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Assertion failure in set_iterator_to_next

From: Stefan Monnier
Subject: Assertion failure in set_iterator_to_next
Date: Sat, 10 Apr 2010 20:52:45 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.0.50 (gnu/linux)

While playing with my nhexl-mode.el I bumped into some assertion failure
in the redisplay code.  More specifically the assertion

          xassert (IT_BYTEPOS (*it) == CHAR_TO_BYTE (IT_CHARPOS (*it)));

at line 6183.
I think the problem is that the non-bidi code above that line uses
it->len assuming it holds the length of the char at point (well, at
IT_CHARPOS(it)), whereas IIUC it holds the length of "the" glyph.
In my case it->c holds sometimes the "lf symbol" char (u240a) which comes
from a display property and it->len is then 3, which is obviously not
the length of the char at IT_CHARPOS since the buffer I was looking at
only happens to contain ASCII chars.

I use the quick patch below, which seems to fix the problem, but I don't
know enough of the redisplay to know whether that's the right fix, or
whether the right fix is to make sure it->len holds some other value.


--- src/xdisp.c 2010-04-06 16:00:42 +0000
+++ src/xdisp.c 2010-04-10 01:35:11 +0000
@@ -6151,7 +6151,8 @@
          if (!it->bidi_p)
-             IT_BYTEPOS (*it) += it->len;
+             IT_BYTEPOS (*it)
+               += BYTES_BY_CHAR_HEAD (*BUF_BYTE_ADDRESS (XBUFFER (it->object), 
IT_BYTEPOS (*it)));
              IT_CHARPOS (*it) += 1;

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