Re: curl_multi_timeout and the multi_socket API

From: Henrik Holst via curl-library <curl-library_at_cool.haxx.se>
Date: Mon, 5 Apr 2021 16:54:00 +0200

Den sön 4 apr. 2021 kl 12:28 skrev Daniel Stenberg <daniel_at_haxx.se>:

> On Sat, 3 Apr 2021, Henrik Holst wrote:
>
> (Let me just add some meat to this and explain this described behavior.)
>
> > As I wrote earlier, usually when you call curl_multi_add_handle(), the
> Timer
> > Function CB is called once by curl with timeout set to 0. Then when you
> call
> > curl_multi_socket_action() with CURL_SOCKET_TIMEOUT the Timer Function
> CB is
> > usually called once more but this time with a timeout set to 1.
>
> This description matches how libcurl *did* behave. More recent versions
> don't
> need to poll the name resolve results from the threaded resolver so the
> timeout pattern will be slightly different.
>

On a side note (and perhaps I should have written this in a separate mail)
I did notice that the timer function callback returns far too low values
for long polling http servers if the user haven't set CURLOPT_TIMEOUT.
Caveat here is that I have only tested this with 7.58 and 7.68 as of yet
and not with HEAD.

So one of the external services that I connect to uses long polling to
avoid having people like me polling the service several times per second in
order to get the latest update with minimal latency. Normally I set
CURLOPT_TIMEOUT for all connections, but yesterday I experimented by using
TCP Keep-alive instead so I commented out the setting of CURLOPT_TIMEOUT
and noticed that the application took 100% cpu, later examination revealed
that this was because the last timeout from the Timer Function callback was
1 (and sometimes 0).

Here is one such run with CURLOPT_VERBOSE set, unfortunately the server is
not public so replication might be hard:

For this I simply print out the given timeout with this callback:

static int CURL_timer_callback (__attribute__((unused)) CURLM *multi, long
timeout, void *userp)
{
   printf ("timeout: %ld\n", timeout);
   return 0;
}

timeout: -1
timeout: 0
timeout: 1
timeout: 1
timeout: 1
timeout: 4
timeout: 7
timeout: 1
timeout: 15
timeout: 1
timeout: 31
timeout: 1
* Trying xxx.170.239.163:80...
* TCP_NODELAY set
timeout: 49
* Connected to press.xxx.com (xxx.170.239.163) port 80 (#0)
> GET /items&lastId=126599 HTTP/1.1
Host: press.xxx.com
Accept: */*

timeout: 119
timeout: 1
^C* Closing connection 0

As you can see the last timeout given here was 1 and then about 10s later I
hit ctrl-c to quit. I was rather expecting it to be -1 since the default
timeout should be infinite.

If I however set CURLOPT_TIMEOUT to say 1800L then:
timeout: -1
timeout: 0
timeout: 1
timeout: 2
timeout: 3
timeout: 1
timeout: 8
timeout: 15
timeout: 1
timeout: 31
* Trying xxx.170.239.163:80...
* TCP_NODELAY set
timeout: 10
timeout: 1
timeout: 188
* Connected to press.xxx.com (xxx.170.239.163) port 80 (#0)
> GET /items&lastId=126599 HTTP/1.1
Host: press.xxx.com
Accept: */*

timeout: 1799747
^C* Closing connection 0

Here the timeout it set to the configured value of ~1800000ms

/HH


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Received on 2021-04-05