Cache locks initial implementation.

New directives: proxy_cache_lock on/off, proxy_cache_lock_timeout.  With
proxy_cache_lock set to on, only one request will be allowed to go to
upstream for a particular cache item.  Others will wait for a response
to appear in cache (or cache lock released) up to proxy_cache_lock_timeout.

Waiting requests will recheck if they have cached response ready (or are
allowed to run) every 500ms.

Note: we intentionally don't intercept NGX_DECLINED possibly returned by
ngx_http_file_cache_read().  This needs more work (possibly safe, but needs
further investigation).  Anyway, it's exceptional situation.

Note: probably there should be a way to disable caching of responses
if there is already one request fetching resource to cache (without waiting
at all).  Two possible ways include another cache lock option ("no_cache")
or using proxy_no_cache with some supplied variable.

Note: probably there should be a way to lock updating requests as well.  For
now "proxy_cache_use_stale updating" is available.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/http/ngx_http_cache.h b/src/http/ngx_http_cache.h
index bd02100da..ae8b8c376 100644
--- a/src/http/ngx_http_cache.h
+++ b/src/http/ngx_http_cache.h
@@ -79,6 +79,14 @@ struct ngx_http_cache_s {
     ngx_http_file_cache_t           *file_cache;
     ngx_http_file_cache_node_t      *node;
 
+    ngx_msec_t                       lock_timeout;
+    ngx_msec_t                       wait_time;
+
+    ngx_event_t                      wait_event;
+
+    unsigned                         lock:1;
+    unsigned                         waiting:1;
+
     unsigned                         updated:1;
     unsigned                         updating:1;
     unsigned                         exists:1;