GNU bug report logs - #33414
27.0.50; inhibit-changing-match-data can be t in syntax-propertize functions, breaking backtrace and looking-at

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Package: emacs; Reported by: Pip Cet <pipcet@HIDDEN>; dated Sat, 17 Nov 2018 13:31:02 UTC; Maintainer for emacs is bug-gnu-emacs@HIDDEN.

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From: Eli Zaretskii <eliz@HIDDEN>
To: Pip Cet <pipcet@HIDDEN>
In-reply-to: <CAOqdjBdLoibvn4MPHndr5hoZ_oZOPPecDpei71s2PvegGkeCVA@HIDDEN>
 (message from Pip Cet on Sat, 17 Nov 2018 14:14:04 +0000)
Subject: Re: bug#33414: 27.0.50; inhibit-changing-match-data can be t in
 syntax-propertize functions, breaking backtrace and looking-at
References: <CAOqdjBemRE0Vpe_KDG3iZ+dMgoB-BZYnkxjA5Yoe-HW7vttpdg@HIDDEN>
 <83lg5ru88i.fsf@HIDDEN>
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> From: Pip Cet <pipcet@HIDDEN>
> Date: Sat, 17 Nov 2018 14:14:04 +0000
> Cc: 33414 <at> debbugs.gnu.org
> 
> > > (let ((inhibit-changing-match-data t)) (backtrace))
> > >
> > > into the *scratch* buffer and evaluate with C-M-x.)
> >
> > This works on the release branch, so I think it's a regression that
> > needs to be fixed.
> 
> If I'm testing correctly,
> 
> (let ((inhibit-changing-match-data t)) (debug))
> 
> fails on both branches

You are right, not sure what I did wrong when trying it a few moments
ago.




Information forwarded to bug-gnu-emacs@HIDDEN:
bug#33414; Package emacs. Full text available.

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From: Pip Cet <pipcet@HIDDEN>
Date: Sat, 17 Nov 2018 14:14:04 +0000
Message-ID: <CAOqdjBdLoibvn4MPHndr5hoZ_oZOPPecDpei71s2PvegGkeCVA@HIDDEN>
Subject: Re: bug#33414: 27.0.50; inhibit-changing-match-data can be t in
 syntax-propertize functions, breaking backtrace and looking-at
To: eliz@HIDDEN
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On Sat, Nov 17, 2018 at 1:47 PM Eli Zaretskii <eliz@HIDDEN> wrote:
>
> > From: Pip Cet <pipcet@HIDDEN>
> > Date: Sat, 17 Nov 2018 13:30:04 +0000
> >
> > This was a bit tricky to debug because (backtrace) doesn't work when
> > inhibit-changing-match-data is t.  (In emacs -Q, enter
> >
> > (let ((inhibit-changing-match-data t)) (backtrace))
> >
> > into the *scratch* buffer and evaluate with C-M-x.)
>
> This works on the release branch, so I think it's a regression that
> needs to be fixed.

If I'm testing correctly,

(let ((inhibit-changing-match-data t)) (debug))

fails on both branches, which is also bad; I think that means the
issue is present in some form on both branches.

Maybe `save-match-data' should save/restore
`inhibit-changing-match-data' along with the actual match data?




Information forwarded to bug-gnu-emacs@HIDDEN:
bug#33414; Package emacs. Full text available.

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 (message from Pip Cet on Sat, 17 Nov 2018 13:30:04 +0000)
Subject: Re: bug#33414: 27.0.50;
 inhibit-changing-match-data can be t in syntax-propertize functions,
 breaking backtrace and looking-at
References: <CAOqdjBemRE0Vpe_KDG3iZ+dMgoB-BZYnkxjA5Yoe-HW7vttpdg@HIDDEN>
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> From: Pip Cet <pipcet@HIDDEN>
> Date: Sat, 17 Nov 2018 13:30:04 +0000
> 
> This was a bit tricky to debug because (backtrace) doesn't work when
> inhibit-changing-match-data is t.  (In emacs -Q, enter
> 
> (let ((inhibit-changing-match-data t)) (backtrace))
> 
> into the *scratch* buffer and evaluate with C-M-x.)

This works on the release branch, so I think it's a regression that
needs to be fixed.

Thanks.




Information forwarded to bug-gnu-emacs@HIDDEN:
bug#33414; Package emacs. Full text available.

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Subject: 27.0.50; inhibit-changing-match-data can be t in syntax-propertize
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I'm debugging an issue in a custom mode that happened because my
syntax-propertize function, which is complex, assumed it could use the
match data and looking-at. However, the function itself was called
from within looking-at-p, via a regexp generated by (regexp-opt "..."
'symbols), so inhibit-changing-match-data was still bound to t,
yielding unexpected results.

This was a bit tricky to debug because (backtrace) doesn't work when
inhibit-changing-match-data is t.  (In emacs -Q, enter

(let ((inhibit-changing-match-data t)) (backtrace))

into the *scratch* buffer and evaluate with C-M-x.)

At the very least, we should document these unfortunate limitations,
though I think it would be better to remove them.




Acknowledgement sent to Pip Cet <pipcet@HIDDEN>:
New bug report received and forwarded. Copy sent to bug-gnu-emacs@HIDDEN. Full text available.
Report forwarded to bug-gnu-emacs@HIDDEN:
bug#33414; Package emacs. Full text available.
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