Entity tags: explicit flag to skip not modified filter.

Previously, last_modified_time was tested against -1 to check if the
not modified filter should be skipped.  Notably, this prevented nginx
from additional If-Modified-Since (et al.) checks on proxied responses.
Such behaviour is suboptimal in some cases though, as checks are always
skipped on responses from a cache with ETag only (without Last-Modified),
resulting in If-None-Match being ignored in such cases.  Additionally,
it was not possible to return 412 from the If-Unmodified-Since if last
modification time was not known for some reason.

This change introduces explicit r->disable_not_modified flag instead,
which is set by ngx_http_upstream_process_headers().

1 files changed, 2 insertions, 0 deletions

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 8922dbc8b..5b55b263b 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -2238,6 +2238,8 @@ ngx_http_upstream_process_headers(ngx_http_request_t *r, ngx_http_upstream_t *u)
 
     r->headers_out.content_length_n = u->headers_in.content_length_n;
 
+    r->disable_not_modified = !u->cacheable;
+
     u->length = -1;
 
     return NGX_OK;