Inconsistency about cache's valid_sec

[Peng Fang]

Hello, Maxim and other developers,

In the latest official release, ngx_http_file_cache_set_header will be
called in ngx_http_upstream_send_response to realy set a cached file's
valid_sec.
before calling ngx_http_file_cache_set_header, there will be a chance for
user defined modules to change the valid_sec in their header filter
hooks,no matter what the value is set by
Cache-Control/Expires/X-Accel-Expires previously.

And that's reasonable.

When a cached file is expired, that logic will work fine too.

Inconsistency happens, however, when proxy_cache_revalidate
<http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_revalidate>
is enabled. when we enable proxy_cache_revalidate
<http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_revalidate>,
ngx_http_file_cache_update_header will be called in
ngx_http_upstream_test_next. And in ngx_http_upstream_test_next, there is
no chance for user defined modules to change the valid_sec finally in their
header filter hooks. Because before calling ngx_http_upstream_cache_send in
ngx_http_upstream_test_next, valid_sec will be cached firstly, and set to a
cached file after calling ngx_http_upstream_cache_send and
ngx_http_file_cache_update_header.

If we get valid_sec after ngx_http_upstream_cache_send in
ngx_http_upstream_test_next, user defined module's modification of
valid_sec will take effect.And the consistency will be kept.

I don't konw if i missed something, so what do you think about this
situation?
Thanks.
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