. > > > > Some preliminary code below: it disables X-Accel-Redirect > > processing altogether if ngx_http_upstream_process_headers() is > > called when returning a cached response (this essentially means > > that "proxy_ignore_headers X-Accel-Expires" is preserved in the > > cache file, which seems to be the right thing to do as we don't > > save responses with X-Accel-Redirect to cache unless it is > > ignored), and returns NGX_ERROR in other places to trigger > > appropriate error handling instead of calling > > ngx_http_upstream_finalize_request() directly (this no longer > > tries to return 500 Internal Server Error response though, as > > doing so might be unsafe after copying some of the cached headers > > to the response). > > > > Please take a look if it works for you. > > The provided patch works as expected, with no observed issues. > > Considering that proxy_ignore_headers for caching headers is preserved with > the > cached file, it seems reasonable to extend the same behavior to > X-Accel-Redirect. Yes, such handling is (mostly) in line with some proxy_ignore_headers handling, that is, X-Accel-Expires, Expires, Cache-Control, Set-Cookie, Vary, and X-Accel-Buffering, as these affect creation of a cache file, but not sending an already cached response to clients. Still, X-Accel-Limit-Rate from a cache file will be applied to the response if not ignored by the current configuration. Similarly, X-Accel-Charset is also applied as long as no longer ignored. As such, I mostly consider this to be a neutral argument. Further, we might reconsider X-Accel-Redirect handling if caching of X-Accel-Redirect responses will be introduced (see https://trac.nginx.org/nginx/ticket/407 for a feature request). > From my perspective, the updated code in ngx_http_upstream_process_headers() > is > a bit confusing. The function can return NGX_DONE, but this return code is > only > handled in one place where ngx_http_upstream_process_headers() is called. I've removed NGX_DONE handling from the other call since NGX_DONE return code isn't possible there due to r->cached being set just before the call. We can instead assume it can be returned and handle appropriately: this will also make handling X-Accel-Redirect from cached files easier if we'll decide to (instead of checking r->cached, we'll have to call ngx_http_upstream_finalize_request(NGX_DECLINED) conditionally, only if u->cleanup is set). > If I may suggest, splitting the function might be helpful – redirect > processing > would only occur for direct upstream responses, while the rest of the header > processing would be called always (i.e., also for cached responses). I can't say I like this idea. Processing of X-Accel-Redirect is a part of headers processing, and quite naturally handled in ngx_http_upstream_process_headers(). Moving it to a separate function will needlessly complicate things. > Additionally, I believe the special handling of NGX_DECLINED in > ngx_http_upstream_finalize_request() can be removed. The updated patch is > provided below. Not really. The ngx_http_upstream_finalize_request(NGX_DECLINED) call ensures that the upstream handling is properly finalized, notably the upstream connection is closed. For short responses after X-Accel-Redirect, this might not be important, because the upstream connection will be closed anyway during request finalization. But if the redirected request processing takes a while, the upstream connection will still be open, and might receive further events - leading to unexpected behaviour (not to mention that various upstream timing variables, such as $upstream_response_time, will be wrong). Below is a patch which preserves proper NGX_DONE processing, and handles X-Accel-Redirect from cached files by checking r->cleanup when calling ngx_http_upstream_finalize_request(NGX_DECLINED). I tend to think this might be the best solution after all, providing better compatibility for further improvements. # HG changeset patch # User Maxim Dounin <mdou...@mdounin.ru> # Date 1707167064 -10800 # Tue Feb 06 00:04:24 2024 +0300 # Node ID 6e7f0d6d857473517048b8838923253d5230ace0 # Parent 631ee3c6d38cfdf97dec67c3d2c457af5d91db01 Upstream: fixed X-Accel-Redirect handling from cache files. The X-Accel-Redirect header might appear in cache files if its handling is ignored with the "proxy_ignore_headers" directive. If the cache file is later served with different settings, ngx_http_upstream_process_headers() used to call ngx_http_upstream_finalize_request(NGX_DECLINED), which is not expected to happen before the cleanup handler is installed and resulted in ngx_http_finalize_request(NGX_DONE), leading to unexpected request counter decrement, "request count is zero" alerts, and segmentation faults. Similarly, errors in ngx_http_upstream_process_headers() resulted in ngx_http_upstream_finalize_request(NGX_HTTP_INTERNAL_SERVER_ERROR) being called. This is also not expected to happen before the cleanup handler is installed, and resulted in ngx_http_finalize_request(NGX_DONE) without proper request finalization. Fix is to avoid calling ngx_http_upstream_finalize_request() from ngx_http_upstream_process_headers(), notably when the cleanup handler is not yet installed. Errors are now simply return NGX_ERROR, so the caller is responsible for proper finalization by calling either ngx_http_finalize_request() or ngx_http_upstream_finalize_request(). And X-Accel-Redirect handling now does not call ngx_http_upstream_finalize_request(NGX_DECLINED) if no cleanup handler is installed. Reported by Jiří Setnička (https://mailman.nginx.org/pipermail/nginx-devel/2024-February/HWLYHOO3DDB3XTFT6X3GRMXIEJ3SJRUA.html). 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 @@ -1087,8 +1087,10 @@ ngx_http_upstream_cache_send(ngx_http_re if (rc == NGX_OK) { - if (ngx_http_upstream_process_headers(r, u) != NGX_OK) { - return NGX_DONE; + rc = ngx_http_upstream_process_headers(r, u); + + if (rc != NGX_OK) { + return rc; } return ngx_http_cache_send(r); @@ -2516,7 +2518,14 @@ ngx_http_upstream_process_header(ngx_htt } } - if (ngx_http_upstream_process_headers(r, u) != NGX_OK) { + rc = ngx_http_upstream_process_headers(r, u); + + if (rc == NGX_DONE) { + return; + } + + if (rc == NGX_ERROR) { + ngx_http_upstream_finalize_request(r, u, NGX_ERROR); return; } @@ -2829,7 +2838,9 @@ ngx_http_upstream_process_headers(ngx_ht if (u->headers_in.x_accel_redirect && !(u->conf->ignore_headers & NGX_HTTP_UPSTREAM_IGN_XA_REDIRECT)) { - ngx_http_upstream_finalize_request(r, u, NGX_DECLINED); + if (u->cleanup) { + ngx_http_upstream_finalize_request(r, u, NGX_DECLINED); + } part = &u->headers_in.headers.part; h = part->elts; @@ -2918,18 +2929,14 @@ ngx_http_upstream_process_headers(ngx_ht if (hh) { if (hh->copy_handler(r, &h[i], hh->conf) != NGX_OK) { - ngx_http_upstream_finalize_request(r, u, - NGX_HTTP_INTERNAL_SERVER_ERROR); - return NGX_DONE; + return NGX_ERROR; } continue; } if (ngx_http_upstream_copy_header_line(r, &h[i], 0) != NGX_OK) { - ngx_http_upstream_finalize_request(r, u, - NGX_HTTP_INTERNAL_SERVER_ERROR); - return NGX_DONE; + return NGX_ERROR; } } -- Maxim Dounin http://mdounin.ru/ _______________________________________________ nginx-devel mailing list nginx-devel@nginx.org https://mailman.nginx.org/mailman/listinfo/nginx-devel