HTTP/2: made it possible to flush response headers (ticket #1743). Response headers can be buffered in the SSL buffer. But stream's fake connection buffered flag did not reflect this, so any attempts to flush the buffer without sending additional data were stopped by the write filter. It does not seem to be possible to reflect this in fc->buffered though, as we never known if main connection's c->buffered corresponds to the particular stream or not. As such, fc->buffered might prevent request finalization due to sending data on some other stream. Fix is to implement handling of flush buffers when the c->need_flush_buf flag is set, similarly to the existing last buffer handling. The same flag is now used for UDP sockets in the stream module instead of explicit checking of c->type.

diff --git a/src/core/ngx_connection.h b/src/core/ngx_connection.h
index 8cc1475..6d3348d 100644
--- a/src/core/ngx_connection.h
+++ b/src/core/ngx_connection.h

@@ -184,6 +184,7 @@
     unsigned            tcp_nopush:2;    /* ngx_connection_tcp_nopush_e */
 
     unsigned            need_last_buf:1;
+    unsigned            need_flush_buf:1;
 
 #if (NGX_HAVE_SENDFILE_NODISKIO || NGX_COMPAT)
     unsigned            busy_count:2;

diff --git a/src/event/ngx_event_connect.c b/src/event/ngx_event_connect.c
index adbbde6..668084a 100644
--- a/src/event/ngx_event_connect.c
+++ b/src/event/ngx_event_connect.c

@@ -179,6 +179,8 @@
         c->recv = ngx_udp_recv;
         c->send = ngx_send;
         c->send_chain = ngx_udp_send_chain;
+
+        c->need_flush_buf = 1;
     }
 
     c->log_error = pc->log_error;

diff --git a/src/event/ngx_event_udp.c b/src/event/ngx_event_udp.c
index 640b377..1ac82c7 100644
--- a/src/event/ngx_event_udp.c
+++ b/src/event/ngx_event_udp.c

@@ -246,6 +246,8 @@
         c->send = ngx_udp_send;
         c->send_chain = ngx_udp_send_chain;
 
+        c->need_flush_buf = 1;
+
         c->log = log;
         c->pool->log = log;
         c->listening = ls;

diff --git a/src/http/ngx_http_write_filter_module.c b/src/http/ngx_http_write_filter_module.c
index 932f26d..89ab49e 100644
--- a/src/http/ngx_http_write_filter_module.c
+++ b/src/http/ngx_http_write_filter_module.c

@@ -227,7 +227,8 @@
 
     if (size == 0
         && !(c->buffered & NGX_LOWLEVEL_BUFFERED)
-        && !(last && c->need_last_buf))
+        && !(last && c->need_last_buf)
+        && !(flush && c->need_flush_buf))
     {
         if (last || flush || sync) {
             for (cl = r->out; cl; /* void */) {

diff --git a/src/http/v2/ngx_http_v2_filter_module.c b/src/http/v2/ngx_http_v2_filter_module.c
index 9ffb155..62b6a04 100644
--- a/src/http/v2/ngx_http_v2_filter_module.c
+++ b/src/http/v2/ngx_http_v2_filter_module.c

@@ -665,6 +665,7 @@
 
     fc->send_chain = ngx_http_v2_send_chain;
     fc->need_last_buf = 1;
+    fc->need_flush_buf = 1;
 
     return ngx_http_v2_filter_send(fc, stream);
 }
@@ -1815,7 +1816,11 @@
 static ngx_inline ngx_int_t
 ngx_http_v2_filter_send(ngx_connection_t *fc, ngx_http_v2_stream_t *stream)
 {
-    if (stream->queued == 0) {
+    ngx_connection_t  *c;
+
+    c = stream->connection->connection;
+
+    if (stream->queued == 0 && !c->buffered) {
         fc->buffered &= ~NGX_HTTP_V2_BUFFERED;
         return NGX_OK;
     }

diff --git a/src/stream/ngx_stream_write_filter_module.c b/src/stream/ngx_stream_write_filter_module.c
index 156a61c..07dc7b5 100644
--- a/src/stream/ngx_stream_write_filter_module.c
+++ b/src/stream/ngx_stream_write_filter_module.c

@@ -235,7 +235,7 @@
     if (size == 0
         && !(c->buffered & NGX_LOWLEVEL_BUFFERED)
         && !(last && c->need_last_buf)
-        && !(c->type == SOCK_DGRAM && flush))
+        && !(flush && c->need_flush_buf))
     {
         if (last || flush || sync) {
             for (cl = *out; cl; /* void */) {