GNU bug report logs - #73801
31.0.50; project-try-vc sometimes set wrong cache project-vc-extra-root-markers

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Dmitry Gutov <dmitry <at> gutov.dev>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#73801: closed (31.0.50; project-try-vc sometimes set wrong
 cache project-vc-extra-root-markers)
Date: Fri, 01 Nov 2024 00:50:02 +0000

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Your message dated Fri, 1 Nov 2024 02:49:24 +0200
with message-id <f7957dc7-3ded-433d-a0d9-27f20235b461 <at> gutov.dev>
and subject line Re: bug#73801: 31.0.50; project-try-vc sometimes set wrong cache project-vc-extra-root-markers
has caused the debbugs.gnu.org bug report #73801,
regarding 31.0.50; project-try-vc sometimes set wrong cache project-vc-extra-root-markers
to be marked as done.

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From: Zhengyi Fu <i <at> fuzy.me>
To: bug-gnu-emacs <at> gnu.org
Subject: 31.0.50; project-try-vc sometimes set wrong cache
 project-vc-extra-root-markers
Date: Mon, 14 Oct 2024 14:29:48 +0800

Consider the following project:

   test-project/
   ├── .git/
   └── subproject/
       ├── marker
       └── subdir/

If `project-vc-extra-root-markers' is set to `("marker")' and
`project-try-vc' is invoked with `test-project/subproject/subdir', it will set the `project-vc'
VC property of the `test-project/subproject' directory to
`(".../test-project" project-vc nil)', so if later `project-try-vc' is
invoked with that directory, it will return a wrong result.

This is because project-vc tries to detect the VC backend by invoking
`project-try-vc' on the subproject while let binding
`project-vc-extra-root-markers' to nil and the result is cached.

The following patch can fix the problem:

--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -595,7 +595,7 @@ project-try-vc
             (let* ((project-vc-extra-root-markers nil)
                    ;; Avoid submodules scan.
                    (enable-dir-local-variables nil)
-                   (parent (project-try-vc root)))
+                   (parent (project-try-vc dir)))
               (and parent (setq backend (nth 1 parent)))))
           (setq project (list 'vc backend root))
           ;; FIXME: Cache for a shorter time.

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From: Dmitry Gutov <dmitry <at> gutov.dev>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: i <at> fuzy.me, 73801-done <at> debbugs.gnu.org
Subject: Re: bug#73801: 31.0.50; project-try-vc sometimes set wrong cache
 project-vc-extra-root-markers
Date: Fri, 1 Nov 2024 02:49:24 +0200

Version: 30.1

On 31/10/2024 12:06, Eli Zaretskii wrote:
>>> How do we know that catering to this corner case will not screw
>>> other corner cases?
>> Difficult to guarantee that 100%, but this specific case seems important
>> enough, while at the same time we can infer that the change won't affect
>> the majority scenario because the code is guarded by these conditions:
>>
>>       (when root
>>         (when (not backend)
>>            ...
> FWIW, I have a bad feeling about this, but if you are confident, feel
> free to backport.

Thanks, I've cherry-picked the longer (but more transparent) fix and a 
new regression test as well, to the release branch.

And thanks to Zhengyi Fu for trying it out.

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