Merge of r5113, r5114: upstream: resolve errors handling.

Upstream: call ngx_http_run_posted_requests() on resolve errors.

If proxy_pass to a host with dynamic resolution was used to handle
a subrequest, and host resolution failed, the main request wasn't run
till something else happened on the connection.  E.g. request to "/zzz"
with the following configuration hanged:

    addition_types *;
    resolver 8.8.8.8;

    location /test {
        set $ihost xxx;
        proxy_pass http://$ihost;
    }

    location /zzz {
        add_after_body /test;
        return 200 "test";
    }

Report and original version of the patch by Lanshun Zhou,
http://mailman.nginx.org/pipermail/nginx-devel/2013-March/003476.html.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 3436c6b..84fd8b1 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -865,11 +865,13 @@
 static void
 ngx_http_upstream_resolve_handler(ngx_resolver_ctx_t *ctx)
 {
+    ngx_connection_t              *c;
     ngx_http_request_t            *r;
     ngx_http_upstream_t           *u;
     ngx_http_upstream_resolved_t  *ur;
 
     r = ctx->data;
+    c = r->connection;
 
     u = r->upstream;
     ur = u->resolved;
@@ -881,7 +883,7 @@
                       ngx_resolver_strerror(ctx->state));
 
         ngx_http_upstream_finalize_request(r, u, NGX_HTTP_BAD_GATEWAY);
-        return;
+        goto failed;
     }
 
     ur->naddrs = ctx->naddrs;
@@ -906,13 +908,17 @@
     if (ngx_http_upstream_create_round_robin_peer(r, ur) != NGX_OK) {
         ngx_http_upstream_finalize_request(r, u,
                                            NGX_HTTP_INTERNAL_SERVER_ERROR);
-        return;
+        goto failed;
     }
 
     ngx_resolve_name_done(ctx);
     ur->ctx = NULL;
 
     ngx_http_upstream_connect(r, u);
+
+failed:
+
+    ngx_http_run_posted_requests(c);
 }