bug#17817: 24.3.91; Assertion failure in bidi.c (Cygwin-w32 build)

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 20 Jun 2014 14:42:05 +0100
> From: Ken Brown <kbrown@cornell.edu>
> 
> I just got the following assertion failure:
> 
>    bidi.c:329: Emacs fatal error: assertion failed: UNKNOWN_BT <= type 
> && type <= NEUTRAL_ON

Is this the same 64-bit Cygwin-w32 build that was reported lately to
produce nonsensical backtraces?

> #0  terminate_due_to_signal (sig=6, backtrace_limit=2147483647) at emacs.c:351
> No locals.
> #1  0x00000001005ba95d in die (
>     msg=0x100a2e538 <DEFAULT_REHASH_SIZE+64> "UNKNOWN_BT <= type && type <= NEUTRAL_ON", file=0x100a2e530 <DEFAULT_REHASH_SIZE+56> "bidi.c", line=329)
>     at alloc.c:6826
> No locals.
> #2  0x00000001004fb4fe in bidi_check_type (type=STRONG_L) at bidi.c:329
> No locals.
> #3  0x0000000100500630 in bidi_level_of_next_char (bidi_it=0x2267d8)
>     at bidi.c:2430
>         type = STRONG_L
>         level = 0
>         prev_level = 0
>         next_for_neutral = {
>           bytepos = 0, 
>           charpos = -1, 
>           type = UNKNOWN_BT, 
>           type_after_w1 = UNKNOWN_BT, 
>           orig_type = UNKNOWN_BT
>         }
>         next_char_pos = 1

This makes no sense at all: STRONG_L is one of the bidi types defined
by 'enum bidi_type_t' (see dispextern.h), and therefore its value
_must_ be between UNKNOWN_BT (whose value is zero) and NEUTRAL_ON, the
last tag in the enumeration type.

Can you see the numerical value of 'type' in frame #2?  Like this:

 (gdb) fr 2
 (gdb) p type + 0

Also, using a similar technique, display the values of UNKNOWN_BT and
of NEUTRAL_ON.

Other than that, the backtrace you show is just a normal redisplay
cycle.  Nothing catches my eye.  In particular, the crash was while
the display engine was recomputing the mode-line display.