Client support for non-leader connections

[cole-miller]

This is the go-dqlite counterpart of canonical/dqlite#709, tests are expected to fail since that hasn't merged. We expose the new functionality with a query parameter in the filename passed to OpenConnector. The usage is

connector := dqlitedriver.OpenConnector("file:my_db_name?dqlite_direct=TARGET")
conn, err := connector.Connect(ctx)
// conn is a database/sql/driver.Conn, needs some type-asserting to use

This gives you a usable connection to the node with address TARGET, which need not be the leader. You can prepare statements and run queries.

Signed-off-by: Cole Miller [email protected]

[sabaini]

Would this API make it possible to force leader reads for specific queries?

[cole-miller]

@sabaini With the current state of the PR you'd have to store two DB connections, a "normal" one that talks to the currrent leader and a special one created using the new API that talks to a specific (possibly non-leader) node. I've implemented it this way because go-dqlite is built on a model where one DB connection = one network connection to a dqlite server; moving away from that would introduce a lot more complexity, especially with the database/sql connection pool in the mix.