[PATCH] shell: avoid segfault on ${0::0/0~09J}. Closes 15216

[Steffen Nurpmeso]

Ron Yorston wrote in
 <63ad9a0f.MQrBKX7zM8Pqj639%rmy at pobox.com>:
 |Both ash and hush segfault when asked to evaluate ${0::0/0~09J}.
 ...
 | shell/math.c | 3 ++-
 ...

Without hurry or pressure or any of such sort, but to be more
explicit than in the patch commit message of the thing i had sent,
it must be said that the $(()) evaluator currently in use is
broken in some ways.  There are explicitly disabled tests.  But
more than what is tested.  (And the ?: problem is not easy fixable
with Dijkstra, a recursive descendent thing like bash is however
also not a small solution.)

I am sorry that the one i posted (too often) adds 2000 bytes,
i would have thought it shrinks a bit when fully integrated.  All
i could offer for that is to introduce another configuration
switch which disables the very costy and large ?: conditional
branch evaluation, i would assume doing so would bring costs down
by a large amount, while keeping the benefit of the (hopefully)
graceful parser and evaluator.  (And ?: is not standardized (yet);
like &&, || etc. etc., but these are all simple binary operators.)

P.S.: the only change since then is that i added some more tests,
maybe grazy stuff

  e "<$((0||0||0||0||0||0||0||0||0||0))>"
  e "<$((0||0||0||0||0||0||0||0||0||3||0||0))>"
  e "<$((0||0||0||0||0||0||0||0||0||3||0&&0))>"
  e "<$((0||0||0||0||0||0||0||0||0||3&&0))>"
  e "<$(((0||0||0||0||0||0||0||0||0||3||0)&&0))>"
  e "<$(((0||0||0||0||0||0||0||0||0||3||0)&&3))>"
  s I1=10 I2=20;p "<$((I1+=I2+=I2))>";e "<$I1><$I2>"
  s I1=10 I2=20;p "<$((I1+=I2+=I1))>";e "<$I1><$I2>"
  s I1=10 I2=20;p "<$((I1+=I2+=I1+=I2))>";e "<$I1><$I2>"
  s I1=10 I2=20;p "<$((I1+=I2+=I1+=I1))>";e "<$I1><$I2>"

--steffen
|
|Der Kragenbaer,                The moon bear,
|der holt sich munter           he cheerfully and one by one
|einen nach dem anderen runter  wa.ks himself off
|(By Robert Gernhardt)