Migrations may fail if started from replica

[opomuc]

So, exact bug reproduction scenario is thw following:

• some time-consuming ddl-changing migration is added in production (e. g. changing space format on an empty space)
• admin triggers migrator.up on a replica node (let's call is coordinator)
• coordinator triggers migrations on all replicaset leaders (including leader of its own replicaset)
• all leaders apply migrations and respond 'ok' to coordinator
• space format change is sent to coordinator from its leader via replication channel, and it takes considerable time to apply, so coordinator's actual ddl remains unchanged for some time
• upon receiving 'ok's from all leaders, coordinator triggers config.patch_clusterwide with supposedly new ddl schema, which it collects from local spaces
• BUT since schema is not yet changed on coordinator itself (since coordinator is async lagging replica), it tries to apply "old" ddl, and fails total operation with smth like CheckSchemaError: Incompatible schema: spaces["somespace"] //format/3 (expected table, got nil)

[Totktonada]

Just checking of box.info.ro is okay? Should this behaviour work under an option or by default?

[sharonovd]

Proposed fix (in linked PR) is that coordinator should fetch ddl schema from a master, instead of colleting it locally.