Fix migrations check at startup with a read-only connection

[kind84]

Closes #1653.

Background

The rpc node is supposed to connect to the state database using a read-only connection, in order to not overwhelm the RW connection. This is made possible by the changes introduced in #1531 that allow the rpc node to not run migrations.

What does this PR do?

Unfortunately it turns out that the sql-migrate library requires a RW connection even for tasks that do not actually run the migrations. In our case we were calling migrate.PlanMigration and migrate.GetMigrationRecords but both fail on a RO connection.

To overcome this, here the proposal is to directly query the migrations table and compare the number of migrations found there with the number of Up migrations in our source code (by inspecting list the returned by migrationSource.FindMigrations()).

The solution has been tested to be working on the develop testnet. It may sound a bit "hacky" and thus alternative ideas/approaches are welcome.

Reviewers

Main reviewers:

• @tclemos
• @ARR552
• @arnaubennassar

[arnaubennassar]

LGTM, but I think that this check is only done for the StateDB, it would be nice to do it for the PoolDB as well

EDIT: you can create issue and do this in a future PR if you prefere

[ARR552]

LGTM, but I think that this check is only done for the StateDB, it would be nice to do it for the PoolDB as well

EDIT: you can create issue and do this in a future PR if you prefere

@arnaubennassar As far as I know we can not have the pool db in readonly mode because the rpc needs to store new transactions on the pool db. Maybe we need to allow the rpc to handle read and write replicas. For instance, if the request is for getting if the gas price, then it should use a read replica, if the method is eth_sendRawTransaction, then it should be able to write into the pool db

[kind84]

@ARR552 I tested this on develop testnet against all the 3 possible scenarios:

• Clean start from an empty state DB
• Start coming from a previous version, with some migrations to apply
• Start from an up to date version with all migrations already applied

I didn't face any issue with this implementation.

In addition, I added a log message to inform that the check has been successful b4aebed

WDYT?