bug#73771: 30.0.91; etags generates broken TAGS file for multi-line regex match

[Eli Zaretskii <eliz@gnu.org>]

> From: Francesco Potortì <pot@gnu.org>
> Date: Sun, 13 Oct 2024 12:48:40 +0200
> Cc: 73771@debbugs.gnu.org,
> 	Morgan Willcock <morgan@ice9.digital>
> 
> >This is because etags always records one extra character with the
> >regexp match, which is harmless, unless that extra character is a
> >newline.  The patch below fixes it:
> >
> >diff --git a/lib-src/etags.c b/lib-src/etags.c
> >index a822a82..848d8ea 100644
> >--- a/lib-src/etags.c
> >+++ b/lib-src/etags.c
> >@@ -7420,7 +7420,7 @@ regex_tag_multiline (void)
> > 
> > 	      /* Force explicit tag name, if a name is there. */
> > 	      pfnote (name, true, buffer + linecharno,
> >-		      charno - linecharno + 1, lineno, linecharno);
> >+		      charno - linecharno, lineno, linecharno);
> > 
> > 	      if (debug)
> > 		fprintf (stderr, "%s on %s:%"PRIdMAX": %s\n",
> >
> >Francesco, why does the code add one more character there?
> 
> While I can't remember the reason, I am sure it was not done by chance.
> 
> Sure, I may have been wrong at that time, or maybe that reason is now obsolete.  To check, one should run the etags regression test and check the results.

I already did.  The only differences I see are in that 1 extra
character, which is now no longer written to TAGS in the tests which
use multi-line regexps.

Note that the change is only in this single call to pfnote, from the
code that is part of generating tags from multi-line regexp matches.

> Off the top of my head, it may have to do with implicitly named tags that are initial substring of a different tag, but this may be a fake reconstruction of my mind,

The tags we produce from regexp matches are norally explicitly named,
AFAICT:

	      /* Match occurred.  Construct a tag. */
	      while (charno < rp->regs.end[0])
		if (buffer[charno++] == '\n')
		  lineno++, linecharno = charno;
	      name = rp->name;
	      if (name[0] == '\0')
		name = NULL;
	      else /* make a named tag */
		name = substitute (buffer, rp->name, &rp->regs);

	      /* Force explicit tag name, if a name is there. */
	      pfnote (name, true, buffer + linecharno,
		      charno - linecharno, lineno, linecharno);

But if you say that we should add that offset of 1 when 'name' is
NULL, I'm okay with doing that.