[PATCH 3 of 4] QUIC: fixed congesion control in GSO mode

[Sergey Kandaurov]

> On 27 Jul 2023, at 16:42, Roman Arutyunyan <arut at nginx.com> wrote:
> 
> # HG changeset patch
> # User Roman Arutyunyan <arut at nginx.com>
> # Date 1690450542 -14400
> #      Thu Jul 27 13:35:42 2023 +0400
> # Node ID d8a6ec55938e9a4f1a84c825d9c6abe13aa8b791
> # Parent  0d12ada84c168c62e9bae847af2725641da583d0
> QUIC: fixed congesion control in GSO mode.
> 
> In non-GSO mode, a datagram is sent if congestion window is not exceeded by the
> time of send.  The window could be exceeded by a small amount after the send.
> In GSO mode, congestion window was checked in a similar way, but for all
> concatenated datagrams as a whole.  This could result in exceeding congestion
> window by a lot.  Now congestion window is checked for every datagram in GSO
> mode as well.
> 
> diff --git a/src/event/quic/ngx_event_quic_output.c b/src/event/quic/ngx_event_quic_output.c
> --- a/src/event/quic/ngx_event_quic_output.c
> +++ b/src/event/quic/ngx_event_quic_output.c
> @@ -358,7 +358,7 @@ ngx_quic_create_segments(ngx_connection_
> 
>         len = ngx_min(segsize, (size_t) (end - p));
> 
> -        if (len && cg->in_flight < cg->window) {
> +        if (len && cg->in_flight + (p - dst) < cg->window) {
> 
>             n = ngx_quic_output_packet(c, ctx, p, len, len);
>             if (n == NGX_ERROR) {

So ngx_quic_commit_send() is called just once all data
is prepared to be sent in ngx_quic_send_segments().

I think the change is good, though it introduces a subtle diff
compared to cg->in_flight check in ngx_quic_create_datagrams(),
now we will need to take into account.

-- 
Sergey Kandaurov