bug#46701: [PATCH] small cleanups related to `unlock-buffer'

[Matt Armstrong]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Matt Armstrong <matt@rfc20.org>
>> Date: Sun, 21 Feb 2021 20:18:44 -0800
>> 
>> Subject: [PATCH 1/4] Remove unecessary (unlock-buffer) calls.
>> 
>> * lisp/type-break.el (type-break-mode): Remove an (unlock-buffer) call
>> implied by (set-buffer-modified nil).
>> * lisp/simple.el (primitive-undo): ditto.
>
> My reading of the code is that the above is true only if
> inhibit-modification-hooks is nil.  Otherwise, these calls are not
> no-ops.

Thanks, good catch.  I think the change to type-break.el is probably
fine, but I will drop it for now.  In simple.el I just removed the
unecessary fboundp check (new patch attached).


>> --- a/lisp/files.el
>> +++ b/lisp/files.el
>> @@ -6234,11 +6234,8 @@ revert-buffer-insert-file-contents--default-function
>>               "Cannot revert unreadable file %s")
>>             file-name))
>>     (t
>> -    ;; Bind buffer-file-name to nil
>> -    ;; so that we don't try to lock the file.
>> -    (let ((buffer-file-name nil))
>> -      (or auto-save-p
>> -          (unlock-buffer)))
>> +    (unless auto-save-p
>> +      (unlock-buffer))
>
> And here, I think we just forgot to update the Lisp code when
> unlock-buffer started to look at buffer-file-truename instead of
> buffer-file-name.  But otherwise, I see no reason why we should remove
> the call to unlock-buffer; what did I miss?

This is some very old code.

When originally written the `unlock-buffer' was a nop because it was
well before file-buffer-truename existed -- the let bind disabled it.
The let bind was intended to prevent locking the buffer by
`erase-buffer'.  See Roland's commit below, which is the first commit
for this file that we've got.


b4da00e92a0 (Roland McGrath      1991-07-19 1804)

;; Bind buffer-file-name to nil
;; so that we don't try to lock the file.
(let ((buffer-file-name nil))
  (or auto-save-p
      (unlock-buffer))
  (erase-buffer))
(insert-file-contents file-name (not auto-save-p))))


A few years later Richard moved the erase logic to insert-file-contents,
but left the no-op call to unlock-buffer.  Note that the comment no
longer makes sense.

f9456b0a5b5 (Richard M. Stallman 1994-02-17 2020)

;; Bind buffer-file-name to nil
;; so that we don't try to lock the file.
(let ((buffer-file-name nil))
  (or auto-save-p
      (unlock-buffer))
(insert-file-contents file-name (not auto-save-p)
                      nil nil t)))


insert-file-contents has logic to unlock the buffer. I must admit that I
don't understand how that logic works.

When unlock-buffer switched to using buffer-file-tempfile the
unlock-buffer call here became active for the first time, probably by
accident?  Removing it now is a possible behavior change, but it
restores the original behavior.

I did a manual test.  I edited a file, then changed the file outside
emacs, then ran revert-buffer.  The file's lock file was still removed,
even with the patch below applied.

>From 3b569a6b9139d2b350745bebc64db506728cf994 Mon Sep 17 00:00:00 2001
From: Matt Armstrong <matt@rfc20.org>
Date: Mon, 22 Feb 2021 16:40:05 -0800
Subject: [PATCH 1/2] Remove unecessary unlock-buffer call.

* lisp/files.el (revert-buffer-insert-file-contents--default-function):
Remove vestigial call to `unlock-buffer'.
---
 lisp/files.el | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/lisp/files.el b/lisp/files.el
index 68e883513c..b5d92cd841 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -6234,11 +6234,6 @@ revert-buffer-insert-file-contents--default-function
              "Cannot revert unreadable file %s")
            file-name))
    (t
-    ;; Bind buffer-file-name to nil
-    ;; so that we don't try to lock the file.
-    (let ((buffer-file-name nil))
-      (or auto-save-p
-          (unlock-buffer)))
     (widen)
     (let ((coding-system-for-read
            ;; Auto-saved file should be read by Emacs's
-- 
2.30.0

>From dc9cc451e0ab6a84d839f3ed9d1b552da3c43373 Mon Sep 17 00:00:00 2001
From: Matt Armstrong <matt@rfc20.org>
Date: Mon, 22 Feb 2021 16:41:44 -0800
Subject: [PATCH 2/2] Assume unlock-buffer is always bound.

* lisp/simple.el (primitive-undo): Assume unlock-buffer is always
bound.
---
 lisp/simple.el | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/lisp/simple.el b/lisp/simple.el
index 1dfc3374ad..c4062d97cc 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -3043,8 +3043,7 @@ primitive-undo
                      (and (consp time)
                           (equal (list (car time) (cdr time))
                                  (visited-file-modtime))))
-             (when (fboundp 'unlock-buffer)
-               (unlock-buffer))
+             (unlock-buffer)
              (set-buffer-modified-p nil)))
           ;; Element (nil PROP VAL BEG . END) is property change.
           (`(nil . ,(or `(,prop ,val ,beg . ,end) pcase--dontcare))
-- 
2.30.0