[PATCH 21 of 31] Fix cpu hog with all upstream servers marked "down"

[Maxim Dounin]

# HG changeset patch
# User Maxim Dounin <mdounin at mdounin.ru>
# Date 1309187262 -14400
# Node ID a36e93979fbdcf882ba8cfa3803ac55df10d2cc4
# Parent  d11450dffd2221fbb525e6aeab506d7a4239ac23
Fix cpu hog with all upstream servers marked "down".

The following configuration causes nginx to hog cpu due to infinite loop
in ngx_http_upstream_get_peer():

    upstream backend {
        server 127.0.0.1:8080 down;
        server 127.0.0.1:8080 down;
    }

    server {
       ...
       location / {
           proxy_pass http://backend;
       }
    }

Make sure we don't loop infinitely in ngx_http_upstream_get_peer() but stop
after resetting peer weights once.

Return 0 if we are stuck.  This is guaranteed to work as peer 0 always exists,
and eventually ngx_http_upstream_get_round_robin_peer() will do the right
thing falling back to backup servers or returning NGX_BUSY.

Initially posted here:
http://nginx.org/pipermail/nginx/2010-January/018332.html

diff --git a/src/http/ngx_http_upstream_round_robin.c b/src/http/ngx_http_upstream_round_robin.c
--- a/src/http/ngx_http_upstream_round_robin.c
+++ b/src/http/ngx_http_upstream_round_robin.c
@@ -583,7 +583,7 @@ failed:
 static ngx_uint_t
 ngx_http_upstream_get_peer(ngx_http_upstream_rr_peers_t *peers)
 {
-    ngx_uint_t                    i, n;
+    ngx_uint_t                    i, n, reset = 0;
     ngx_http_upstream_rr_peer_t  *peer;
 
     peer = &peers->peer[0];
@@ -622,6 +622,10 @@ ngx_http_upstream_get_peer(ngx_http_upst
             return n;
         }
 
+        if (reset++) {
+            return 0;
+        }
+
         for (i = 0; i < peers->number; i++) {
             peer[i].current_weight = peer[i].weight;
         }