bug#49066: 26.3; Segmentation fault on specific utf8 string

[Robert Pluim <rpluim@gmail.com>]

>>>>> On Sat, 03 Jul 2021 11:05:05 +0900, handa <handa@gnu.org> said:

    handa> In article <83bl7qp52q.fsf@gnu.org>, Eli Zaretskii <eliz@gnu.org> writes:
    >> > With the patch it still crashes for me in emacs-master with harfbuzz disabled:

    >> Too bad.
    >> Kenichi, any suggestions?

    handa> I checked the code again, and found that it was a fault of m17n-lib
    handa> which was not robust enough to handle an OTF table that is different
    handa> from what the library expects.

    handa> Here is a revised patch to handle such a case.  Could you please try it?

Thanks, that fixes the crash, and results in the ZWNJ being composed.

    >> Btw, I think there's a bug in those patterns: ZWJ and ZWNJ shouldn't
    >> compose unless they are followed by a character.  See section 12.2 in
    >> the Unicode Standard.

    handa> Even if they should not be composed with, we must include them in the
    handa> string to shape because their existence may change the glyph of the
    handa> previous character.  A shaper (m17n-lib or harfbuzz) must return a glyph
    handa> string that has an independent grapheme cluster for the last ZWJ/ZWNJ.

    handa> At the time of developing m17n-lib, the above rule was not clear.  To
    handa> conform to that rule, please to put the attached BNG2-OTF.flt under the
    handa> directory ~/.m17n.d/.

I believe you, but I did not test this specifically.

Robert
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