Better behaviour for interactive apps on leaf overflow?

Hello!

We’ve been using libgmp for our bignum support in XEmacs for a decade or so
now, thanks to Jerry James, one of our more active developers. XEmacs Lisp is
in spirit a Lisp-2 close in philosophy and history to Common Lisp, and it’s
correct and pleasant to have good bignum support.

One of our design (and implementation!) goals for XEmacs Lisp is that it
should be impossible for Lisp to crash XEmacs, something very important in
preserving our users’ data. We supply our own allocation functions to gmp and
handle our failures ourselves to minimise the risk of that happening. But, it
is still perfectly possible for Lisp to do something like this, where lsh is
the bitwise left shift operation, and most-positive-fixnum is 2 to the 30th
minus one:

(lsh (lsh most-positive-fixnum #xFFFFFF80) #xFFFFFF80)

On a 32-bit machine this will eventually fail with the following message:

(gdb) r -batch -eval "(lsh (lsh most-positive-fixnum #xFFFFFF80) #xFFFFFF80)"
Starting program: /Sources/xemacs-21.5-integer-length/src/xemacs -batch -eval "(lsh (lsh most-positive-fixnum #xFFFFFF80) #xFFFFFF80)"
gmp: overflow in mpz type
Program received signal SIGABRT, Aborted.
0x95187c5a in __kill () from /usr/lib/libSystem.B.dylib
(gdb) 

Now, this abort isn’t wrapped, gmp calls it unconditionally in
realloc.c:_mpz_realloc(). This is frustrating for XEmacs; from our
perspective, we’d like to throw an error in this situation in the same way we
do when we divide by zero, rather than crash the entire process and have the
user lose so much of his or her state. Our garbage collector will take care of
the lost memory. 

Best,

Aidan Kehoe

-- 
‘Tramadol is further fed to cattle […] when working them […] (as draft
animals) so that the animals do not get tired quickly. …’
	— Angewandte Chemie, Sept 2014, describing the social context of
	(synthetic) tramadol having been found in Cameroon tree roots.

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