Subject: bug#40317: 27.0.90; Reverting a buffer that visits C file signals an error

From: Jeff Norden <[email-masked]>
Date: Fri, 18 Sep 2020 17:03:07 -0500

> I've spent quite a long time looking at this, trying various means to
> trigger the error (via `insert-file-contents' and `revert-buffer').
>
> Then it suddenly dawned on me that the (setq c-new-END (.....)) is OK.
> If the body of the the last `unless' has been run, (- end beg) and
> old-len are equal to each other, and to the buffer length.  So c-new-END
> doesn't get changed in this case.

Yup, I guess I was more tired than I realized when I sent that off last
night, and jumped to a conclusion.  So, I'll fall back to my original
theory, which I changed when noticed the code that precedes the
(setq c-new-END ...) line.

Somehow, and I sure don't know how, I think that c-after-change gets
called with: c-new-END already set to the value of point-max after the
insertion; and with the other variables set so that that beg, end, and
old-len remain unchanged.  It's the only scenario that I can see that
fits the backtrace that Eli posted.

If Damien and/or Eli can temporarily try out the test that I suggested
and get it to trigger, I think that would verify this.  In fact, maybe
warn would be even better: 

  (if (> c-new-END (point-max))
    (warn "c-new-END is too big! %d > %d" c-new-END (point-max)))

This should produce a warnings window *and* a backtrace with the
args-out-of-range error.  Don't change the line above yet if the goal is
to diagnose this.  Assuming this does cause a combination warning and
backtrace to occur, then I guess there are two choices: 

1) Try to figure out how the after-change function gets called in this
   way, or
2) Just add a min to prevent c-new-END from exceeding point-max, and
   leave it at that. 

Regards,
-Jeff