From 308a69a9878212bc2f96279023ce16070dac9214 Mon Sep 17 00:00:00 2001
From: Dean Rasheed <dean.a.rasheed@gmail.com>
Date: Thu, 9 Nov 2023 09:54:22 +0000
Subject: [PATCH] Fix corner-case 64-bit integer subtraction bug on some
 platforms.

When computing "0 - INT64_MIN", most platforms would report an
overflow error, which is correct. However, platforms without integer
overflow builtins or 128-bit integers would fail to spot the overflow,
and incorrectly return INT64_MIN.

Back-patch to all supported branches.

Patch be me. Thanks to Jian He for initial investigation, and Laurenz
Albe and Tom Lane for review.

Discussion: https://postgr.es/m/CAEZATCUNK-AZSD0jVdgkk0N%3DNcAXBWeAEX-QU9AnJPensikmdQ%40mail.gmail.com
---
 src/include/common/int.h           | 6 +++++-
 src/test/regress/expected/int8.out | 2 ++
 src/test/regress/sql/int8.sql      | 1 +
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/include/common/int.h b/src/include/common/int.h
index 12a269d952..e2617fbc5d 100644
--- a/src/include/common/int.h
+++ b/src/include/common/int.h
@@ -200,8 +200,12 @@ pg_sub_s64_overflow(int64 a, int64 b, int64 *result)
 	*result = (int64) res;
 	return false;
 #else
+	/*
+	 * Note: overflow is also possible when a == 0 and b < 0 (specifically,
+	 * when b == PG_INT64_MIN).
+	 */
 	if ((a < 0 && b > 0 && a < PG_INT64_MIN + b) ||
-		(a > 0 && b < 0 && a > PG_INT64_MAX + b))
+		(a >= 0 && b < 0 && a > PG_INT64_MAX + b))
 	{
 		*result = 0x5EED;		/* to avoid spurious warnings */
 		return true;
diff --git a/src/test/regress/expected/int8.out b/src/test/regress/expected/int8.out
index 1ae23cf3f9..329f3911dd 100644
--- a/src/test/regress/expected/int8.out
+++ b/src/test/regress/expected/int8.out
@@ -654,6 +654,8 @@ select -('-9223372036854775807'::int8);
 
 select -('-9223372036854775808'::int8);
 ERROR:  bigint out of range
+select 0::int8 - '-9223372036854775808'::int8;
+ERROR:  bigint out of range
 select '9223372036854775800'::int8 + '9223372036854775800'::int8;
 ERROR:  bigint out of range
 select '-9223372036854775800'::int8 + '-9223372036854775800'::int8;
diff --git a/src/test/regress/sql/int8.sql b/src/test/regress/sql/int8.sql
index 38b771964d..8a3d5371d6 100644
--- a/src/test/regress/sql/int8.sql
+++ b/src/test/regress/sql/int8.sql
@@ -126,6 +126,7 @@ select '9223372036854775808'::int8;
 
 select -('-9223372036854775807'::int8);
 select -('-9223372036854775808'::int8);
+select 0::int8 - '-9223372036854775808'::int8;
 
 select '9223372036854775800'::int8 + '9223372036854775800'::int8;
 select '-9223372036854775800'::int8 + '-9223372036854775800'::int8;