Cache: ignore stale-if-error for 4xx and 5xx codes. Previously the stale-if-error extension of the Cache-Control upstream header triggered the return of a stale response for all error conditions that can be specified in the proxy_cache_use_stale directive. The list of these errors includes both network/timeout/format errors, as well as some HTTP codes like 503, 504, 403, 429 etc. The latter prevented a cache entry from being updated by a response with any of these HTTP codes during the stale-if-error period. Now stale-if-error only works for network/timeout/format errors and ignores the upstream HTTP code. The return of a stale response for certain HTTP codes is still possible using the proxy_cache_use_stale directive. This change also applies to the stale-while-revalidate extension of the Cache-Control header, which triggers stale-if-error if it is missing. Reported at http://mailman.nginx.org/pipermail/nginx/2020-July/059723.html.

/* XPM */
static char * nginx_xpm[] = {
"16 16 2 2",
/* colors */
"   c none",
"GG c #009900",
/* pixels */
"                                ",
"        GGGGGGGGGGGGGGGG        ",
"        GGGGGGGGGGGGGGGG        ",
"      GGGGGGGGGGGGGGGGGGGG      ",
"      GGGGGG        GGGGGG      ",
"    GGGGGG            GGGGGG    ",
"    GGGGGG                      ",
"  GGGGGG      GGGGGGGGGGGGGGGG  ",
"  GGGGGG    GGGGGGGGGGGGGGGGGG  ",
"    GGGGGG    GGGGGGGGGGGGGG    ",
"    GGGGGG            GGGGGG    ",
"      GGGGGG        GGGGGG      ",
"      GGGGGGGGGGGGGGGGGGGG      ",
"        GGGGGGGGGGGGGGGG        ",
"        GGGGGGGGGGGGGGGG        ",
"                                "
};