On Sat, Jun 04, 2022 at 12:13:05AM +0300, Maxim Dounin wrote:
> Hello!
>
> On Thu, Jun 02, 2022 at 03:20:54AM +0000, Aleksei Bavshin via nginx-devel wrote:
>
> > > I don't think it's the way to go. Semantically, flush and
> > > last_buf are mutually exclusive: last_buf implies flushing of all
> > > remaining data, and there should be no flush set if last_buf is
> > > set.
> > >
> > > The "cl->buf->flush = !src->read->eof" approach might be actually
> > > better and more obvious: it simply ensures that last_buf and flush
> > > are not set at the same time. I don't immediately see any cases
> > > where it can break things, and this should fix the original
> > > problem with empty UDP packets being sent.
> >
> > Thanks for confirming that, I wasn't sure it is the right approach exactly because I had doubts regarding the mutual exclusivity of the flags.
> > Updated patch below; tests are passing on both branches and all systems we have in CI.
> >
> > # HG changeset patch
> > # User Aleksei Bavshin <a.bavshin@f5.com>
> > # Date 1653330584 25200
> > # Mon May 23 11:29:44 2022 -0700
> > # Node ID 40926829be83986a3a5a5f941d2014000a0acd2e
> > # Parent e0cfab501dd11fdd23ad492419692269d3a01fc7
> > Stream: don't flush empty buffers created for read errors.
> >
> > When we generate the last_buf buffer for an UDP upstream recv error, it does
> > not contain any data from the wire. ngx_stream_write_filter attempts to forward
> > it anyways, which is incorrect (e.g., UDP upstream ECONNREFUSED will be
> > translated to an empty packet).
> >
> > This happens because we mark the buffer as both 'flush' and 'last_buf', and
> > ngx_stream_write_filter has special handling for flush with certain types of
> > connections (see d127837c714f, 32b0ba4855a6). The flags are meant to be
> > mutually exclusive, so the fix is to ensure that flush and last_buf are not set
> > at the same time.
> >
> > Reproduction:
> >
> > stream {
> > upstream unreachable {
> > server 127.0.0.1:8880;
> > }
> > server {
> > listen 127.0.0.1:8998 udp;
> > proxy_pass unreachable;
> > }
> > }
> >
> > 1 0.000000000 127.0.0.1 → 127.0.0.1 UDP 47 45588 → 8998 Len=5
> > 2 0.000166300 127.0.0.1 → 127.0.0.1 UDP 47 51149 → 8880 Len=5
> > 3 0.000172600 127.0.0.1 → 127.0.0.1 ICMP 75 Destination unreachable (Port
> > unreachable)
> > 4 0.000202400 127.0.0.1 → 127.0.0.1 UDP 42 8998 → 45588 Len=0
> >
> > Fixes d127837c714f.
> >
> > diff --git a/src/stream/ngx_stream_proxy_module.c b/src/stream/ngx_stream_proxy_module.c
> > --- a/src/stream/ngx_stream_proxy_module.c
> > +++ b/src/stream/ngx_stream_proxy_module.c
> > @@ -1735,7 +1735,7 @@ ngx_stream_proxy_process(ngx_stream_sess
> >
> > cl->buf->temporary = (n ? 1 : 0);
> > cl->buf->last_buf = src->read->eof;
> > - cl->buf->flush = 1;
> > + cl->buf->flush = !src->read->eof;
> >
> > (*packets)++;
> > *received += n;
>
> Looks good to me.
>
> Roman, please take another look.

Good to me too.

This version is equivalent to the original patch for UDP. The only difference
is a side-effect for TCP. Now flush is not set in the last buffer.

>
> --
> Maxim Dounin
> http://mdounin.ru/
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