Upstream keepalive: "single" parameter deprecated.

The original idea was to optimize edge cases in case of interchangeable
backends, i.e. don't establish a new connection if we have any one
cached.  This causes more harm than good though, as it screws up
underlying balancer's idea about backends used and may result in
various unexpected problems.

src/http/modules/ngx_http_upstream_keepalive_module.c

@@ -12,7 +12,6 @@
 
 typedef struct {
     ngx_uint_t                         max_cached;
-    ngx_uint_t                         single;       /* unsigned:1 */
 
     ngx_queue_t                        cache;
     ngx_queue_t                        free;
@@ -223,36 +222,11 @@ ngx_http_upstream_get_keepalive_peer(ngx_peer_connection_t *pc, void *data)
 
     kp->failed = 0;
 
-    /* single pool of cached connections */
-
-    if (kp->conf->single && !ngx_queue_empty(&kp->conf->cache)) {
-
-        q = ngx_queue_head(&kp->conf->cache);
-
-        item = ngx_queue_data(q, ngx_http_upstream_keepalive_cache_t, queue);
-        c = item->connection;
-
-        ngx_queue_remove(q);
-        ngx_queue_insert_head(&kp->conf->free, q);
-
-        ngx_log_debug1(NGX_LOG_DEBUG_HTTP, pc->log, 0,
-                       "get keepalive peer: using connection %p", c);
-
-        c->idle = 0;
-        c->log = pc->log;
-        c->read->log = pc->log;
-        c->write->log = pc->log;
-        c->pool->log = pc->log;
-
-        pc->connection = c;
-        pc->cached = 1;
-
-        return NGX_DONE;
-    }
+    /* ask balancer */
 
     rc = kp->original_get_peer(pc, kp->data);
 
-    if (kp->conf->single || rc != NGX_OK) {
+    if (rc != NGX_OK) {
         return rc;
     }
 
@@ -552,7 +526,8 @@ ngx_http_upstream_keepalive(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
     for (i = 2; i < cf->args->nelts; i++) {
 
         if (ngx_strcmp(value[i].data, "single") == 0) {
-            kcf->single = 1;
+            ngx_conf_log_error(NGX_LOG_WARN, cf, 0,
+                               "the \"single\" parameter is deprecated");
             continue;
         }