Fix integer overflow in array_agg(), when the array grows too large

If you accumulate many arrays full of NULLs, you could overflow
'nitems', before reaching the MaxAllocSize limit on the allocations.
Add an explicit check that the number of items doesn't grow too large.
With more than MaxArraySize items, getting the final result with
makeArrayResultArr() would fail anyway, so better to error out early.

Reported-by: Xint Code
Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Backpatch-through: 14
Security: CVE-2026-6473
(cherry picked from commit e24fb3247644a9baef72758806d83ec59d914781)

Author: hlinnaka

src/backend/utils/adt/arrayfuncs.c

@@ -5585,6 +5585,7 @@ accumArrayResultArr(ArrayBuildStateArr *astate,
 				ndatabytes;
 	char	   *data;
 	int			i;
+	int			newnitems;
 
 	/*
 	 * We disallow accumulating null subarrays.  Another plausible definition
@@ -5614,6 +5615,14 @@ accumArrayResultArr(ArrayBuildStateArr *astate,
 	nitems = ArrayGetNItems(ndims, dims);
 	ndatabytes = ARR_SIZE(arg) - ARR_DATA_OFFSET(arg);
 
+	/* Check that the array doesn't grow too large */
+	newnitems = astate->nitems + nitems;
+	if (newnitems > MaxArraySize)
+		ereport(ERROR,
+				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
+				 errmsg("array size exceeds the maximum allowed (%zu)",
+						MaxArraySize)));
+
 	if (astate->ndims == 0)
 	{
 		/* First input; check/save the dimensionality info */
@@ -5679,8 +5688,6 @@ accumArrayResultArr(ArrayBuildStateArr *astate,
 	/* Deal with null bitmap if needed */
 	if (astate->nullbitmap || ARR_HASNULL(arg))
 	{
-		int			newnitems = astate->nitems + nitems;
-
 		if (astate->nullbitmap == NULL)
 		{
 			/*
@@ -5704,7 +5711,7 @@ accumArrayResultArr(ArrayBuildStateArr *astate,
 						  nitems);
 	}
 
-	astate->nitems += nitems;
+	astate->nitems = newnitems;
 	astate->dims[0] += 1;
 
 	MemoryContextSwitchTo(oldcontext);