Current codebase does not handle 0 source rows well

[atc0005]

I recreated the source MySQL database in a test LXD container, but initially forgot to insert the test data. When I ran the prototype mysql2sqlite binary I got this output (debug logging):

  INFO[0000] no entries in table access_check_clients for database mailserver, skipping rows retrieval and SQLite database insert
 DEBUG[0000] reading table access_check_recipients from source database mailserver
 DEBUG[0000] Rows retrieved from source MySQL database
 DEBUG[0000] Successfully ran DROP TABLE query for table access_check_recipients
 DEBUG[0000] Successfully ran DROP INDEX query for table access_check_recipients
 DEBUG[0000] Created table access_check_recipients in SQLite database
 DEBUG[0000] Created index for table access_check_recipients in SQLite database
 DEBUG[0000] SQLite database is ready
 DEBUG[0000] Before mysqlRows.Next()
  INFO[0000] no entries in table access_check_recipients for database mailserver, skipping rows retrieval and SQLite database insert
 DEBUG[0000] reading table sender_bcc_maps from source database mailserver

and the application went no further.

The Nagios plugin currently applies a rows count check to both source and mirror databases and exits early if there isn't a match. The main app does check for zero rows, but its later in the code and just before attempting the fetch/insert process.

I'm going to defer fixing this until #19 has been completed so I can track the specific problem instead of continuing to tweak/refine the current code before it ever lands here.

[atc0005]

I found a workaround.

Modify the config file to allow a greater number of open connections:

   # Maximum number of connections to the MySQL allowed for this application.
-  max_open_connections: 10
+  max_open_connections: 15

The max open connections value got up to 12 before the application completed.

Once I inserted the test data, the open connections dropped to 1. This is testing against a local MariaDB 10.0.x instance, so the behavior is likely different against a remote database server, or even a newer version of MariaDB.