Re: Fix potential hang in https handshake.

[Shawn Pearce <spearce@spearce.org>]

On Fri, Oct 19, 2012 at 3:36 AM, Jeff King <peff@peff.net> wrote:
> On Thu, Oct 18, 2012 at 03:59:41PM -0700, Junio C Hamano wrote:
>
>> > It will sometimes happen that curl_multi_fdset() doesn't
>> > return any file descriptors.  In that case, it's recommended
>> > that the application sleep for a short time before running
>> > curl_multi_perform() again.
>> >
>> > http://curl.haxx.se/libcurl/c/curl_multi_fdset.html
>> >
>> > Signed-off-by: Stefan Zager <szager@google.com>
>> > ---
>>
>> Thanks.  Would it be a better idea to "patch up" in problematic
>> case, instead of making this logic too deeply nested, like this
>> instead, I have to wonder...
>>
>>
>>       ... all the existing code above unchanged ...
>>       curl_multi_fdset(..., &max_fd);
>> +     if (max_fd < 0) {
>> +             /* nothing actionable??? */
>> +             select_timeout.tv_sec = 0;
>> +             select_timeout.tv_usec = 50000;
>> +     }
>>
>>       select(max_fd+1, ..., &select_timeout);
>
> But wouldn't that override a potentially shorter timeout that curl gave
> us via curl_multi_timeout, making us unnecessarily slow to hand control
> back to curl?
>
> The current logic is:
>
>   - if curl says there is something to do now (timeout == 0), do it
>     immediately
>
>   - if curl gives us a timeout, use it with select
>
>   - otherwise, feed 50ms to selection
>
> It should not matter what we get from curl_multi_fdset. If there are
> fds, great, we will feed them to select with the timeout, and we may
> break out early if there is work to do. If not, then we are already
> doing this wait.
>
> IOW, it seems like we are _already_ following the advice referenced in
> curl's manpage. Is there some case I am missing? Confused...

The issue with the current code is sometimes when libcurl is opening a
CONNECT style connection through an HTTP proxy it returns a crazy high
timeout (>240 seconds) and no fds. In this case Git waits forever.
Stefan observed that using a timeout of 50 ms in this situation to
poll libcurl is better, as it figures out a lot more quickly that it
is connected to the proxy and can issue the request.