Update 004_pending_and_more migration to add pending value to river_job_state only if it does not exists

[umed]

Hi guys,

004_pending_and_more.down.sql migration does not remove river_job_state's pending value. It brings us to situation where we can't apply this migration after rollback.

Steps to reproduce:

1. apply migration 004 to database
2. rollback
3. apply migration 004

Expected behavior:

All migrations applied without error

Actual:

failed: error applying version 004 [UP]: ERROR: enum label "pending" already exists (SQLSTATE 42710)
exit status 1

This PR is meant to fix this behavior by optionally adding pending value during migration application.

[umed]

I tested it just by executing queries on locally running postgres. I would gladly follow if you point out to how properly test it.

[bgentry]

Thoughts @brandur ?

[bgentry]

Normally we don’t want to use IF NOT EXISTS in these migrations because they indicate a bug or something has been manually manipulated. But, in this case I’m reminded that you can’t safely remove an enum value in Postgres once it’s been added, which is why it was skipped in the down migration.

Maybe this is this best we can do in this case?

[brandur]

Yeah, I don't see any other viable options.

The migration in its current form isn't right because it makes the rollback unusable given it can't be used to roll forward again.

A possibility is to to an enum swap by creating a whole new enum without pending, then swap it into place on state, and remove the old enum. I'm sure that'd need a full table scan to check the type though.

So yep, looks like this is the path.

[brandur]

I'll add a changelog separately.