I propose adding a new sql.DB.Close behavior/side effect: closing driver.Connector if it implements an optional io.Closer interface.

See CL 258360 / #41710

We would like to be able to manage the resources in driver.Connector, e.g. to share the same underlying database handle between multiple connections. This is required for some embedded databases with in-memory backends like SQLite and Genji so that we don’t create a new in-memory store whenever sql.DB decides to ask for a new connection by calling driver.Connector.Connect.

For in-memory storage go-sqlite3 driver docs currently advice manually setting cache=shared DSN parameter, and non-zero max idle connection limit and infinite connection lifetime on sql.DB. (see FAQ: Why I'm getting no such table error?).

It get worse with Genji since both on-disk BoltDB and BadgerDB engines lock the database. That is, connector can’t open the database until the existing handle is closed. With database/sql it means that either user must set connection limit to 1, or we have to share the handle between driver.Conn instances, which also allows us to support concurrent transactions with BadgerDB (see genjidb/genji#210).

That said, we need to release the resources held by driver.Connector (e.g. close file handle, unlock database or free unmanaged memory) when sql.DB is no longer needed.