id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	uname	nginx_version
2153	Race in proxy_cache_background_update with proxy_cache_use_stale = updating	koshikov@…		"Hello nginx developers,

the main issue - nginx server stale content from cache forever
setup
 http_host:
   proxy_to_main_backend ->
     in_case_50x_error - error_page_to_fallback ->
   fallback:
     proxy_to_fallback_backend

Attached nginx config could repro issue with nginx serving stale cache ""forever"", even update process in the background is allowed. The issue present when all servers from 'main_backend' return error( in this case 502), but fallback servers are good and could serve requests.

Current behavior observed with debug logs:

when proxy_cache_use_stale includes 'updating' nginx replies to client with stale cache, as it should. Then it started connection to ""backend_main"", that returns 502 -> then fallbacks (second request) to ""backend_fallback"" and got reply. And that reply doesn't update cache.
Next request from the client would repeat above process and got stale reply + 2 connections without cache update.

when proxy_cache_use_stale doesn't have 'updating'. Nginx on client request starts 1 req to ""backend_main"" - got 502, then fallback (2 req) - got reply - then replies to client and save response to disk cache.
Next request from the client would be server from disk cache until proxy_cache_valid allows.

Repro:
curl -v  http://localhost:88/url
touch 81 // 502 from main backend
curl -v  http://localhost:88/url

"	defect	closed	minor		documentation	1.14.x	invalid	proxy_cache_background_update		Linux devvm4787 5.2.9-229_ #1 SMP Mon Aug 10 09:34:07 PDT 2020 x86_64 x86_64 x86_64 GNU/Linux	"nginx version: nginx/1.14.1
built by gcc 8.2.1 20180905 (Red Hat 8.2.1-3) (GCC)
built with OpenSSL 1.1.1 FIPS  11 Sep 2018 (running with OpenSSL 1.1.1g FIPS  21 Apr 2020)
TLS SNI support enabled
configure arguments: --prefix=/usr/share/nginx --sbin-path=/usr/sbin/nginx --modules-path=/usr/lib64/nginx/modules --conf-path=/etc/nginx/nginx.conf --error-log-path=/var/log/nginx/error.log --http-log-path=/var/log/nginx/access.log --http-client-body-temp-path=/var/lib/nginx/tmp/client_body --http-proxy-temp-path=/var/lib/nginx/tmp/proxy --http-fastcgi-temp-path=/var/lib/nginx/tmp/fastcgi --http-uwsgi-temp-path=/var/lib/nginx/tmp/uwsgi --http-scgi-temp-path=/var/lib/nginx/tmp/scgi --pid-path=/run/nginx.pid --lock-path=/run/lock/subsys/nginx --user=nginx --group=nginx --with-file-aio --with-ipv6 --with-http_ssl_module --with-http_v2_module --with-http_realip_module --with-http_addition_module --with-http_xslt_module=dynamic --with-http_image_filter_module=dynamic --with-http_sub_module --with-http_dav_module --with-http_flv_module --with-http_mp4_module --with-http_gunzip_module --with-http_gzip_static_module --with-http_random_index_module --with-http_secure_link_module --with-http_degradation_module --with-http_slice_module --with-http_stub_status_module --with-http_perl_module=dynamic --with-http_auth_request_module --with-mail=dynamic --with-mail_ssl_module --with-pcre --with-pcre-jit --with-stream=dynamic --with-stream_ssl_module --with-debug --with-cc-opt='-O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection' --with-ld-opt='-Wl,-z,relro -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -Wl,-E'"