Fix possible internal overflow in numeric multiplication.

mul_var() postpones propagating carries until it risks overflow in its
internal digit array.  However, the logic failed to account for the
possibility of overflow in the carry propagation step, allowing wrong
results to be generated in corner cases.  We must slightly reduce the
when-to-propagate-carries threshold to avoid that.

Discovered and fixed by Dean Rasheed, with small adjustments by me.

This has been wrong since commit d72f6c75038d8d37e64a29a04b911f728044d83b,
so back-patch to all supported branches.

3 files changed, 49 insertions, 4 deletions

diff --git a/src/backend/utils/adt/numeric.c b/src/backend/utils/adt/numeric.c
index a5dec22d9c..2e61355f12 100644
--- a/src/backend/utils/adt/numeric.c
+++ b/src/backend/utils/adt/numeric.c
@@ -4191,9 +4191,15 @@ mul_var(NumericVar *var1, NumericVar *var2, NumericVar *result,
 	 * to avoid normalizing carries immediately.
 	 *
 	 * maxdig tracks the maximum possible value of any dig[] entry; when this
-	 * threatens to exceed INT_MAX, we take the time to propagate carries. To
-	 * avoid overflow in maxdig itself, it actually represents the max
-	 * possible value divided by NBASE-1.
+	 * threatens to exceed INT_MAX, we take the time to propagate carries.
+	 * Furthermore, we need to ensure that overflow doesn't occur during the
+	 * carry propagation passes either.  The carry values could be as much as
+	 * INT_MAX/NBASE, so really we must normalize when digits threaten to
+	 * exceed INT_MAX - INT_MAX/NBASE.
+	 *
+	 * To avoid overflow in maxdig itself, it actually represents the max
+	 * possible value divided by NBASE-1, ie, at the top of the loop it is
+	 * known that no dig[] entry exceeds maxdig * (NBASE-1).
 	 */
 	dig = (int *) palloc0(res_ndigits * sizeof(int));
 	maxdig = 0;
@@ -4208,7 +4214,7 @@ mul_var(NumericVar *var1, NumericVar *var2, NumericVar *result,
 
 		/* Time to normalize? */
 		maxdig += var1digit;
-		if (maxdig > INT_MAX / (NBASE - 1))
+		if (maxdig > (INT_MAX - INT_MAX / NBASE) / (NBASE - 1))
 		{
 			/* Yes, do it */
 			carry = 0;
diff --git a/src/test/regress/expected/numeric.out b/src/test/regress/expected/numeric.out
index 635c7d9840..4abcb02d11 100644
--- a/src/test/regress/expected/numeric.out
+++ b/src/test/regress/expected/numeric.out
@@ -1310,6 +1310,33 @@ SELECT * FROM num_input_test;
 (7 rows)
 
 --
+-- Test some corner cases for multiplication
+--
+select 4790999999999999999999999999999999999999999999999999999999999999999999999999999999999999 * 9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999;
+                                                                                     ?column?                                                                                     
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ 47909999999999999999999999999999999999999999999999999999999999999999999999999999999999985209000000000000000000000000000000000000000000000000000000000000000000000000000000000001
+(1 row)
+
+select 4789999999999999999999999999999999999999999999999999999999999999999999999999999999999999 * 9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999;
+                                                                                     ?column?                                                                                     
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ 47899999999999999999999999999999999999999999999999999999999999999999999999999999999999985210000000000000000000000000000000000000000000000000000000000000000000000000000000000001
+(1 row)
+
+select 4770999999999999999999999999999999999999999999999999999999999999999999999999999999999999 * 9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999;
+                                                                                     ?column?                                                                                     
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ 47709999999999999999999999999999999999999999999999999999999999999999999999999999999999985229000000000000000000000000000000000000000000000000000000000000000000000000000000000001
+(1 row)
+
+select 4769999999999999999999999999999999999999999999999999999999999999999999999999999999999999 * 9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999;
+                                                                                     ?column?                                                                                     
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ 47699999999999999999999999999999999999999999999999999999999999999999999999999999999999985230000000000000000000000000000000000000000000000000000000000000000000000000000000000001
+(1 row)
+
+--
 -- Test some corner cases for division
 --
 select 999999999999999999999::numeric/1000000000000000000000;
diff --git a/src/test/regress/sql/numeric.sql b/src/test/regress/sql/numeric.sql
index 1d380face2..fc1953601a 100644
--- a/src/test/regress/sql/numeric.sql
+++ b/src/test/regress/sql/numeric.sql
@@ -812,6 +812,18 @@ INSERT INTO num_input_test(n1) VALUES (' N aN ');
 SELECT * FROM num_input_test;
 
 --
+-- Test some corner cases for multiplication
+--
+
+select 4790999999999999999999999999999999999999999999999999999999999999999999999999999999999999 * 9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999;
+
+select 4789999999999999999999999999999999999999999999999999999999999999999999999999999999999999 * 9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999;
+
+select 4770999999999999999999999999999999999999999999999999999999999999999999999999999999999999 * 9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999;
+
+select 4769999999999999999999999999999999999999999999999999999999999999999999999999999999999999 * 9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999;
+
+--
 -- Test some corner cases for division
 --