pt-table-checksum fails if a database is dropped while the tool is running

[Agustín]

If a database is dropped while pt-table-checksum is running, the tool exists with a fatal error:

$ pt-table-checksum --no-check-binlog-format h=127.0.0.1,u=root,p=msandbox,P=9870
            TS ERRORS DIFFS ROWS CHUNKS SKIPPED TIME TABLE
01-23T16:40:10 0 0 0 1 0 0.012 mysql.columns_priv
<...output trimmed>

DBD::mysql::db selectall_arrayref failed: Unknown database 'pttctest98' [for Statement "SHOW /*!50002 FULL*/ TABLES FROM `pttctest98`"] at /usr/bin/pt-table-checksum line 7535.

$ echo $?
255

How to reproduce:

In one session in the master server run the following:

for i in `seq 1000`; do
mysql -e "CREATE SCHEMA pttctest$i";
mysql -e "DROP SCHEMA pttctest$(($i-1))";
done;

In another one (after the previous command is run), run the pt-table-checksum. The tool will exit with an error like seen above.

Suggested fix:

Check if the database exists before running the SHOW FULL TABLES FROM ... command.

[Agustín (agustin-gallego)]

Sorry, I forgot to mention versions.

MySQL 5.7.17, and pt-table-checksum 2.2.19

[markus_albe (markus-albe)]

The technique of observing whether a db exists before using SHOW FULL TABLES FROM... is bound to same type of race conditions. Am not sure it's possible to provide a solution until DDL and schemas become transactional.

[Agustín (agustin-gallego)]

Agreed, but two comments on this:

1- Even if it's bound to some race conditions, we should at least check.

2- The tool should not error out. Instead (as being done with tables that don't exist), it should show a warning, set the error number bitmask to 'non-fatal error', and continue with other schemas.

[Carlos Salguero (carlos-salguero)]

Since the change goes to SchemaIterator, all these programs also get the fix:
 modified: bin/pt-duplicate-key-checker
 modified: bin/pt-index-usage
 modified: bin/pt-table-checksum
 modified: bin/pt-table-sync

[Carlos Salguero (carlos-salguero)]

Resolved: https://github.com/percona/percona-toolkit/pull/184

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PT-727