bug#70824: 29.3; Run time error in `dabbrev-completion'

bug#70824: 29.3; Run time error in `dabbrev-completion'

[Matt Armstrong via Bug reports for GNU Emacs, the Swiss army knife of text editors]

I'm in a state where `dabbrev--last-buffer' is set to #<killed buffer>,
which confuses the `dabbrev-completion' command as shown by the 
backtrace below.

If I find a way to reproduce I will follow up with that.

I suspect that dabbrev.el should use `(buffer-live-p
dabbrev--last-buffer)' instead of checking for non-nil.

First, the value of dabbrev--last-buffer:

     dabbrev--last-buffer is a variable defined in ‘dabbrev.el’.
     Its value is #<killed buffer>
     Not documented as a variable.

Second, what happens when I run `dabbrev-completion':

Debugger entered--Lisp error: (error "Selecting deleted buffer")
   set-buffer(#<killed buffer>)
   (progn (set-buffer dabbrev--last-buffer))
   (if dabbrev--last-buffer (progn (set-buffer dabbrev--last-buffer)))
   (save-excursion (if dabbrev--last-buffer (progn (set-buffer 
dabbrev--last-buffer))) (or (and (not dabbrev-search-these-buffers-only) 
(>= direction 0) (setq dabbrev--last-direction (min 1 direction)) 
(dabbrev--try-find abbrev t (max 1 direction) ignore-case)) (and (or 
(not dabbrev-search-these-buffers-only) dabbrev--last-buffer) (<= 
direction 0) (setq dabbrev--last-direction -1) (dabbrev--try-find abbrev 
nil (max 1 (- direction)) ignore-case)) (progn (setq 
dabbrev--last-direction -1) (if dabbrev--last-buffer nil (if 
dabbrev--friend-buffer-list nil (setq dabbrev--friend-buffer-list 
(dabbrev--make-friend-buffer-list)) (setq dabbrev--progress-reporter 
(make-progress-reporter "Scanning for dabbrevs..." (- ...) 0 0 1 1.5)))) 
(let ((file-name (buffer-file-name)) file-name-buffer) (unwind-protect 
(progn (if file-name (progn ... ... ...)) (let (expansion) (while ... 
... ... ... ... ...) (progress-reporter-done dabbrev--progress-reporter) 
expansion)) (if (buffer-live-p file-name-buffer) (progn (kill-buffer 
file-name-buffer))) (setq dabbrev--friend-buffer-list (seq-filter 
#'buffer-live-p dabbrev--friend-buffer-list)))))))
   dabbrev--find-expansion("p" -1 t)
   dabbrev--find-all-expansions("p" t)
   #f(compiled-function (s p a) #<bytecode -0x1773fde4def7ae30>)(#("p" 0 
1 (fontified t)) nil (boundaries . ""))
   completion-boundaries(#("p" 0 1 (fontified t)) #f(compiled-function 
(s p a) #<bytecode -0x1773fde4def7ae30>) nil "")
   completion-basic-try-completion(#("p" 0 1 (fontified t)) 
#f(compiled-function (s p a) #<bytecode -0x1773fde4def7ae30>) nil 1)
   #f(compiled-function (style) #<bytecode -0x1bb324416e7d7fbf>)(basic)
   completion--some(#f(compiled-function (style) #<bytecode 
-0x1bb324416e7d7fbf>) (basic partial-completion emacs22))
   completion--nth-completion(1 #("p" 0 1 (fontified t)) 
#f(compiled-function (s p a) #<bytecode -0x1773fde4def7ae30>) nil 1 
(metadata (cycle-sort-function . identity) (category . dabbrev)))
   completion-try-completion(#("p" 0 1 (fontified t)) 
#f(compiled-function (s p a) #<bytecode -0x1773fde4def7ae30>) nil 1 
(metadata (cycle-sort-function . identity) (category . dabbrev)))
   completion--do-completion(#<marker at 531 in persist.rs> 532)
   completion--in-region-1(#<marker at 531 in persist.rs> 532)
   #f(compiled-function (start end collection predicate) #<bytecode 
-0x16fb47f8fb6a4d3d>)(#<marker at 531 in persist.rs> 532 
#f(compiled-function (s p a) #<bytecode -0x1773fde4def7ae30>) nil)
   apply(#f(compiled-function (start end collection predicate) 
#<bytecode -0x16fb47f8fb6a4d3d>) (#<marker at 531 in persist.rs> 532 
#f(compiled-function (s p a) #<bytecode -0x1773fde4def7ae30>) nil))
   #f(compiled-function (funs global args) #<bytecode 
-0x73c1f01704730a2>)(nil nil (#<marker at 531 in persist.rs> 532 
#f(compiled-function (s p a) #<bytecode -0x1773fde4def7ae30>) nil))
   completion--in-region(#<marker at 531 in persist.rs> 532 
#f(compiled-function (s p a) #<bytecode -0x1773fde4def7ae30>) nil)
   completion-in-region(#<marker at 531 in persist.rs> 532 
#f(compiled-function (s p a) #<bytecode -0x1773fde4def7ae30>) nil)
   completion-at-point()
   dabbrev-completion(nil)
   funcall-interactively(dabbrev-completion nil)
   command-execute(dabbrev-completion)




In GNU Emacs 29.3 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.41,
cairo version 1.18.0)
System Description: Debian GNU/Linux 12 (bookworm)

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Load-path shadows:
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bug#70824: 29.3; Run time error in `dabbrev-completion'

[Matt Armstrong via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Debugging this further (sorry, still no repro so I'm debugging my live 
image), I think this was introduced a few years ago in commit 
2d0085f756572856a2ed8d1bf043b59195a3e3f3 and partially fixed in commit 
d0975d7db03c231a3db5a1cd0edaf41094d43f0d.

Those commits introduced a feature where `dabbrev--find-expansion' 
considers the components of a buffer's file name as completion 
candidates. It does this by creating a temp buffer containing the 
components.

The problem is that this temp buffer is immediately killed, yet dabbrev 
variables still hold references to it. The second commit above correctly 
removes the killed buffer from `dabbrev--friend-buffer-list', but did 
not handle the case where `dabbrev--last-buffer' or 
`dabbrev--last-buffer-found' happened to be the just-killed buffer.

I can think of two approaches:

  - continual with the "manual GC" approach and scrub more dabbrev vars 
of this killed buffer.
  - handle the killed buffer gracefully by using buffer-live-p 
consistently, instead using checks against nil.

bug#70824: 29.3; Run time error in `dabbrev-completion'

[Eli Zaretskii]

> Date: Tue, 7 May 2024 12:52:56 -0700
> From:  Matt Armstrong via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> Debugging this further (sorry, still no repro so I'm debugging my live 
> image), I think this was introduced a few years ago in commit 
> 2d0085f756572856a2ed8d1bf043b59195a3e3f3 and partially fixed in commit 
> d0975d7db03c231a3db5a1cd0edaf41094d43f0d.
> 
> Those commits introduced a feature where `dabbrev--find-expansion' 
> considers the components of a buffer's file name as completion 
> candidates. It does this by creating a temp buffer containing the 
> components.
> 
> The problem is that this temp buffer is immediately killed, yet dabbrev 
> variables still hold references to it. The second commit above correctly 
> removes the killed buffer from `dabbrev--friend-buffer-list', but did 
> not handle the case where `dabbrev--last-buffer' or 
> `dabbrev--last-buffer-found' happened to be the just-killed buffer.

Thanks for the analysis.

> I can think of two approaches:
> 
>   - continual with the "manual GC" approach and scrub more dabbrev vars 
> of this killed buffer.
>   - handle the killed buffer gracefully by using buffer-live-p 
> consistently, instead using checks against nil.

I think we should do both.

Can you suggest a patch along these lines?

bug#70824: 29.3; Run time error in `dabbrev-completion'

[Eli Zaretskii]

> Date: Tue, 7 May 2024 12:19:43 -0700
> From:  Matt Armstrong via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> I'm in a state where `dabbrev--last-buffer' is set to #<killed buffer>,
> which confuses the `dabbrev-completion' command as shown by the 
> backtrace below.
> 
> If I find a way to reproduce I will follow up with that.
> 
> I suspect that dabbrev.el should use `(buffer-live-p
> dabbrev--last-buffer)' instead of checking for non-nil.

Thanks, I added the buffer-live-p tests where appropriate in
dabbrev.el and installed the changes on the master branch.  I'm now
boldly closing this bug.