[PATCH] HTTP: trigger lingering close when keepalive connection will be closed

[Miao Wang]

> 2023年1月23日 12:05，Maxim Dounin <mdounin at mdounin.ru> 写道：
> 
> Hello!
> 
> On Wed, Jan 18, 2023 at 11:28:52PM +0800, Miao Wang wrote:
> 
>> # HG changeset patch
>> # User Miao Wang <shankerwangmiao at gmail.com>
>> # Date 1674055068 -28800
>> #      Wed Jan 18 23:17:48 2023 +0800
>> # Node ID 73aa64bd29f3dec9e43e97560d6b5a07cdf40063
>> # Parent  07b0bee87f32be91a33210bc06973e07c4c1dac9
>> HTTP: trigger lingering close when keepalive connection will be closed
>> 
>> When finalizing a request, if the request is not keepalive but
>> its connection has served more than one request, then the connection
>> has been a keepalive connection previously and this connection will
>> be closed after this response. In this condition, it is likely that
>> there are pipelined requests following this request, which we should
>> ignore. As a result, lingering close is necessary in this case.
>> 
>> Without this patch, nginx (with its default configuration) will send
>> out TCP RST when there are more pipelined requests. The symptom is
>> obvious when nginx is serving a debian repository and apt is
>> downloading massive of packages. See [1]. It becomes more obvious
>> when `keepalive_requests` is lower or nginx is under a relative
>> higher load, and it disappears when specifying
>> `lingering_close always`.
>> 
>> [1]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=973861#10
>> 
>> diff -r 07b0bee87f32 -r 73aa64bd29f3 src/http/ngx_http_request.c
>> --- a/src/http/ngx_http_request.c Wed Dec 21 14:53:27 2022 +0300
>> +++ b/src/http/ngx_http_request.c Wed Jan 18 23:17:48 2023 +0800
>> @@ -2749,6 +2749,10 @@
>>         return;
>>     }
>> 
>> +    if (!r->keepalive && r->connection->requests > 1) {
>> +        r->lingering_close = 1;
>> +    }
>> +
>>     if (clcf->lingering_close == NGX_HTTP_LINGERING_ALWAYS
>>         || (clcf->lingering_close == NGX_HTTP_LINGERING_ON
>>             && (r->lingering_close
> 
> Thanks for the patch and the link to the Debian bug report.
> 
> Lingering close implies noticeable additional resource usage: even 
> if nothing happens on the connection, it will be kept open for 
> lingering_timeout, which is 5 seconds by default.  Given that 
> pipelining is not used by most of the clients, forcing lingering 
> close for all clients which are using keepalive does not look like 
> a good solution to me.
> 
> In general, nginx tries hard to determine if any additional data 
> are expected on the connection, and uses lingering close if there 
> is a good chance there will be some, but avoids lingering close by 
> default if additional data are unlikely.  If this logic does not 
> work for some reason, lingering close can be explicitly requested 
> with "lingering_close always;".

That's true since the symptom I described can be worked around with
that option.

> 
> In particular, note the "r->header_in->pos < r->header_in->last" 
> and "r->connection->read->ready" checks - these are expected to 
> catch connections with additional pipelined requests (see revision
> 3981:77604e9a1ed8).  And from the log provided in the report it 
> looks like it works most of the time - there are more than 6k HTTP 
> requests, and 60+ connections.  But sometimes it fails - there are 
> two RST errors logged (and one "Undetermined Error", which looks 
> like a bug in apt, but might be related).
> 
> It looks like when apt is downloading many resources, it does not 
> send all the requests at once (or in batches), but instead tries 
> to maintain a constant "depth", a number of pipelined requests in 
> flight.  This essentially means that after reading of a response 
> it sends an additional request.

That's right. From a traffic dump, I can see apt first sends one
request, and after receiving the response, it will send out 10
more requests, and maintain a depth of 10, since by default
Acquire::http::Pipeline-Depth is 10.

> 
> I see at least two possible cases which can result in nginx not 
> using lingering close with such a load:
> 
> 1.  If a response where keepalive_requests is reached happens to 
> be the last request in the r->header_in buffer (so the 
> "r->header_in->pos < r->header_in->last" won't be effective), and 
> there is a chance that nginx wasn't yet got an event from kernel 
> about additional data (and therefore "r->connection->read->ready" 
> will not be set).  As such, nginx won't use lingering close, and 
> might close connection with unread data in the socket buffer, 
> resulting in RST.
> 
> 2.  Similarly, if nginx happens to be faster than apt, and socket 
> buffers are large enough, it might sent all the responses, 
> including the last one with "Connection: close", and close the 
> connection (since there are no pending pipelined requests at the 
> moment) even before an additional request is sent by apt.  When 
> later apt will send an additional request after reading some of 
> the responses, it will send the request to already closed 
> connection, again resulting in RST.

Actually, comparing the debug log and the pcap, nginx calls
close() after writing the last response. However, at that time,
that response is not fully transmitted to the client and there
seems to be more requests not processed in the kernel buffer.
Thus close() triggers an immediate RST.

> 
> It would be interesting to see more details, such as tcpdump 
> and/or nginx debug logs, to find out what actually goes on here.

The tcpdump and debug logs are too large to send in this mail list.
I wonder if I can directly email it to you.

> 
> Overall, given how apt uses pipelining, I tend to think that at 
> least (2) is unavoidable and can happen with certain sizes of the 
> responses.
> 
> A good enough solution might be check for r->pipeline, which is 
> set by nginx as long as it reads a pipelined request.  It might 
> not be enough though, since r->pipeline is only set for requests 
> seen by nginx as pipelined, which might not be true for the last 
> request.
> 
> A more complete solution might be to introduce something like 
> c->pipeline flag and use lingering close if any pipelined requests 
> were seen on the connection.
> 
> -- 
> Maxim Dounin
> http://mdounin.ru/
> _______________________________________________
> nginx-devel mailing list
> nginx-devel at nginx.org
> https://mailman.nginx.org/mailman/listinfo/nginx-devel

Many thanks for looking into this annoying issue.

Cheers,

Miao Wang