Added database file bases lock

[pokehanai]

Current version of sqlite.so(~3.8.5) it seems that it needs to lock entire a database file during SQL access.

• not only row nor table but a file
• not only INSERT, UPDATE but also SELECT

Without lock, app will have either:

• database file disruption
• BUSY exception with much time overhead

To ensure that, I put logic around SQL execution to lock entire file automatically.

And to prepend deadlock I added more:

• RunInTransaction(Action) locks the database during the transaction (for insert/update)
• RunInDatabaseLock(Action) similarly locks the database but no transaction (for query)

P.S. lock() should have overheads, IMHO, it's small. It depends on runtime environment; I tested on several gadgets and it was 300ns or so.

[pokehanai]

I tested sqlite 3.9.2 on Android, still it needs the db file base lock.

[robertohuertasm]

Thanks @pokehanai! :+1: