BACKUP sql command does not verify schema exists

[wwar]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| INFORMATION_SCHEMA |
| METRICS_SCHEMA     |
| PERFORMANCE_SCHEMA |
| mysql              |
| test               |
+--------------------+
5 rows in set (0.00 sec)

mysql> BACKUP DATABASE `abc123` TO 'local:///tmp/';
+---------------+------+--------------------+---------------------+---------------------+
| Destination   | Size | BackupTS           | Queue Time          | Execution Time      |
+---------------+------+--------------------+---------------------+---------------------+
| local:///tmp/ |    0 | 417741498863583234 | 2020-06-30 16:49:13 | 2020-06-30 16:49:13 |
+---------------+------+--------------------+---------------------+---------------------+
1 row in set (0.01 sec)

mysql> RESTORE DATABASE `abc123` FROM 'local:///tmp/';
+---------------+------+----------+---------------------+---------------------+
| Destination   | Size | BackupTS | Queue Time          | Execution Time      |
+---------------+------+----------+---------------------+---------------------+
| local:///tmp/ |    6 |        0 | 2020-06-30 16:54:50 | 2020-06-30 16:54:50 |
+---------------+------+----------+---------------------+---------------------+
1 row in set (0.00 sec)

mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| INFORMATION_SCHEMA |
| METRICS_SCHEMA     |
| PERFORMANCE_SCHEMA |
| mysql              |
| test               |
+--------------------+
5 rows in set (0.00 sec)

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

It should provide an error, since this could lead to a case where a typo causes a backup not to be generated.

3. What did you see instead (Required)

Statement succeeds:

mysql> BACKUP DATABASE `abc123` TO 'local:///tmp/';
+---------------+------+--------------------+---------------------+---------------------+
| Destination   | Size | BackupTS           | Queue Time          | Execution Time      |
+---------------+------+--------------------+---------------------+---------------------+
| local:///tmp/ |    0 | 417741498863583234 | 2020-06-30 16:49:13 | 2020-06-30 16:49:13 |
+---------------+------+--------------------+---------------------+---------------------+
1 row in set (0.01 sec)

4. Affected version (Required)

mysql> SELECT tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v4.0.0-beta.2-688-g28cc5e6ff
Edition: Community
Git Commit Hash: 28cc5e6ff7f9fe1d7b989b238a12aff62280682d
Git Branch: master
UTC Build Time: 2020-06-28 02:04:29
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
1 row in set (0.00 sec)

5. Root Cause Analysis

[zz-jason]

@kennytm PTAL