This patch adds "before_send" to proxy_next_upstream directive. When "before_send" is specified, a request is passed to next upstream if an error or a timeout has occurred before sending the first byte of the request.

nginx_before_send.patch

diff --git a/src/http/modules/ngx_http_proxy_module.c b/src/http/modules/ngx_http_proxy_module.c
index a869e74..78b5305 100644
--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -220,6 +220,7 @@ static ngx_conf_bitmask_t  ngx_http_proxy_next_upstream_masks[] = {
     { ngx_string("http_403"), NGX_HTTP_UPSTREAM_FT_HTTP_403 },
     { ngx_string("http_404"), NGX_HTTP_UPSTREAM_FT_HTTP_404 },
     { ngx_string("updating"), NGX_HTTP_UPSTREAM_FT_UPDATING },
+    { ngx_string("before_send"), NGX_HTTP_UPSTREAM_FT_BEFORE_SEND },
     { ngx_string("off"), NGX_HTTP_UPSTREAM_FT_OFF },
     { ngx_null_string, 0 }
 };
diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index d5720f4..c45050e 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -3919,11 +3919,15 @@ ngx_http_upstream_next(ngx_http_request_t *r, ngx_http_upstream_t *u,
     }
 
     if (status) {
+        int match_condition = u->conf->next_upstream & ft_type;
+        if (u->conf->next_upstream & NGX_HTTP_UPSTREAM_FT_BEFORE_SEND)
+            match_condition |= !u->request_sent;
+
         u->state->status = status;
         timeout = u->conf->next_upstream_timeout;
 
         if (u->peer.tries == 0
-            || !(u->conf->next_upstream & ft_type)
+            || !match_condition
             || (u->request_sent && r->request_body_no_buffering)
             || (timeout && ngx_current_msec - u->peer.start_time >= timeout))
         {
diff --git a/src/http/ngx_http_upstream.h b/src/http/ngx_http_upstream.h
index 7ecbba4..564201e 100644
--- a/src/http/ngx_http_upstream.h
+++ b/src/http/ngx_http_upstream.h
@@ -29,6 +29,7 @@
 #define NGX_HTTP_UPSTREAM_FT_UPDATING        0x00000400
 #define NGX_HTTP_UPSTREAM_FT_BUSY_LOCK       0x00000800
 #define NGX_HTTP_UPSTREAM_FT_MAX_WAITING     0x00001000
+#define NGX_HTTP_UPSTREAM_FT_BEFORE_SEND     0x00002000
 #define NGX_HTTP_UPSTREAM_FT_NOLIVE          0x40000000
 #define NGX_HTTP_UPSTREAM_FT_OFF             0x80000000