Clean up AppFramework database layer

[BernhardPosselt]

The current approach adds more abstraction that is currently needed which makes it harder to debug database related issues. Furthermore there are problems with our internal API (Statement Wrapper) whose multiple return types (bool, Statement) cause some headaches when trying to close cursors properly for the most common cases, see #14459

Current status

Currently the classes that you typically use are called in the following order when running a method on the mapper class

• Mapper
• IDb
• Db
• StatementWrapper
• IDBConnection
• Connection

The goal is to get it down to this:

• Mapper
• IDBConnection
• Connection

How can we do this?

Instead of requiring an instance of IDb for the Mapper, we are going to require an IDBConnection. In order to stay compatible until this can be removed after deprecation, we are going to do the following:

• Deprecate IDb and Db
• IDb inherits from IDBConnection
• Db will forward calls to IDBConnection using the __call method (much like this here: https://github.com/owncloud/core/blob/master/lib/private/db/statementwrapper.php#L47) or just proxy it if required by the interface. Previous methods on the class itself will keep working, but will be deprecated
• Mapper class will be adjusted to use IDBConnection and the methods on that class
• Statementwrapper will be removed from Db

At some point we will be able to return an instance of IDBConnection from the ServerContainer instead of IDb to finish the migration and finally remove the code.

@DeepDiver1975 @MorrisJobke @PVince81 @LukasReschke @butonic @icewind1991 @georgehrke @jbtbnl

[BernhardPosselt]

PS: as for the statement wrapper class:

• Logging is already done in the connection class: https://github.com/owncloud/core/blob/master/lib/private/db/connection.php#L105
• MSSQL stuff in there is weird. @bartv2 do you know if this is still needed? Should be handled in doctrine

[BernhardPosselt]

Hm, migration is even easier, just use OCP\IDBConnection in the mapper constructor instead of OCP\IDb ^^