Adding an "extras" column to transport databases

[ashwinraghav]

1. We add an extras column to the transport_contexts table.
2. We introduce migrations to the new state for existing clients. Table creates are also modeled as migrations.
3. Migrations are upgrade only. Downgrades are lossy and tables are dropped and recreated. Here's why
   1. When app developers downgrade their app, our SDKs potentially get downgraded (from say Vn to Vn-1). Since SDKs cannot know in advance what migrations are coming, Vn-1 will not know that (say) renaming a column will restore the database to the state expected by code in Vn-1.
   2. The other option would be to enforce all database migrations to be indefinitely backward compatible going forward. This will allow us to downgrade lossless-ly.
      Both options do not yield us enough value, with the potential to introduce bugs.
4. We test migrations using a combination of high level operations on the SQLiteEventStore and operations on SchemaManager.
5. We no longer need to inject SQL statements, since the behavior of the store can be controlled by injecting only the version of the database. (Sorry! 😞 )

[ashwinraghav]

/retest

[ashwinraghav]

/retest

[vkryachko]

I think this is wrong, and should instead be onUpgrade(0, newVersion), am I missing something?

[ashwinraghav]

Net effect is the same.

@Override
  public void onCreate(SQLiteDatabase db) {
    ensureConfigured(db);
    // schemaVersion here should be the same as the newVersion in onDowngrade.
    upgrade(db, 0, schemaVersion);
  }