GNU bug report logs - #10190
eventp can give incorrect results (subr.el), Emacs 23 and 24

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Christopher Genovese <genovese.cr <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: eventp can give incorrect results (subr.el), Emacs 23 and 24
Date: Fri, 2 Dec 2011 00:55:11 -0500

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This is not a major problem, but it did come up for me in a real program.
In Emacs 23 and 24, the function eventp in subr.el can give incorrect
results
for symbolic events, depending on the timing of the call.  This seems to
occurs because event symbol elements (mask, modifiers, basic type) are
stored in a plist
associated with symbolic events, but the list is set in the function
internal-event-symbol-parse-modifiers, which is only called in the function
event-modifiers not in the inline function eventp.  Hence, code that tests
for
an event before checking the modifiers can give the wrong results, e.g.,

           (cond  ((integerp c) ...)
                       ....
                      ((eventp )  (do-something-with (event-modfiers c)))
                      (t
                          (not-what-we-want-with c)))

will fail when c is a symbolic event that has not been parsed before.

Consider the following sequence, which illustrates the error in a new Emacs
24
session with -Q (on Mac OS X 10.5.8). Although the event M-S-f5 is a rather
obscure one, it is the event used to illustrate the event- fucntions in the
Elisp Info documentation.

;; #1
(eventp 'M-S-f5)
> nil
(symbol-plist 'M-S-f5)
> nil

;; #2
(event-modifiers 'M-S-f5)
> (meta shift)

;; #3
(eventp 'M-S-f5)
> (f5 meta shift)
(symbol-plist 'M-S-f5)
> (event-symbol-element-mask (f5 167772160) event-symbol-elements (f5 meta
shift))

A similar example occurs with, say, M-s-z, even if M-s-z has been defined
in the global map.

This problem holds with event-basic-type too because it also just checks
the symbol plist.
In #1, for instance, (event-basic-type 'M-S-f5) would be nil, but not in
#3.
Another side of the problem(?) is that in #1  calling (event-modifier
'foobar) would make
subsequent (eventp 'foobar) calls true.

Neither of these seems like desirable behaviors.

Not a really big deal, but I thought I'd pass it along.

Thanks,

   Chris

P.S. I haven't had a chance to look over the C code on this too closely,
but the function
parse_modifiers, which is doing the work in
internal-event-symbol-parse-modifiers,
is called at some points when reading key sequences. Caching the events when
they are read would seem to be a good idea, although it is not happening in
the
M-s-z example.

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