infoschema: panic occurred when applying default charset modification for databases named in uppercase.

[bb7133]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

tidb> DROP DATABASE IF EXISTS A;
Query OK, 0 rows affected (0.02 sec)

tidb> CREATE DATABASE A DEFAULT CHARACTER SET utf8;
Query OK, 0 rows affected (0.01 sec)

tidb> ALTER DATABASE A DEFAULT CHARACTER SET utf8mb4;

2. What did you expect to see? (Required)

tidb> ALTER DATABASE A DEFAULT CHARACTER SET utf8mb4;
Query OK, 0 rows affected (0.01 sec)

3. What did you see instead (Required)

ALTER DATABASE A DEFAULT CHARACTER SET utf8mb4;
ERROR 1105 (HY000): runtime error: invalid memory address or nil pointer dereference

4. What is your TiDB version? (Required)

v4.0.4

Others

TiDB panic stack:

[2020/08/19 00:16:16.337 +08:00] [ERROR] [conn.go:660] ["connection running loop panic"] [conn=1] [lastSQL="ALTER DATABASE A DEFAULT CHARACTER SET utf8mb4"] [err="runtime error: invalid memory address or nil pointer dereference"] [stack="goroutine 315 [running]:
github.com/pingcap/tidb/server.(*clientConn).Run.func1(0x3763880, 0xc00098f770, 0xc00073cea0)
	/home/bb7133/Projects/gopath/src/github.com/pingcap/tidb/server/conn.go:658 +0xee
panic(0x2e71a00, 0x4b550d0)
	/home/bb7133/Softwares/go/src/runtime/panic.go:679 +0x1b2
github.com/pingcap/tidb/executor.(*ExecStmt).Exec.func1(0xc00113c480, 0xc000b287c8, 0xc000b287a8)
	/home/bb7133/Projects/gopath/src/github.com/pingcap/tidb/executor/adapter.go:283 +0x544
panic(0x2e71a00, 0x4b550d0)
	/home/bb7133/Softwares/go/src/runtime/panic.go:679 +0x1b2
github.com/pingcap/tidb/infoschema.(*Builder).copySchemaTables(0xc000b27930, 0x4be55c1, 0x1, 0x0)
	/home/bb7133/Projects/gopath/src/github.com/pingcap/tidb/infoschema/builder.go:368 +0x79
github.com/pingcap/tidb/infoschema.(*Builder).applyModifySchemaCharsetAndCollate(0xc000b27930, 0xc000a89620, 0xc000fb6540, 0x1, 0xc000502590)
	/home/bb7133/Projects/gopath/src/github.com/pingcap/tidb/infoschema/builder.go:190 +0x8c
github.com/pingcap/tidb/infoschema.(*Builder).ApplyDiff(0xc000b27930, 0xc000a89620, 0xc000fb6540, 0xc000ba7230, 0x1, 0xc0011d2928, 0x0, 0x1)
	/home/bb7133/Projects/gopath/src/github.com/pingcap/tidb/infoschema/builder.go:49 +0x931
github.com/pingcap/tidb/domain.(*Domain).tryLoadSchemaDiffs(0xc00052b440, 0xc000a89620, 0xa1, 0xa1, 0xa1, 0x0, 0x0, 0xc000b27a20, 0x14a53a4, 0xc000751540)
	/home/bb7133/Projects/gopath/src/github.com/pingcap/tidb/domain/domain.go:260 +0x384
github.com/pingcap/tidb/domain.(*Domain).loadInfoSchema(0xc00052b440, 0xc0002ed940, 0xa0, 0x5d0096d77400000, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
	/home/bb7133/Projects/gopath/src/github.com/pingcap/tidb/domain/domain.go:121 +0x305
github.com/pingcap/tidb/domain.(*Domain).Reload(0xc00052b440, 0x0, 0x0)
	/home/bb7133/Projects/gopath/src/github.com/pingcap/tidb/domain/domain.go:360 +0x1a5
github.com/pingcap/tidb/domain.(*ddlCallback).OnChanged(0xc0005766d8, 0x0, 0x0, 0x0, 0x1)
	/home/bb7133/Projects/gopath/src/github.com/pingcap/tidb/domain/domain.go:626 +0x8d
github.com/pingcap/tidb/ddl.(*ddl).callHookOnChanged(0xc000b84420, 0x0, 0x0, 0x0, 0x0)
	/home/bb7133/Projects/gopath/src/github.com/pingcap/tidb/ddl/ddl.go:524 +0xbc
github.com/pingcap/tidb/ddl.(*ddl).AlterSchema(0xc000b84420, 0x37a6e00, 0xc0009e4d00, 0xc00054b580, 0xc00052b440, 0x306c720)
...

• Looks that if the name of database is lower-case only, the ALTER ... statement runs just fine.
• The panic happened when 'incremental' diff is applied for the information schema builder.