[nginx] Additional connections reuse.

[Maxim Dounin]

details:   https://hg.nginx.org/nginx/rev/6d98f29867e8
branches:  
changeset: 7767:6d98f29867e8
user:      Maxim Dounin <mdounin at mdounin.ru>
date:      Thu Feb 11 21:52:11 2021 +0300
description:
Additional connections reuse.

If ngx_drain_connections() fails to immediately reuse any connections
and there are no free connections, it now additionally tries to reuse
a connection again.  This helps to provide at least one free connection
in case of HTTP/2 with lingering close, where merely trying to reuse
a connection once does not free it, but makes it reusable again,
waiting for lingering close.

diffstat:

 src/core/ngx_connection.c |  16 ++++++++++++++++
 1 files changed, 16 insertions(+), 0 deletions(-)

diffs (33 lines):

diff -r 3e83336cda5b -r 6d98f29867e8 src/core/ngx_connection.c
--- a/src/core/ngx_connection.c	Thu Feb 11 21:52:09 2021 +0300
+++ b/src/core/ngx_connection.c	Thu Feb 11 21:52:11 2021 +0300
@@ -1310,6 +1310,7 @@ ngx_drain_connections(ngx_cycle_t *cycle
                       cycle->connection_n);
     }
 
+    c = NULL;
     n = ngx_max(ngx_min(32, cycle->reusable_connections_n / 8), 1);
 
     for (i = 0; i < n; i++) {
@@ -1326,6 +1327,21 @@ ngx_drain_connections(ngx_cycle_t *cycle
         c->close = 1;
         c->read->handler(c->read);
     }
+
+    if (cycle->free_connection_n == 0 && c && c->reusable) {
+
+        /*
+         * if no connections were freed, try to reuse the last
+         * connection again: this should free it as long as
+         * previous reuse moved it to lingering close
+         */
+
+        ngx_log_debug0(NGX_LOG_DEBUG_CORE, c->log, 0,
+                       "reusing connection again");
+
+        c->close = 1;
+        c->read->handler(c->read);
+    }
 }