Callbacks or pre-migration scripts to aid testing

[rajeevs]

Is your feature request related to a problem? Please describe.
I have some migrations that backfill data into a new table using another table.

I want to be able to test that the backfill migration worked correctly, but in order to test it, I need to insert data into the original table before the migrations run.

Overall, there are situations in test where you want to insert data into the DB to test migrations are correct. Testing on empty DB is one use case, but testing when data is not empty mirrors production more.

Describe the solution you'd like
It would be ideal if there was a callback mechanism / event handler that we can register with golang-migrate that executes before/after a migration runs. (event params could be eventType:[Before/After], and migrationName and a db connection/session object).

We would use this during test only to intercept pre-migration event for backfill migration, and insert the data we want into the original table. Then we would check at the end of all migrations being run that the state of DB is what is expected.

This is similar to FlyWay Java callbacks. https://documentation.red-gate.com/fd/callback-concept-184127466.html

This would also address situations like #136 which could be achieved using a callback (to insert data).

Note that what I propose is an optional way to add callbacks in special situations. It should not affect existing use cases.

Describe alternatives you've considered
Other solutions, could be

1. A SQL callback: You provide a SQL file (named in a suitable way to indicate which migration it is connected to). This is similar to Flyway SQL callbacks. I think programmatic is better, so we can insert it only in test code.
2. Another would be for Up to run till a specific migration (like Up(sentinelMigrationId, inclusive)). Then we could run

Up(BakfillMigrationId, false)
// insert some test data
Up() // run everything
// test backfill was correct

But I think my proposed solution is better.

Additional context
Of course, we are opening up a way for people to run code in the migration sequence.
But it is not magical, and someone would have to explicitly register a callback.

[rajeevs]

@dhui : Your thoughts?