Subject: Re: bug#36803: 27.0.50; Update mode-line of every window when compilation ends

From: Stefan Monnier <[email-masked]>
Date: Fri, 26 Jul 2019 17:10:40 -0400

>> Right: there are 2 windows displaying the original buffer, plus a third
>> one displaying the compilation buffer.  The C code for process sentinels
>> makes sure that the mode-line of the window showing the compilation
>> buffer get updated (for the benefit of mode-line-process, presumably),
>> but none of the others.
>> As it so happens, one of the others also gets updated because it's the
>> currently selected_window.
>
> That's not what happens.  What happens is that one of the windows
> still shows "Compiling" after the process exits.

AFAIK this is indeed just what I describe in the previous paragraph
(i.e. at the end of the compilation, when the sentinel is run, the
C part of the sentinel processing causes the refresh of the mode-line
of the window showing the compilation buffer, the redisplay itself will
on its own accord decide to refresh the selected window's mode-line,
but the third window's mode-line is not refreshed because noone tells
the C code that all mode-lines indicated the status of this specific
process).

> If you are saying that that buffer's mode line wasn't supposed to show
> the process status in the first place, then that's the bug we should
> solve.

No, I only indicated as a side-comment that this default of showing the
status of the compilation process in all mode lines is a behavior I dislike.

> But right now the mode line of *scratch* does show "Compiling"
> in all of its windows, so there's a global setting that is updated
> when the process starts and ends,

Indeed.

> and that change isn't triggering the update of mode lines in all the
> windows showing *scratch*.  I still don't understand how you explain
> that inconsistency.

Very simple: the change in the mode-line is caused by a change to the
`compilation-in-progress` global variable.  Yet, the mode-line
infrastructure is not setup to track dependencies on variables; instead
it's traditionally the responsability of the code which sets this
variable to explicitly call force-mode-line-update (as is done, for
example in all minor-modes (from within `define-minor-mode`)).

> If the call to force-mode-line-update is the solution you suggest,
> then I think it is not a good solution.  We cannot rely in a call to
> force-mode-line-update, because there's the default sentinel, and

The default sentinel does not modify `compilation-in-progress` (nor any
other global variable for that matter), so there's never any need to
update all the mode-lines at the end of a process using the default sentinel.

> because it's unreasonable to request each sentinel written in Lisp to
> make that call.  If every sentinel must do that, then the core should
> do that for them.

Not all sentinels must so that.  Only those which set global variables
whose content affects the mode-line of "all" windows (not necessarily
all, tho, but at least more than just the mode lines of the current
buffer).

There might be a few beside compile.el's sentinel, but they are the
exception rather than the rule.

        Stefan