'work-buffer--release' kill locals silently and reset the buffer (bug#81111)

This change helps ensure that work buffers are "clean" for reuse
by arbitrary callers.

* lisp/emacs-lisp/subr-x.el (work-buffer--release): Call
'kill-all-local-variables' with KILL-PERMANENT 'reset.
* src/buffer.c (Fkill_all_local_variables): If KILL-PERMANENT is
'permanent-local, kill all locals without prejudice.  If
KILL-PERMANENT is 'reset, also call 'reset_buffer'.
* test/lisp/emacs-lisp/subr-x-tests.el
(subr-x-with-work-buffer-locals-killed): New test.
* doc/lispref/variables.texi: Update 'kill-all-local-variables'
documentation.
* etc/NEWS: Announce the change to 'kill-all-local-variables'.

doc/lispref/variables.texi

@@ -1748,6 +1748,14 @@ local hook functions that have a non-@code{nil}
 killed, but if the optional @var{kill-permanent} argument is
 non-@code{nil}, even those variables will be killed.
 
+If @var{kill-permanent} is the symbol @code{permanent-local} the
+function @code{kill-all-local-variables} kills local variables and
+ignores any variable watchers.  If it is the symbol @code{reset},
+variable watchers are ignored and the buffer is reset as if the buffer
+was newly created.  Use the foregoing with caution: For example,
+@code{reset} sets buffer variables such as @code{default-directory} to
+@code{nil} and may result in unexpected behavior.
+
 This function also resets certain other information pertaining to the
 buffer: it sets the local keymap to @code{nil}, the syntax table to the
 value of @code{(standard-syntax-table)}, the case table to

etc/NEWS

@@ -137,6 +137,20 @@ To install the grammars, use 'M-x markdown-ts-mode-install-parsers'.
 
 * Lisp Changes in Emacs 32.1
 
++++
+** 'kill-all-local-variables' can kill locals silently and reset the buffer.
+This function's KILL-PERMANENT argument now accepts 'permanent-local'
+which kills all locals ignoring any variable watchers, and also 'reset'
+which does what 'permanent-local' does and also resets the buffer as if
+newly created.  Use these with caution to avoid unexpected behavior such
+as 'default-directory' being reset to nil.
+
+---
+** 'with-work-buffer' kills all locals silently and resets its buffers.
+This macro, when returning a buffer to its share buffer pool, now kills
+all buffer locals silently, ignoring any variable watchers, and resets
+the buffer as if newly created.
+
 +++
 ** The new function 'markers-in' returns the set of markers in a region.
 

lisp/emacs-lisp/subr-x.el

@@ -334,10 +334,10 @@ automatically killed, which means that in a such case
           (erase-buffer)
           (delete-all-overlays))
         (let (change-major-mode-hook)
-          ;; `kill-all-local-variables' does not kill permanent locals
-          ;; like `buffer-read-only'.
-          (setq buffer-read-only nil)
-          (kill-all-local-variables t))
+          ;; Ensure `kill-all-local-variables' kills *all* permanent locals
+          ;; rather than exempting any, and resets the buffer to pristine
+          ;; state.
+          (kill-all-local-variables 'reset))
         ;; Make the buffer available again.
         (push buffer work-buffer--list)))
   ;; If the maximum number of reusable work buffers is exceeded, kill

src/buffer.c

@@ -3031,15 +3031,33 @@ As a special exception, local variables whose names have a non-nil
 the optional KILL-PERMANENT argument is non-nil, clear out these local
 variables, too.
 
+If KILL-PERMANENT is the symbol `permanent-local', kill local variables
+and ignore variable watchers.  If KILL-PERMANENT is the symbol `reset',
+ignore variable watchers and reset the buffer as if newly created.  Use
+these with caution.  For example, `reset' sets buffer variables such as
+`default-directory' to nil and may result in unexpected behavior.
+
 The first thing this function does is run
 the normal hook `change-major-mode-hook'.  */)
   (Lisp_Object kill_permanent)
 {
   run_hook (Qchange_major_mode_hook);
 
-  /* Actually eliminate all local bindings of this buffer.  */
+  int permanent_too = 0;
+  if (!NILP (kill_permanent))
+    {
+      permanent_too = 2;
+      if (EQ (kill_permanent, Qpermanent_local))
+        permanent_too = 1;
+      else if (EQ (kill_permanent, Qreset))
+        {
+          permanent_too = 1;
+          reset_buffer (current_buffer);
+        }
+    }
 
-  reset_buffer_local_variables (current_buffer, !NILP (kill_permanent) ? 2 : 0);
+  /* Actually eliminate all local bindings of this buffer.  */
+  reset_buffer_local_variables (current_buffer, permanent_too);
 
   /* Force mode-line redisplay.  Useful here because all major mode
      commands call this function.  */

test/lisp/emacs-lisp/subr-x-tests.el

@@ -815,5 +815,19 @@
   (should (equal (string-truncate-left "band" 2) "...d"))
   (should (equal (string-truncate-left "longstring" 8) "...tring")))
 
+(ert-deftest subr-x-with-work-buffer-locals-killed ()
+  (let (b)
+    (with-work-buffer
+      (setq b (current-buffer))
+      (setq-local test-buffer-local 123)
+      (setq mark-active t)
+      (setq buffer-read-only t))
+    (with-work-buffer
+      ;; Sanity check `with-work-buffer' yields the same buffer.
+      (should (eq b (current-buffer)))
+      (should-not (buffer-local-boundp 'test-buffer-local (current-buffer)))
+      (should-not mark-active)
+      (should-not buffer-read-only))))
+
 (provide 'subr-x-tests)
 ;;; subr-x-tests.el ends here