bug#22789: 25.1.50; In last master build https connections stop working

[Eli Zaretskii]

> From: Lars Magne Ingebrigtsen <larsi@gnus.org>
> Cc: a.s@realize.ch,  j_l_domenech@yahoo.com,  22789@debbugs.gnu.org
> Date: Sat, 05 Mar 2016 13:26:50 +0100
> 
> This is probably totally unrelated, but I saw a "hang" in Emacs for the
> first time in weeks.  shr was downloading an image over https, and Emacs
> became unresponsive.  strace showed the following:
> 
> [pid 12890] pselect6(39, [3 6 7 12 13 14 15 16 17 18 19 20 38], [], NULL, {0, 
> 0}, {NULL, 8}) = 2 (in [18 19], left {0, 0})
> [pid 12890] pselect6(39, [3 6 7 12 13 14 15 16 17 18 19 20 38], [], NULL, {0, 
> 0}, {NULL, 8}) = 2 (in [18 19], left {0, 0})
> [pid 12890] pselect6(39, [3 6 7 12 13 14 15 16 17 18 19 20 38], [], NULL, {0, 
> 0}, {NULL, 8}) = 2 (in [18 19], left {0, 0})
> [pid 12890] pselect6(39, [3 6 7 12 13 14 15 16 17 18 19 20 38], [], NULL, {0, 
> 0}, {NULL, 8}) = 2 (in [18 19], left {0, 0})
> [pid 12890] pselect6(39, [3 6 7 12 13 14 15 16 17 18 19 20 38], [], NULL, {0, 
> 0}, {NULL, 8}) = 2 (in [18 19], left {0, 0})
> [pid 12890] pselect6(39, [3 6 7 12 13 14 15 16 17 18 19 20 38], [], NULL, {0, 
> 0}, {NULL, 8}) = 2 (in [18 19], left {0, 0})
> [pid 12890] pselect6(39, [3 6 7 12 13 14 15 16 17 18 19 20 38], [], NULL, {0, 
> 0}, {NULL, 8}) = 2 (in [18 19], left {0, 0})
> [pid 12890] pselect6(39, [3 6 7 12 13 14 15 16 17 18 19 20 38], [], NULL, {0, 
> 0}, {NULL, 8}) = 2 (in [18 19], left {0, 0})
> [pid 12890] pselect6(39, [3 6 7 12 13 14 15 16 17 18 19 20 38], [], NULL, {0, 
> 0}, {NULL, 8}) = 2 (in [18 19], left {0, 0})
> 
> This went on for perhaps 30 seconds?  (Hm.  Which is what I have
> `url-queue-timeout' set to...  Hm...) Then it stopped without
> downloading the image.

Hard to say anything intelligent without knowing what are descriptors
18 and 19.

> Now, the pselect6 call has a timeout of {0, 0}?  If I'm reading the man
> page right.
> 
> I think it's probably this code?
> 
>       /* If status of something has changed, and no input is
>        available, notify the user of the change right away.  After
>        this explicit check, we'll let the SIGCHLD handler zap
>        timeout to get our attention.  */
>       if (update_tick != process_tick)
>       {
>         fd_set Atemp;
>         fd_set Ctemp;
> 
>           if (kbd_on_hold_p ())
>             FD_ZERO (&Atemp);
>           else
>             Atemp = input_wait_mask;
>         Ctemp = write_mask;
> 
>         timeout = make_timespec (0, 0);
>         if ((pselect (max (max_process_desc, max_input_desc) + 1,
>                       &Atemp,
> #ifdef NON_BLOCKING_CONNECT
>                       (num_pending_connects > 0 ? &Ctemp : NULL),
> #else
>                       NULL,
> #endif
>                       NULL, &timeout, NULL)
>              <= 0))
> 
> I'm not quite sure what it's trying to do...  Hm...

The comment above seems to explain what it does, no?