bug#38179: 26.1; clone-buffer globally unsets locally void variable

bug#38179: 26.1; clone-buffer globally unsets locally void variable

[Braun Gábor]

Hi,

* Idea:

Let's create a test, which sets the default value of a variable but
undefines it locally in some buffer.  Then clone the buffer and check
that the default value of the variable is unchanged, and it is locally
undefined in both the original buffer and the clone.

* Recipe to reproduce the bug:

Create test.el with the following content
(there is no empty line in the file):

(ert-deftest clone-buffer-unbound-local-variable ()
  "Test `clone-buffer' on locally unbound variables."
  (let ((var (make-symbol "test")))
    (set-default var 1)
    (with-temp-buffer
      (makunbound (make-local-variable var))
      (let ((buffer (current-buffer))
            (clone (clone-buffer)))
        (unwind-protect
            (progn
              (should (eq (default-value var) 1))
              (should-not (boundp var))
              (with-current-buffer clone
                (should-not (boundp var))))
          (when (buffer-live-p clone)
            (kill-buffer clone)))))))

Run the test:

$ emacs -Q -batch -l ert -l test.el -f ert-run-tests-batch-and-exit

Running 1 tests (2019-11-12 15:55:39+0100)
Test clone-buffer-unbound-local-variable backtrace:
  signal(void-variable (test))
  apply(signal (void-variable (test)))
  (setq value-2 (apply fn-0 args-1))
  (unwind-protect (setq value-2 (apply fn-0 args-1)) (setq form-descri
  (if (unwind-protect (setq value-2 (apply fn-0 args-1)) (setq form-de
  (let (form-description-4) (if (unwind-protect (setq value-2 (apply f
  (let ((value-2 'ert-form-evaluation-aborted-3)) (let (form-descripti
  (let* ((fn-0 (function eq)) (args-1 (condition-case err (let ((signa
  (progn (let* ((fn-0 (function eq)) (args-1 (condition-case err (let
  (unwind-protect (progn (let* ((fn-0 (function eq)) (args-1 (conditio
  (let ((buffer (current-buffer)) (clone (clone-buffer))) (unwind-prot
  (progn (makunbound (make-local-variable var)) (let ((buffer (current
  (unwind-protect (progn (makunbound (make-local-variable var)) (let (
  (save-current-buffer (set-buffer temp-buffer) (unwind-protect (progn
  (let ((temp-buffer (generate-new-buffer " *temp*"))) (save-current-b
  (let ((var (make-symbol "test"))) (set-default var 1) (let ((temp-bu
  (lambda nil (let ((var (make-symbol "test"))) (set-default var 1) (l
  ert--run-test-internal(#s(ert--test-execution-info :test #s(ert-test
  ert-run-test(#s(ert-test :name clone-buffer-unbound-local-variable :
  ert-run-or-rerun-test(#s(ert--stats :selector t :tests [#s(ert-test
  ert-run-tests(t #f(compiled-function (event-type &rest event-args) #
  ert-run-tests-batch(nil)
  ert-run-tests-batch-and-exit()
  command-line-1(("-l" "ert" "-l" "test.el" "-f" "ert-run-tests-batch-
  command-line()
  normal-top-level()
Test clone-buffer-unbound-local-variable condition:
    (void-variable test)
   FAILED  1/1  clone-buffer-unbound-local-variable

Ran 1 tests, 0 results as expected, 1 unexpected (2019-11-12 15:55:40+0100)

1 unexpected results:
   FAILED  clone-buffer-unbound-local-variable


The test failed but it should pass.

* Patch to fix the problem

When copying buffer local variables, make even undefined variables
buffer local before undefining them:

--- lisp/simple.el
+++ lisp/simple.el
@@ -8535,7 +8535,7 @@
       (mapc (lambda (v)
 	      (condition-case ()	;in case var is read-only
 		  (if (symbolp v)
-		      (makunbound v)
+		      (makunbound (make-local-variable v))
 		    (set (make-local-variable (car v)) (cdr v)))
 		(error nil)))
 	    lvars)

Best wishes,

     Gábor


In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.4)
 of 2019-02-03, modified by Debian built on zam904
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description:	Debian GNU/Linux 10 (buster)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Ran 1 tests, 0 results were as expected, 1 unexpected
scroll-up-command: End of buffer

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
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 --libexecdir=/usr/lib --localstatedir=/var/lib
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bug#38179: 26.1; clone-buffer globally unsets locally void variable

[Noam Postavsky]

There is no way to make a "locally void" variable.

> Let's create a test, which sets the default value of a variable but
> undefines it locally in some buffer.

>     (with-temp-buffer
>       (makunbound (make-local-variable var))

That will unbind the global value and local values, not just the local
one.

bug#38179: 26.1; clone-buffer globally unsets locally void variable

[Braun Gábor]

Hi Noam,

> > (with-temp-buffer
> > (makunbound (make-local-variable var))
> 
> That will unbind the global value and local values, not just the local
> one.

My Emacs 26.1 does not seem to behave that way.
For example, evaluating the following form returns 2:

(with-temp-buffer
  (setq-default test 2)
  (makunbound (make-local-variable 'test))
  (default-value 'test))

Neither the info documentation nor the docstring of makunbound say how 
makunbound behaves for buffer-local variables, so my assumption was that 
like other Lisp forms it acts on the buffer-local value of local 
variables.

Best wishes,

	Gábor

bug#38179: 26.1; clone-buffer globally unsets locally void variable

[Noam Postavsky]

Braun Gábor <braungb88@gmail.com> writes:

>> > (with-temp-buffer
>> > (makunbound (make-local-variable var))
>> 
>> That will unbind the global value and local values, not just the local
>> one.
>
> My Emacs 26.1 does not seem to behave that way.
> For example, evaluating the following form returns 2:

Oh, you're right.  I was confused because describe-variable doesn't see
the global value when the local value is void.

> Neither the info documentation nor the docstring of makunbound say how 
> makunbound behaves for buffer-local variables, so my assumption was that 
> like other Lisp forms it acts on the buffer-local value of local 
> variables.

Yeah, I think the current behaviour is a bit strange, but changing it is
probably too risky with regards to backwards compatibility.

So I think your patch is correct.

bug#38179: 26.1; clone-buffer globally unsets locally void variable

[Lars Ingebrigtsen]

Noam Postavsky <npostavs@gmail.com> writes:

>> Neither the info documentation nor the docstring of makunbound say how 
>> makunbound behaves for buffer-local variables, so my assumption was that 
>> like other Lisp forms it acts on the buffer-local value of local 
>> variables.
>
> Yeah, I think the current behaviour is a bit strange, but changing it is
> probably too risky with regards to backwards compatibility.
>
> So I think your patch is correct.

OK; I've now applied the patch to Emacs 27.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no