bug#41002: Undo breaks inhibit-read-only text property

[Filipp Gunbin <fgunbin@fastmail.fm>]

> On 5/2/20 8:33 PM, Filipp Gunbin wrote:
[...]
>> Cannot reproduce on current master.  Can you reproduce it with emacs -Q?
>> Do you have any custom undo functionality? (like a package)

I see I wasn't trying right, it's actually reproducible on master too.

I think this comes from verify_interval_modification (textprop.c).  For
each interval, we check INTERVAL_WRITABLE_P, and then:

commit d1b04a9e7ada7070dbd84bb450411c1f169b3739
Author: Lars Magne Ingebrigtsen <larsi@gnus.org>
Date:   Sun Nov 16 23:36:58 2014 +0100
[...]
diff --git a/src/textprop.c b/src/textprop.c
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -2301,0 +2301,4 @@
+	  if (i->position + LENGTH (i) < end
+	      && (!NILP (BVAR (current_buffer, read_only))
+		  && NILP (Vinhibit_read_only)))
+	    xsignal1 (Qbuffer_read_only, Fcurrent_buffer ());

So if there happens to be an (writeable) interval ending before "end"
(the end of the region we're killing with C-w), we will signal that the
buffer is read-only.  This may be read as "if we're going over the
boundary of an interval, then check the _buffer_ read-onliness".  Maybe
Lars, as the author of this code, could comment on this.

Filipp