Predict integer overflow to avoid buffer overruns.

Several functions, mostly type input functions, calculated an allocation
size such that the calculation wrapped to a small positive value when
arguments implied a sufficiently-large requirement.  Writes past the end
of the inadvertent small allocation followed shortly thereafter.
Coverity identified the path_in() vulnerability; code inspection led to
the rest.  In passing, add check_stack_depth() to prevent stack overflow
in related functions.

Back-patch to 8.4 (all supported versions).  The non-comment hstore
changes touch code that did not exist in 8.4, so that part stops at 9.0.

Noah Misch and Heikki Linnakangas, reviewed by Tom Lane.

Security: CVE-2014-0064
txid_current_snapshot(PG_FUNCTION_ARGS)
{
	TxidSnapshot *snap;
	uint32		nxip,
				i,
				size;
	TxidEpoch	state;
	Snapshot	cur;

	cur = GetActiveSnapshot();
	if (cur == NULL)
		elog(ERROR, "no active snapshot set");
 
 	load_xid_epoch(&state);
 
	/*
	 * Compile-time limits on the procarray (MAX_BACKENDS processes plus
	 * MAX_BACKENDS prepared transactions) guarantee nxip won't be too large.
	 */
	StaticAssertStmt(MAX_BACKENDS * 2 <= TXID_SNAPSHOT_MAX_NXIP,
					 "possible overflow in txid_current_snapshot()");

 	/* allocate */
 	nxip = cur->xcnt;
 	size = TXID_SNAPSHOT_SIZE(nxip);
	snap = palloc(size);
	SET_VARSIZE(snap, size);

	/* fill */
	snap->xmin = convert_xid(cur->xmin, &state);
	snap->xmax = convert_xid(cur->xmax, &state);
	snap->nxip = nxip;
	for (i = 0; i < nxip; i++)
		snap->xip[i] = convert_xid(cur->xip[i], &state);

	/* we want them guaranteed to be in ascending order */
	sort_snapshot(snap);

	PG_RETURN_POINTER(snap);
}

txid_current_snapshot(PG_FUNCTION_ARGS)
{
	TxidSnapshot *snap;
	uint32		nxip,
				i,
				size;
	TxidEpoch	state;
	Snapshot	cur;

	cur = GetActiveSnapshot();
	if (cur == NULL)
		elog(ERROR, "no active snapshot set");
 
 	load_xid_epoch(&state);
 
 	/* allocate */
 	nxip = cur->xcnt;
 	size = TXID_SNAPSHOT_SIZE(nxip);
	snap = palloc(size);
	SET_VARSIZE(snap, size);

	/* fill */
	snap->xmin = convert_xid(cur->xmin, &state);
	snap->xmax = convert_xid(cur->xmax, &state);
	snap->nxip = nxip;
	for (i = 0; i < nxip; i++)
		snap->xip[i] = convert_xid(cur->xip[i], &state);

	/* we want them guaranteed to be in ascending order */
	sort_snapshot(snap);

	PG_RETURN_POINTER(snap);
}
