[PATCH 2 of 4] Simplified sendfile_max_chunk handling

[Maxim Dounin]

# HG changeset patch
# User Maxim Dounin <mdounin at mdounin.ru>
# Date 1633978587 -10800
#      Mon Oct 11 21:56:27 2021 +0300
# Node ID 489323e194e4c3b1a7937c51bd4e1671c70f52f8
# Parent  d175cd09ac9d2bab7f7226eac3bfce196a296cc0
Simplified sendfile_max_chunk handling.

Previously, it was checked that sendfile_max_chunk was enabled and
almost whole sendfile_max_chunk was sent (see e67ef50c3176), to avoid
delaying connections where sendfile_max_chunk wasn't reached (for example,
when sending responses smaller than sendfile_max_chunk).  Now we instead
check if there are unsent data, and the connection is still ready for writing.
Additionally we also check c->write->delayed to ignore connections already
delayed by limit_rate.

This approach is believed to be more robust, and correctly handles
not only sendfile_max_chunk, but also internal limits of c->send_chain(),
such as sendfile() maximum supported length (ticket #1870).

diff --git a/src/http/ngx_http_write_filter_module.c b/src/http/ngx_http_write_filter_module.c
--- a/src/http/ngx_http_write_filter_module.c
+++ b/src/http/ngx_http_write_filter_module.c
@@ -321,16 +321,12 @@ ngx_http_write_filter(ngx_http_request_t
         delay = (ngx_msec_t) ((nsent - sent) * 1000 / r->limit_rate);
 
         if (delay > 0) {
-            limit = 0;
             c->write->delayed = 1;
             ngx_add_timer(c->write, delay);
         }
     }
 
-    if (limit
-        && c->write->ready
-        && c->sent - sent >= limit - (off_t) (2 * ngx_pagesize))
-    {
+    if (chain && c->write->ready && !c->write->delayed) {
         ngx_post_event(c->write, &ngx_posted_next_events);
     }