Subject: Re: bug#34525: replace-regexp missing some matches

From: Alan Mackenzie <[email-masked]>
Date: Sun, 24 Feb 2019 17:37:46 +0000

Hello, everybody.

On Thu, Feb 21, 2019 at 05:40:47 +0200, Eli Zaretskii wrote:
> > Date: Wed, 20 Feb 2019 21:30:03 +0000
> > Cc: [email-masked], [email-masked]
> > From: Alan Mackenzie <[email-masked]>

> > > Maybe look at this from a different angle: what do we have in C++ mode
> > > that isn't present in C mode, and could potentially affect this use
> > > case?

> > Well, the most obvious thing is the category text property whose value
> > is the symbol c-<-as-paren-syntax.  This symbol's plist is

> >     (risky-local-variable t syntax-table (4 . 62))

> > .  I can't think of anything else at the moment.

> If you remove that, does the problem go away?

I'm afraid I didn't get around to trying that.

But I've been busy with GDB.

The query-replace word ends up calling re-search-forward.
Fre_search_forward ends up calling re_search_2 (which is called
rpl_re_search_2 in gdb.  :-( ).

This calls re_match_2_internal, which scans through the compiled regexp,
"\<Bitmap\>".

Up till now, we have said yes to replace the first Bitmap with
SharedBitmap in query-replace.  Emacs is now seeking out the second
occurrence of Bitmap, which is on L69 of the OP's test file, and looks
like "Bitmap<", where the < has a syntax-table text property of (4 . 62),
an opening paren which matches ">".

re_natch_2_internal finds its way to case wordbeg: to handle the "\<" of
the regexp.  It invokes UPDATE_SYNTAX_TABLE (charpos) to get the syntax
for the "B" it has already found.

Sadly, UPDATE_SYNTAX_TABLE sets its internal structure gl_state not for
the current contents of position 1948, but the contents of 1948 before
the change at the top of the buffer (Bitmap -> SharedBitmap) was made.
So it picks up the syntax for the "<" rather than the "B".

Since this syntax, (4 . 62) is not the start of a word,
re_match_2_internal returns a failure result.

I think the glitch is in the text property interval handling code.  It is
as though after the replacement of Bitmap by SharedBitmap, the interval
starting positions have not been adjusted for the extra six characters.

I tested this theory by putting a space between the Bitmap and <, and
attempting a query-replace of Bitmap with 1234567Bitmap.  The error still
occurred.  In this buffer, the original replacement then appears to work.

-- 
Alan Mackenzie (Nuremberg, Germany).