From dce26c203d52f92d7daa4cb5eef7b7fafaee40b9 Mon Sep 17 00:00:00 2001
From: Ti Chi Robot <ti-community-prow-bot@tidb.io>
Date: Wed, 22 May 2024 10:32:46 +0800
Subject: [PATCH] expression: wrong result of timestampadd(month,1,date
 '2024-01-31') (#53101) (#53423)

close pingcap/tidb#41052
---
 expression/builtin_time.go      | 19 +++++++++++++------
 expression/builtin_time_test.go | 12 ++++++++++++
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/expression/builtin_time.go b/expression/builtin_time.go
index 9901b9323d060..1d7c2a25dc1b2 100644
--- a/expression/builtin_time.go
+++ b/expression/builtin_time.go
@@ -6969,6 +6969,13 @@ func (b *builtinTimestampAddSig) evalString(row chunk.Row) (string, bool, error)
 		tb = tm1.AddDate(0, 0, 7*int(v))
 	case "MONTH":
 		tb = tm1.AddDate(0, int(v), 0)
+
+		// For corner case: timestampadd(month,1,date '2024-01-31') = "2024-02-29", timestampadd(month,1,date '2024-01-30') = "2024-02-29"
+		// `tb.Month()` refers to the actual result, `t.Month()+v` refers to the expect result.
+		// Actual result may be greater than expect result, we need to judge and modify it.
+		for int(tb.Month())%12 != (int(tm1.Month())+int(v))%12 {
+			tb = tb.AddDate(0, 0, -1)
+		}
 	case "QUARTER":
 		tb = tm1.AddDate(0, 3*int(v), 0)
 	case "YEAR":
@@ -7381,12 +7388,12 @@ func CalAppropriateTime(minTime, maxTime, minSafeTime time.Time) time.Time {
 }
 
 // For a SafeTS t and a time range [t1, t2]:
-//   1. If t < t1, we will use t1 as the result,
-//      and with it, a read request may fail because it's an unreached SafeTS.
-//   2. If t1 <= t <= t2, we will use t as the result, and with it,
-//      a read request won't fail.
-//   2. If t2 < t, we will use t2 as the result,
-//      and with it, a read request won't fail because it's bigger than the latest SafeTS.
+//  1. If t < t1, we will use t1 as the result,
+//     and with it, a read request may fail because it's an unreached SafeTS.
+//  2. If t1 <= t <= t2, we will use t as the result, and with it,
+//     a read request won't fail.
+//  2. If t2 < t, we will use t2 as the result,
+//     and with it, a read request won't fail because it's bigger than the latest SafeTS.
 func calAppropriateTime(minTime, maxTime, minSafeTime time.Time) time.Time {
 	if minSafeTime.Before(minTime) || minSafeTime.After(maxTime) {
 		logutil.BgLogger().Warn("calAppropriateTime",
diff --git a/expression/builtin_time_test.go b/expression/builtin_time_test.go
index 2fcf98d3db3d8..a113644a43f42 100644
--- a/expression/builtin_time_test.go
+++ b/expression/builtin_time_test.go
@@ -2492,6 +2492,18 @@ func TestTimestampAdd(t *testing.T) {
 		{"WEEK", 1, "2003-01-02 23:59:59", "2003-01-09 23:59:59"},
 		{"MICROSECOND", 1, 950501, "1995-05-01 00:00:00.000001"},
 		{"DAY", 28768, 0, ""},
+
+		// issue41052
+		{"MONTH", 1, "2024-01-31", "2024-02-29 00:00:00"},
+		{"MONTH", 1, "2024-01-30", "2024-02-29 00:00:00"},
+		{"MONTH", 1, "2024-01-29", "2024-02-29 00:00:00"},
+		{"MONTH", 1, "2024-01-28", "2024-02-28 00:00:00"},
+		{"MONTH", 1, "2024-10-31", "2024-11-30 00:00:00"},
+		{"MONTH", 3, "2024-01-31", "2024-04-30 00:00:00"},
+		{"MONTH", 15, "2024-01-31", "2025-04-30 00:00:00"},
+		{"MONTH", 10, "2024-10-31", "2025-08-31 00:00:00"},
+		{"MONTH", 1, "2024-11-30", "2024-12-30 00:00:00"},
+		{"MONTH", 13, "2024-11-30", "2025-12-30 00:00:00"},
 	}
 
 	fc := funcs[ast.TimestampAdd]