Check for bad connections

[jethrogb]

There is currently no portable way to tell whether a database connection is still usable. Yes, queries might return an Error, but you can't easily tell from that whether it was just something with the query or database, or the connection. Also, when using connection pooling, you want to know when a connection is returned to the pool whether to retire it or not. This would require just querying the connection without knowing anything about its history.

I suggest adding an fn is_usable to trait Connection or similar. In Postgres, this can be implemented with PQstatus. MySQL hopefully has something similar.

[sgrif]

You can easily test this with connection.execute("SELECT 1").is_ok(). I don't think we need an explicit method for this.

[jethrogb]

That requires a network round-trip to complete. For the connection pooling use, that basically means every query now takes twice as long.

Also, that still doesn't let you distinguish between network errors and other random DB server errors.