Keep Cache and Env alive with Rc

[acrrd]

Fix #462.

• Add a lifetime to Checkpoint to ensure that it does not outlive the DB
• Wrap Cache and Env in an Rc and keep a copy of that in the database to ensure that they are not dropped before the database is.

This add two auxiliary structs where to keep Cache and Env that are set using set_row_cache in Options or set_block_cache, set_block_compressed in BlockBasedOptions. When a DB is open this structs are cloned and kept in the database to ensure the lifetime requirements are met.

This approach allow us to have an unchanged api (with the exception of Checkpoint life time) but the compiler will not help us if we forget to keep track of something. Implementing this with lifetime will require to have four lifetimes parameters in (at least), Options, BlockBasedOptioins, DB, and ColumnFamilyDescriptor.

Do we need to keep track of Cache and Env that are in the Options inside ColumnFamilyDescriptor when we open a database through them?

[stanislav-tkach]

Looking good to me! 👍

[stanislav-tkach]

Do we need to keep track of Cache and Env that are in the Options inside ColumnFamilyDescriptor when we open a database through them?

I'm not one hundred percent sure, but I'm afraid that yes. I need to check the RocksDB code more thoroughly, though.

[aleksuss]

Does it really need to be public rather than pub(crate) ? If so then I'd rename it to BlockBasedCache or something because it seems weird name as for public API.

[acrrd]

It could be private!

[acrrd]

I'm saving also the Options from a ColumnFAmilyDescriptor, we can remove this later if we see that is not needed.
We can have two databases crated with the same Env and Cache and send them to two different thread, so I changed Rc to Arc. I would like to add the test is_sync<OptionsMustOutliveDb> and is_send<OptionsMustOutliveDb> but at the moment Env and Cache are not marker Send/Sync. For Env a comment says that is safe to implemented them, I think would be ok to implement them also for Cache since it can be used from different database in different thread.

[aleksuss]

@acrrd resolve please conflicts. Thanks.

[acrrd]

Done,thanks.

[aleksuss]

LGTM