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Re: [PATCH] HTTP/2: made it possible to flush response headers (ticket #1743)

Maxim Dounin
February 01, 2022 11:06PM
Hello!

On Tue, Feb 01, 2022 at 06:24:47PM +0300, Sergey Kandaurov wrote:

> > On 27 Jan 2022, at 04:24, Maxim Dounin <mdounin@mdounin.ru> wrote:
> >
> > # HG changeset patch
> > # User Maxim Dounin <mdounin@mdounin.ru>
> > # Date 1643225036 -10800
> > # Wed Jan 26 22:23:56 2022 +0300
> > # Node ID f76e0cf8e525996563e5f0092fa48a4fee873e93
> > # Parent 56ead48cfe885e8b89b30017459bf621b21d95f5
> > 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 c->need_last_buf is set,
> > similarly to the existing last buffer handling.
> >
> > diff --git a/src/http/ngx_http_write_filter_module.c b/src/http/ngx_http_write_filter_module.c
> > --- a/src/http/ngx_http_write_filter_module.c
> > +++ b/src/http/ngx_http_write_filter_module.c
> > @@ -227,7 +227,7 @@ ngx_http_write_filter(ngx_http_request_t
> >
> > if (size == 0
> > && !(c->buffered & NGX_LOWLEVEL_BUFFERED)
> > - && !(last && c->need_last_buf))
> > + && !((last || flush) && c->need_last_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
> > --- a/src/http/v2/ngx_http_v2_filter_module.c
> > +++ b/src/http/v2/ngx_http_v2_filter_module.c
> > @@ -1815,7 +1815,11 @@ ngx_http_v2_waiting_queue(ngx_http_v2_co
> > 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;
> > }
> >
>
> Nitpicking:
> Shouldn't NGX_HTTP_V2_BUFFERED still be cleared regardless?
>
> if (stream->queued == 0) {
> fc->buffered &= ~NGX_HTTP_V2_BUFFERED;
>
> if (!c->buffered) {
> return NGX_OK;
> }
> }
>
> It doesn't seem to change anything, at a glance.
> However, this would save a contract that
> (fc->buffered & NGX_HTTP_V2_BUFFERED) is influenced by stream->queued.

No, it shouldn't. The fc->buffered is to be set before returning
from ngx_http_v2_filter_send(), and it is properly set be the
following code.

> Otherwise, looks good.

As discussed privately, it might be good idea to keep the code in
line with UDP handling in the stream module. Here is an updated
patch which introduces the c->need_flush_buf flag, and uses
this flag instead of the c->type checking in the stream module.

# HG changeset patch
# User Maxim Dounin <mdounin@mdounin.ru>
# Date 1643773980 -10800
# Wed Feb 02 06:53:00 2022 +0300
# Node ID 3a695d5d222262cd57c76124d7d0b3e3863a86c6
# Parent dd718d1cef3c2ca2754a21176be3318b00be9e62
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
--- a/src/core/ngx_connection.h
+++ b/src/core/ngx_connection.h
@@ -184,6 +184,7 @@ struct ngx_connection_s {
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
--- a/src/event/ngx_event_connect.c
+++ b/src/event/ngx_event_connect.c
@@ -179,6 +179,8 @@ ngx_event_connect_peer(ngx_peer_connecti
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
--- a/src/event/ngx_event_udp.c
+++ b/src/event/ngx_event_udp.c
@@ -246,6 +246,8 @@ ngx_event_recvmsg(ngx_event_t *ev)
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
--- a/src/http/ngx_http_write_filter_module.c
+++ b/src/http/ngx_http_write_filter_module.c
@@ -227,7 +227,8 @@ ngx_http_write_filter(ngx_http_request_t

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
--- a/src/http/v2/ngx_http_v2_filter_module.c
+++ b/src/http/v2/ngx_http_v2_filter_module.c
@@ -665,6 +665,7 @@ ngx_http_v2_header_filter(ngx_http_reque

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 @@ ngx_http_v2_waiting_queue(ngx_http_v2_co
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
--- a/src/stream/ngx_stream_write_filter_module.c
+++ b/src/stream/ngx_stream_write_filter_module.c
@@ -235,7 +235,7 @@ ngx_stream_write_filter(ngx_stream_sessi
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 */) {

--
Maxim Dounin
http://mdounin.ru/
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Subject Author Views Posted

[PATCH] HTTP/2: made it possible to flush response headers (ticket #1743)

Maxim Dounin 256 January 26, 2022 08:28PM

Re: [PATCH] HTTP/2: made it possible to flush response headers (ticket #1743)

Sergey Kandaurov 44 February 01, 2022 10:28AM

Re: [PATCH] HTTP/2: made it possible to flush response headers (ticket #1743)

Maxim Dounin 40 February 01, 2022 11:06PM

Re: [PATCH] HTTP/2: made it possible to flush response headers (ticket #1743)

Sergey Kandaurov 56 February 02, 2022 11:38AM



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