Upstream: u->conf->preserve_output flag.

The flag can be used to continue sending request body even after we've
got a response from the backend.  In particular, this is needed for gRPC
proxying of bidirectional streaming RPCs, and also to send control frames
in other forms of RPCs.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index bc98f81..680b664 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -2031,7 +2031,9 @@
         c->tcp_nopush = NGX_TCP_NOPUSH_UNSET;
     }
 
-    u->write_event_handler = ngx_http_upstream_dummy_handler;
+    if (!u->conf->preserve_output) {
+        u->write_event_handler = ngx_http_upstream_dummy_handler;
+    }
 
     if (ngx_handle_write_event(c->write, 0) != NGX_OK) {
         ngx_http_upstream_finalize_request(r, u,
@@ -2193,7 +2195,7 @@
 
 #endif
 
-    if (u->header_sent) {
+    if (u->header_sent && !u->conf->preserve_output) {
         u->write_event_handler = ngx_http_upstream_dummy_handler;
 
         (void) ngx_handle_write_event(c->write, 0);

src/http/ngx_http_upstream.h

diff --git a/src/http/ngx_http_upstream.h b/src/http/ngx_http_upstream.h
index 80fa8b3..c2f4dc0 100644
--- a/src/http/ngx_http_upstream.h
+++ b/src/http/ngx_http_upstream.h
@@ -223,6 +223,7 @@
     unsigned                         intercept_404:1;
     unsigned                         change_buffering:1;
     unsigned                         pass_trailers:1;
+    unsigned                         preserve_output:1;
 
 #if (NGX_HTTP_SSL || NGX_COMPAT)
     ngx_ssl_t                       *ssl;