From 0ab49bec1456ca27b9e879fbeb74a068fb416c15 Mon Sep 17 00:00:00 2001
From: Hangjie Mo <mohangjie1995@gmail.com>
Date: Tue, 8 Oct 2024 18:46:56 +0800
Subject: [PATCH] executor: fix `set names` with
 `default_collation_for_utf8mb4` vars (#56469)

close pingcap/tidb#56439
---
 pkg/executor/set.go                           |  4 +++-
 .../r/new_character_set.result                | 20 +++++++++++++++++++
 .../integrationtest/t/new_character_set.test  | 13 ++++++++++++
 3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/pkg/executor/set.go b/pkg/executor/set.go
index 7649027df2abb..8d88a2d782b45 100644
--- a/pkg/executor/set.go
+++ b/pkg/executor/set.go
@@ -254,7 +254,9 @@ func (e *SetExecutor) setCharset(cs, co string, isSetName bool) error {
 	var err error
 	sessionVars := e.Ctx().GetSessionVars()
 	if co == "" {
-		if co, err = charset.GetDefaultCollation(cs); err != nil {
+		if cs == mysql.UTF8MB4Charset {
+			co = sessionVars.DefaultCollationForUTF8MB4
+		} else if co, err = charset.GetDefaultCollation(cs); err != nil {
 			return err
 		}
 	} else {
diff --git a/tests/integrationtest/r/new_character_set.result b/tests/integrationtest/r/new_character_set.result
index 0711afae14891..44ad681270ae6 100644
--- a/tests/integrationtest/r/new_character_set.result
+++ b/tests/integrationtest/r/new_character_set.result
@@ -92,3 +92,23 @@ select hex('一a'), '一a';
 hex('涓?')	涓?
 E4B83F	涓?
 set @@sql_mode=default;
+set @@character_set_client=default;
+set @@character_set_connection=default;
+show variables like 'collation_connection';
+Variable_name	Value
+collation_connection	utf8mb4_bin
+set default_collation_for_utf8mb4 = 'utf8mb4_general_ci';
+set names utf8mb4;
+show variables like 'collation_connection';
+Variable_name	Value
+collation_connection	utf8mb4_general_ci
+set default_collation_for_utf8mb4 = 'utf8mb4_0900_ai_ci';
+set names utf8mb4;
+show variables like 'collation_connection';
+Variable_name	Value
+collation_connection	utf8mb4_0900_ai_ci
+set default_collation_for_utf8mb4 = default;
+set names utf8mb4;
+show variables like 'collation_connection';
+Variable_name	Value
+collation_connection	utf8mb4_bin
diff --git a/tests/integrationtest/t/new_character_set.test b/tests/integrationtest/t/new_character_set.test
index 8b0ee0eb21818..ceb4f5c978d02 100644
--- a/tests/integrationtest/t/new_character_set.test
+++ b/tests/integrationtest/t/new_character_set.test
@@ -70,3 +70,16 @@ set @@character_set_connection=gbk;
 select hex('一a'), '一a';
 
 set @@sql_mode=default;
+set @@character_set_client=default;
+set @@character_set_connection=default;
+
+show variables like 'collation_connection';
+set default_collation_for_utf8mb4 = 'utf8mb4_general_ci';
+set names utf8mb4;
+show variables like 'collation_connection';
+set default_collation_for_utf8mb4 = 'utf8mb4_0900_ai_ci';
+set names utf8mb4;
+show variables like 'collation_connection';
+set default_collation_for_utf8mb4 = default;
+set names utf8mb4;
+show variables like 'collation_connection';