Re: Fix potential hang in https handshake (v2).

From: Junio C Hamano <gitster@pobox.com>
Date: Oct 19, 2012, 00:33 UTC
Message-ID: <7v8vb3l2q5.fsf@alter.siamese.dyndns.org>
In-Reply-To: <5080906b.xT6aRtW1ELoWhZZw%szager@google.com>

szager@google.com writes:

> From aa77ab3dd5b98a5786ac158528f45355fc0ddbc3 Mon Sep 17 00:00:00 2001
> From: Stefan Zager <szager@google.com>
> Date: Thu, 18 Oct 2012 16:23:53 -0700
> Subject: [PATCH] Fix potential hang in https handshake.

Please don't do the above (as usual ;-)

Show 9 quoted lines

> 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>
> ---

The above sounds like "the code is doing something against a recommended practice", but is there a user-visible symptom due to this?

We end up calling select() without any bit set in fds, so it would become micro-sleep of select_timeout in such a case, but as far as I can see, the existing code either

 * does not select() and keeps polling step_active_slots() without
   delay, when curl_timeout gives a 0 return value; or

 * sets 50ms timeout or whatever negative value derived from
   curl_timeout when the returned value is not 0 nor -1.

Is the symptom that select(), when given a negative timeout and no fd to wake it, sleeps forever (or "loooong time, taking that negative value as if it is a large unsigned long") or something?

Thanks.

Show 18 quoted lines

>  http.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
>
> diff --git a/http.c b/http.c
> index df9bb71..e8aba7f 100644
> --- a/http.c
> +++ b/http.c
> @@ -630,6 +630,10 @@ void run_active_slot(struct active_request_slot *slot)
>  			FD_ZERO(&writefds);
>  			FD_ZERO(&excfds);
>  			curl_multi_fdset(curlm, &readfds, &writefds, &excfds, &max_fd);
> +			if (max_fd < 0) {
> +				select_timeout.tv_sec  = 0;
> +				select_timeout.tv_usec = 50000;
> +			}
>  
>  			select(max_fd+1, &readfds, &writefds, &excfds, &select_timeout);
>  		}