Bug Report: vtcombo leaks connections on drop + create database

[brendar]

Overview of the Issue

After executing DROP DATABASE followed by CREATE DATABASE several times queries will start to fail with "Too many connections" errors.

This is because the connection pools for each of the internal tablets are not closed on DROP DATABASE so they reconnect once the database exists again after the next CREATE DATABASE.

PR to fix this: #16606

Reproduction Steps

1. Start vtcombo via vttestserver

vttestserver --keyspaces foo --num_shards 1 --port 20040

2. Connect

mysql --host 127.0.0.1 --port 20043

3. Count connections

mysql> select db, user, count(*) from information_schema.PROCESSLIST group by db, user order by db, user;
+----------+-----------------+----------+
| db       | user            | count(*) |
+----------+-----------------+----------+
| NULL     | event_scheduler |        1 |
| vt_foo_0 | vt_allprivs     |        1 |
| vt_foo_0 | vt_dba          |        8 |
+----------+-----------------+----------+

4. Drop and create the database

mysql> drop database foo;
mysql> create database foo;

5. Count connections

mysql> select db, user, count(*) from information_schema.PROCESSLIST group by db, user order by db, user;
+----------+-----------------+----------+
| db       | user            | count(*) |
+----------+-----------------+----------+
| NULL     | event_scheduler |        1 |
| vt_foo_0 | vt_allprivs     |        2 |
| vt_foo_0 | vt_dba          |       13 |
+----------+-----------------+----------+

Binary Version

Impacts main and previous versions

Operating System and Environment details

n/a

Log Fragments

No response