bug#39585: after-change-functions called with invalid positions in call-process

[Clément Pit-Claudel]

Hi all,

Recent changes in call-process have introduced surprising behavior in 
after-change-functions (and caused bugs in flycheck, e.g. 
https://github.com/flycheck/flycheck/issues/1677).  From the documentation, I 
expected that functions added to the after-change-functions hook would be 
called only with valid positions.  However, the following snippets shows that 
it's not the case:

    (defun ~/after-change (beg end len)
      (message "(after-change %S %S %S); (buffer %S %S %S)"
               beg end len (point-min) (point-max) (buffer-size)))

    (with-current-buffer (get-buffer-create "*tmp*")
      (make-variable-buffer-local 'after-change-functions)
      (add-hook 'after-change-functions #'~/after-change)
      (call-process "echo" nil t t "Hello"))

Running it repeatedly, this is what I observe:

    (after-change 7 13 0); (buffer 1 7 6)
    (after-change 13 19 0); (buffer 1 13 12)
    (after-change 19 25 0); (buffer 1 19 18)
    (after-change 25 31 0); (buffer 1 25 24)

Note how each time the after-change-functions hook is called with a region past 
the end of the buffer.  It's as if after-change-functions was in fact call 
right before the insertion, instead of after.

Previous versions of Emacs didn't call after-change-functions in this case; it 
seems that the new behavior was introduced by this commit:

    commit 224e8d146485ce178086549d41fa8359dcc0e03e
    Author: Alan Mackenzie <acm@muc.de>
    Date:   Wed Jan 22 19:50:30 2020 +0000

        Make call_process call signal_after_change.  This fixes bug #38691.

        Now, functions such as call-proess-region invoke after-change-functions
        correctly.

        * src/callproc.c (call_process): Call prepare_to_modify_buffer in a 
single
        place, no longer delegating the task to insert_1_both, etc.  Call
        signal_after_change in each of two code branches, such that
        before-change-functions and after-change-functions are always called in
        balanced pairs.

Alan, is this behavior expected?

Thanks,
Clément.