Add BeforeConnect callback to configuration object

[ItalyPaleAle]

Description

This can be used to alter the connection options for each connection, right before it's established.

Our use case is that we want to connect to a MySQL server which supports authentication using a JWT (in the specific case, Azure Database for MySQL using tokens from Azure AD). The token is to be used in place of a password. Because the tokens have a limited lifetime, we cannot create a Config object with a hardcoded password, because connections in the pool can be created later in time and the token may be expired. By using the BeforeConnect callback, instead, we can ensure that the Config object used to open the connection has a fresh password every time.

This design is modeled after the "BeforeConnect" property offered by pgx.

Although our use case is for updating the password, there may be other uses for a BeforeConnect callback. The callback can also return an error to abort the connection.

We plan to use this for Dapr: dapr/components-contrib#2955

You can see how we implemented this for Postgres (using pgx) here: https://github.com/dapr/components-contrib/blob/566c7fd31adbd81740ba66995729a7eadeb3444a/internal/authentication/postgresql/metadata.go#L97-L109

Checklist

• [X] Code compiles correctly
• [X] Created tests which fail without the change (if possible)
• [X] All tests passing
• [X] Extended the README / documentation, if necessary
• [X] Added myself / the copyright holder to the AUTHORS file

Summary by CodeRabbit

• New Features

  • Introduced a pre-connection configuration step, allowing custom logic before database connections are established.

• Bug Fixes

  • Ensured the correct database is targeted during connections by implementing additional checks.

• Tests

  • Added new tests to verify the database connection process and pre-connection configurations.

[ItalyPaleAle]

@methane Would it be possible to have this PR considered please? Or please, let me know if there's some other process I should follow.

[methane]

wait. I'm very busy and sick.

[ItalyPaleAle]

Sorry to hear that, hope you get better soon 🙁 No worries, we're fine with waiting

[coderabbitai[bot]]

Walkthrough

The changes involve enhancing a database connection library to support pre-connection configuration adjustments. A new BeforeConnect callback function has been introduced, allowing custom logic to be executed before a connection is established. This functionality is tested with a new test case ensuring the connection is correctly modified before use. The changes affect the connection process, configuration structure, and testing suite to ensure the new feature works as intended.

Changes

File(s)	Change Summary
connector.go, dsn.go	Introduced BeforeConnect callback in Config and updated Connect method to handle pre-connection configuration changes. Added context package import.
driver_test.go	Added TestBeforeConnect function to test the new BeforeConnect feature and ensure proper database connection handling.

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[coveralls]

coverage: 82.055% (+0.1%) from 81.919% when pulling 6a4e24ee11ef6a81c552d1f4bbbe4a8e374bfb74 on ItalyPaleAle:add-beforeconnect into 6964272ffd13a41ad66383cd2ea738fded75ad06 on go-sql-driver:master.

[methane]

@shogo82148 How do you think this new API?

[methane]

Although I like this feature, I want to wait adding more fields to Config object. Please wait for the discussion in https://github.com/go-sql-driver/mysql/discussions/1548

[dolmen]

This seems to be an unnecessary solution to a true problem.

Instead the user should have created its own database/sql.Driver and database/sql.Connector that would build a *mysql.Config on the call to Connector's Connect and call mysql.NewConnector() and wrap its Connect.

See https://github.com/dolmen-go/mylogin-driver/blob/master/driver.go as an example of a database/sql.Driver that wraps github.com/go-sql-driver/mysql that implements a different DSN.