bug#30182: Update

[Eli Zaretskii]

> Date: Tue, 30 Jan 2018 09:30:46 +0100
> From: martin rudalics <rudalics@gmx.at>
> CC: m.sujith@gmail.com, 30182@debbugs.gnu.org
> 
>  > Ideas for how this could happen are welcome.
> 
> The problem should be with calling pos_visible_p.  It's obviously not
> a good idea to calculate the height of the mode line while trying to
> establish the tooltip text for a part of the mode line.  So we
> probably end up evaluating `mode-line-buffer-identification' which
> sets up a timer.

I can believe that pos_visible_p might trigger the :eval form, and
that could add a timer.  But what I cannot understand is how could
pos_visible_p be called between the first and the second call to
Flength inside concat.  This is what we are trying to explain: how
come the first call returns 4, while the second one retuens 5.  That
could only be explained by something that happened between these 2
calls.

> Unfortunately, this remains a wild guess.  It's virtually impossible
> to analyze the problem in more depth: You'd always want the value of
> `timer-list' in order to know which timers have been added to and
> removed from it.

How about running the code with a watchpoint on Vtimer_alist?  Could
that help?