patch for ash mem fault

["Mortimer, Andy" <Andy dot Mortimer at software dot aeat dot com>]

Hi all,

(I'm not sure who to send this to, because I can't quite work 
out where ash comes from!  Hopefully this is a reasonable 
first try ...)

I think I've found and fixed a segfault in ash 20000823 (from 
Cygwin).  I have a feeling that this has been around since b20 
or before -- I definitely saw a similar one there, which I fixed 
by changing to bash for this particular script -- but I don't 
have that installed any more to confirm that it's the same one.

The problem occurs infrequently, when a large string value 
(>504 chars, I think!) is assigned to a variable.  In ash-
source-speak, what happens is that the current stack 
position is saved via a call to setstackmark, but then before 
it is freed again in popstackmark, a call to growstackblock 
moves the stack block which has been saved.  Then when 
the time comes to free it, the free is passed an invalid 
pointer, and basically, it all goes horribly wrong.  (All these
routines are in memalloc.c)

The attached patch adds a new member to the stack_block 
structure which is initially set to zero, but is incremented if 
setstackmark is called on a particular stack_block.  If this 
member is non-zero in growstackblock, even if no data is 
actually allocated in that block, it will still go ahead and 
create another one and leave the previous one in place.

This is all at the cost of an extra 4 bytes in a stack_block, 
but looking at the average length of the block chain for the 
sorts of runs I've been doing (about 4 or 5 for most of the 
time, I think), I don't expect that this is a problem.  Wiser 
minds than mine may prevail, however.

I've checked that this fixes the problem I was having; 
however, I haven't done much more testing than that single 
(admittedly many thousand line) script.

Have fun!

Andy

-- 
Andy Mortimer, CFX-5 Architecture and Infrastructure Team
andy.mortimer@aeat.com

memalloc.diff

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