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.

--- 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;