bug#73232: [PATCH] Allow vc-diff to suggest a default revision in vc-dir

[Spencer Baugh]

Eli Zaretskii <eliz@gnu.org> writes:
>> Cc: Juri Linkov <juri@linkov.net>
>> Date: Fri, 13 Sep 2024 11:51:47 -0400
>> From:  Spencer Baugh via "Bug reports for GNU Emacs,
>>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>> 
>> -     ;; if it's a directory, don't supply any revision default
>> -     ((file-directory-p first)
>> -      nil)
>>       ;; if the file is not up-to-date, use working revision as older 
>> revision
>> -     ((not (vc-up-to-date-p first))
>> -      (setq rev1-default (vc-working-revision first)))
>> +     ((not (and (file-directory-p first) (vc-up-to-date-p first)))
>> +      (setq rev1-default (vc-working-revision first backend)))
>
> Doesn't this change the conditions under which we use
> vc-working-revision for regular files?  Did you perhaps mean
>
>    ((and (not (file-directory-p first)) (vc-up-to-date-p first))
>
> instead?

Oops, yes, fixed.

>From f87b92ce5775d0558b7d9dd502687d1f460bcd2a Mon Sep 17 00:00:00 2001
From: Spencer Baugh <sbaugh@janestreet.com>
Date: Fri, 13 Sep 2024 11:34:55 -0400
Subject: [PATCH] Allow vc-diff to suggest a default revision in vc-dir

C-u M-x vc-root-diff will prompt for the old revision to use for the
diff.  The prompt will have a default calculated by
vc-diff-build-argument-list-internal.  The default is either the
working revision of the current fileset or the revision before that.

vc-diff-build-argument-list-internal contained a check (added in
c0d66cb21bac57f5ec0378e8a04aac8f35c3eb5c) which explicitly avoided
setting a default if the current fileset was a single directory.  This
check was added in 1997 when vc only worked for single files.

This prevents a backend from choosing to return a non-nil value from
'working-revision when passed a directory.  (The vc-hg and vc-git
backends, at least, will do this)

Allow this by moving the file-directory-p check, so that we try
calling 'working-revision when the fileset is a single directory.  The
call is in inside ignore-errors, so if a backend errors when passed a
directory, we'll just get no default, as before.  (Most backends will
just return nil for a directory, rather than erroring)

Also, while we're here, explicitly pass the backend to
vc-working-revision rather than having vc-working-revision recompute
it.

Concretely this has the effect that for the vc-git and vc-hg backends,
running C-u M-x vc-root-diff in vc-dir will have the same behavior as
running C-u M-x vc-root-diff in a clean file: The "Previous revision:"
prompt's default will be the revision before HEAD.

* lisp/vc/vc.el (vc-diff-build-argument-list-internal): Move
file-directory-p check. (bug#73232)
---
 lisp/vc/vc.el | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 597a1622f5a..7b1133565f9 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -2074,17 +2074,14 @@ vc-diff-build-argument-list-internal
      ;; filesets, but not yet.
      ((/= (length files) 1)
       nil)
-     ;; if it's a directory, don't supply any revision default
-     ((file-directory-p first)
-      nil)
      ;; if the file is not up-to-date, use working revision as older revision
-     ((not (vc-up-to-date-p first))
-      (setq rev1-default (vc-working-revision first)))
+     ((and (not (file-directory-p first)) (not (vc-up-to-date-p first)))
+      (setq rev1-default (vc-working-revision first backend)))
      ;; if the file is not locked, use last revision and current source as 
defaults
      (t
       (setq rev1-default (ignore-errors ;If `previous-revision' doesn't work.
                            (vc-call-backend backend 'previous-revision first
-                                            (vc-working-revision first))))
+                                            (vc-working-revision first 
backend))))
       (when (string= rev1-default "") (setq rev1-default nil))))
     ;; construct argument list
     (let* ((rev1-prompt (format-prompt "Older revision" rev1-default))
-- 
2.39.3