bug#30171: 27.0.50; {add-to,remove-from}-invisibility-spec don't treat t specially

bug#30171: 27.0.50; {add-to,remove-from}-invisibility-spec don't treat t specially

[Philipp Stephani]

(with-temp-buffer
  (insert (propertize " " 'invisible 'foo))
  (remove-from-invisibility-spec 'what)
  (invisible-p 1))

is nil.  Commenting out the `remove-from-invisibility-spec' form causes
it to return t.  I'd expect t with `remove-from-invisibility-spec', but
that function and `add-to-invisibility-spec' always convert a
buffer-invisibility-spec of t to (t), which is not the same.


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bug#30171: 27.0.50; {add-to,remove-from}-invisibility-spec don't treat t specially

[Lars Ingebrigtsen]

Philipp Stephani <p.stephani2@gmail.com> writes:

> (with-temp-buffer
>   (insert (propertize " " 'invisible 'foo))
>   (remove-from-invisibility-spec 'what)
>   (invisible-p 1))
>
> is nil.  Commenting out the `remove-from-invisibility-spec' form causes
> it to return t.  I'd expect t with `remove-from-invisibility-spec', but
> that function and `add-to-invisibility-spec' always convert a
> buffer-invisibility-spec of t to (t), which is not the same.

Yes, that's extremely confusing, but poking around, it seems like the
idea is that if you've altered the `buffer-invisibility-spec' in any
way (i.e., made it into a list), then only the properties that are in
the list are the ones that are supposed to be invisible.

In that light, the behaviour here makes sense.

But I'm not sure that's really what's intended here.  Perhaps somebody
who was involved with implementing this could shed some light?

In any case, perhaps calling `remove-from-invisibility-spec' that's not
in the spec anyway shouldn't alter it?  It's certainly surprising.

This definition seems odd to me:

(defun remove-from-invisibility-spec (element)
  "Remove ELEMENT from `buffer-invisibility-spec'."
  (setq buffer-invisibility-spec
        (if (consp buffer-invisibility-spec)
	    (delete element buffer-invisibility-spec)
          (list t))))


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

bug#30171: 27.0.50; {add-to,remove-from}-invisibility-spec don't treat t specially

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Tue, 17 Apr 2018 22:39:29 +0200
> Cc: 30171@debbugs.gnu.org
> 
> In any case, perhaps calling `remove-from-invisibility-spec' that's not
> in the spec anyway shouldn't alter it?  It's certainly surprising.

If the only problem is the surprising behavior, I'd prefer to document
it rather than potentially open a can of worms.

bug#30171: 27.0.50; {add-to,remove-from}-invisibility-spec don't treat t specially

[Lars Ingebrigtsen]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Lars Ingebrigtsen <larsi@gnus.org>
>> Date: Tue, 17 Apr 2018 22:39:29 +0200
>> Cc: 30171@debbugs.gnu.org
>> 
>> In any case, perhaps calling `remove-from-invisibility-spec' that's not
>> in the spec anyway shouldn't alter it?  It's certainly surprising.
>
> If the only problem is the surprising behavior, I'd prefer to document
> it rather than potentially open a can of worms.

The change below is the cause of this odd feature set, and I wonder
whether Stefan (who made the change) meant for it to do what it does.
The relevant bit commit message seems to be "Handle the t case".

Stefan, the issue is that t and `(t)' mean wildly differing things: `t'
means "hide everything that has an invisibility spec" and `(t)' means
"hide the things that has an invisibility spec `eq' to `t'."

So if the spec was t, and you remove `foo', you end up with `(t)', which
means that if you had invisible text that was `eq' to `bar', that
suddenly becomes visible.

If you understand what I mean.  :-)

diff --git a/lisp/subr.el b/lisp/subr.el
index 5d40aaae41..535fa2d3d0 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -4066,9 +4066,10 @@ add-to-invisibility-spec
 
 (defun remove-from-invisibility-spec (element)
   "Remove ELEMENT from `buffer-invisibility-spec'."
-  (if (consp buffer-invisibility-spec)
-      (setq buffer-invisibility-spec
-	    (delete element buffer-invisibility-spec))))
+  (setq buffer-invisibility-spec
+        (if (consp buffer-invisibility-spec)
+	    (delete element buffer-invisibility-spec)
+          (list t))))

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

bug#30171: 27.0.50; {add-to,remove-from}-invisibility-spec don't treat t specially

[Stefan Monnier]

> The change below is the cause of this odd feature set, and I wonder
> whether Stefan (who made the change) meant for it to do what it does.

The commit message refers to bug#20468 where the discussion indicates
that it seems to be on purpose.

One way to look at it is that the patch made

    (remove-from-invisibility-spec FOO)

give the same result as

    (add-to-invisibility-spec FOO)
    (remove-from-invisibility-spec FOO)

I guess the fundamental problem here is that the special behavior for
the value `t` of buffer-invisibility-spec is just not reflected in
(add-to|remove-from)-invisibility-spec.

So if you rely on the current value being `t` and someone comes along
and just does (add-to-invisibility-spec (make-symbol "unused")),
suddenly your invisible stuff becomes visible.

IOW you just can't rely on the special `t` behavior: if you want your
thing to be invisible, you need to either use the special `t` value of
the `invisible` property (which is invisible regardless of
buffer-invisibility-spec), or you need to add your value via
add-to-invisibility-spec.


        Stefan

bug#30171: 27.0.50; {add-to,remove-from}-invisibility-spec don't treat t specially

[Lars Ingebrigtsen]

Stefan Monnier <monnier@IRO.UMontreal.CA> writes:

> IOW you just can't rely on the special `t` behavior: if you want your
> thing to be invisible, you need to either use the special `t` value of
> the `invisible` property (which is invisible regardless of
> buffer-invisibility-spec), or you need to add your value via
> add-to-invisibility-spec.

I see.  Makes sense.  I'll clarify this in the doc strings of those
functions, then.

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

bug#30171: 27.0.50; {add-to,remove-from}-invisibility-spec don't treat t specially

[Lars Ingebrigtsen]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Stefan Monnier <monnier@IRO.UMontreal.CA> writes:
>
>> IOW you just can't rely on the special `t` behavior: if you want your
>> thing to be invisible, you need to either use the special `t` value of
>> the `invisible` property (which is invisible regardless of
>> buffer-invisibility-spec), or you need to add your value via
>> add-to-invisibility-spec.
>
> I see.  Makes sense.  I'll clarify this in the doc strings of those
> functions, then.

I've now done so and am closing this bug report.

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