GNU bug report logs - #77244
30.1; compile.el: `compilation-filter-hook` applied to wrong region with carriage motion

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Erik Arvstedt <erik.arvstedt <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 30.1; compile.el: `compilation-filter-hook` applied to wrong region
 with carriage motion
Date: Mon, 24 Mar 2025 21:34:25 +0100

See here for a markdown-rendered version of this message:
https://gist.github.com/erikarvstedt/a0d838ca38a601da1b04985b01189dc6

When a carriage motion (`comint-carriage-motion`) happens before the call to
`compilation-filter-hook`, the value of `compilation-filter-start` can
 sometimes have an obsolete, too large value,
causing the hook not to filter all of the new input.

Affected Emacs versions:
- 31.0.50
- 30.1

I haven't tested other versions.

Example
=======
Let the following be the last line of the compilation buffer, before the call to
`comint-carriage-motion`:
```
aabb\rcccc
  ^
  compilation-filter-start, (process-mark proc) point to this location

```

`aa` is the existing content of the line, before the current call to
`compilation-filter`. The rest of the line, beginning with `bb...` was
just inserted
by `compilation-filter`.

When `comint-carriage-motion` is called, it removes all chars from the carriage
return (`\r`) to the beginning of the line, but leaves variable
`compilation-filter-start` unchanged:
```
cccc
  ^
  compilation-filter-start

```
`compilation-filter-start` now still points to the old location, but
to correctly
point to the start of new input to filter, it should instead point to
the beginning
of the line.
Because `compilation-filter-start` marks the start of new input for
`compilation-filter-hook`, the hook subsequently doesn't filter all of
the new input.

Reproduce
========
```elisp
(add-hook 'compilation-filter-hook 'ansi-color-compilation-filter)
(compilation-start "bash -c \"echo -n aaa; sleep 0.1; echo '\x0d\x1b[Kbbb'\"")
```

The compilation Bash script outputs the common sequence of a carriage
return (`\x0d`)
followed by an "Erase the line" ANSI escape sequence (`\x1b[K`).
This is often used by terminal apps to clear the last output line.

Compilation output:
```
^[[Kbbb
```
Expected output:
```
bbb
```
Here, the ANSI escape sequence is correctly deleted.
The sequence isn't deleted because the value of
`compilation-filter-start` is too
large, causing `ansi-color-compilation-filter` to not process the whole output.

Workaround
==========
Implemented here:
https://github.com/erikarvstedt/emacs-compilation-carriage-motion-workaround/blob/master/comint-carriage-motion-fixed.el
Diff: https://github.com/erikarvstedt/emacs-compilation-carriage-motion-workaround/commit/450afa4594a260b47143859546f78689f2ce629a
This fixes the bug for me, but I haven't checked it for correctness in
all use cases.

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