Merge of r4619: accept moderation on EMFILE/ENFILE. In case of EMFILE/ENFILE returned from accept() we disable accept events, and (in case of no accept mutex used) arm timer to re-enable them later. With accept mutex we just drop it, and rely on normal accept mutex handling to re-enable accept events once it's acquired again. As we now handle errors in question, logging level was changed to "crit" (instead of "alert" used for unknown errors). Note: the code might call ngx_enable_accept_events() multiple times if there are many listen sockets. The ngx_enable_accept_events() function was modified to check if connection is already active (via c->read->active) and skip it then, thus making multiple calls safe.

--- a/src/os/unix/ngx_errno.h
+++ b/src/os/unix/ngx_errno.h
@@ -29,6 +29,8 @@ typedef int               ngx_err_t;
 #define NGX_ENOTDIR       ENOTDIR
 #define NGX_EISDIR        EISDIR
 #define NGX_EINVAL        EINVAL
+#define NGX_ENFILE        ENFILE
+#define NGX_EMFILE        EMFILE
 #define NGX_ENOSPC        ENOSPC
 #define NGX_EPIPE         EPIPE
 #define NGX_EINPROGRESS   EINPROGRESS