## Issue Addressed
Closes#1866
## Proposed Changes
* Compact the database on finalization. This removes the deleted states from disk completely. Because it happens in the background migrator, it doesn't block other database operations while it runs. On my Medalla node it took about 1 minute and shrank the database from 90GB to 9GB.
* Fix an inefficiency in the pruning algorithm where it would always use the genesis checkpoint as the `old_finalized_checkpoint` when running for the first time after start-up. This would result in loading lots of states one-at-a-time back to genesis, and storing a lot of block roots in memory. The new code stores the old finalized checkpoint on disk and only uses genesis if no checkpoint is already stored. This makes it both backwards compatible _and_ forwards compatible -- no schema change required!
* Introduce two new `INFO` logs to indicate when pruning has started and completed. Users seem to want to know this information without enabling debug logs!
## Issue Addressed
Closes#800Closes#1713
## Proposed Changes
Implement the temporary state storage algorithm described in #800. Specifically:
* Add `DBColumn::BeaconStateTemporary`, for storing 0-length temporary marker values.
* Store intermediate states immediately as they are created, marked temporary. Delete the temporary flag if the block is processed successfully.
* Add a garbage collection process to delete leftover temporary states on start-up.
* Bump the database schema version to 2 so that a DB with temporary states can't accidentally be used with older versions of the software. The auto-migration is a no-op, but puts in place some infra that we can use for future migrations (e.g. #1784)
## Additional Info
There are two known race conditions, one potentially causing permanent faults (hopefully rare), and the other insignificant.
### Race 1: Permanent state marked temporary
EDIT: this has been fixed by the addition of a lock around the relevant critical section
There are 2 threads that are trying to store 2 different blocks that share some intermediate states (e.g. they both skip some slots from the current head). Consider this sequence of events:
1. Thread 1 checks if state `s` already exists, and seeing that it doesn't, prepares an atomic commit of `(s, s_temporary_flag)`.
2. Thread 2 does the same, but also gets as far as committing the state txn, finishing the processing of its block, and _deleting_ the temporary flag.
3. Thread 1 is (finally) scheduled again, and marks `s` as temporary with its transaction.
4.
a) The process is killed, or thread 1's block fails verification and the temp flag is not deleted. This is a permanent failure! Any attempt to load state `s` will fail... hope it isn't on the main chain! Alternatively (4b) happens...
b) Thread 1 finishes, and re-deletes the temporary flag. In this case the failure is transient, state `s` will disappear temporarily, but will come back once thread 1 finishes running.
I _hope_ that steps 1-3 only happen very rarely, and 4a even more rarely. It's hard to know
This once again begs the question of why we're using LevelDB (#483), when it clearly doesn't care about atomicity! A ham-fisted fix would be to wrap the hot and cold DBs in locks, which would bring us closer to how other DBs handle read-write transactions. E.g. [LMDB only allows one R/W transaction at a time](https://docs.rs/lmdb/0.8.0/lmdb/struct.Environment.html#method.begin_rw_txn).
### Race 2: Temporary state returned from `get_state`
I don't think this race really matters, but in `load_hot_state`, if another thread stores a state between when we call `load_state_temporary_flag` and when we call `load_hot_state_summary`, then we could end up returning that state even though it's only a temporary state. I can't think of any case where this would be relevant, and I suspect if it did come up, it would be safe/recoverable (having data is safer than _not_ having data).
This could be fixed by using a LevelDB read snapshot, but that would require substantial changes to how we read all our values, so I don't think it's worth it right now.
* Layer do_atomically() abstractions properly
* Reduce allocs and DRY get_key_for_col()
* Parameterize HotColdDB with hot and cold item stores
* -impl Store for MemoryStore
* Replace Store uses with HotColdDB
* Ditch Store trait
* cargo fmt
* Style fix
* Readd missing dep that broke the build
* Remove redundant method
* Pull out a method out of a struct
* More precise db access abstractions
* Move fake trait method out of it
* cargo fmt
* Fix compilation error after refactoring
* Move another fake method out the Store trait
* Get rid of superfluous method
* Fix refactoring bug
* Rename: SimpleStoreItem -> StoreItem
* Get rid of the confusing DiskStore type alias
* Get rid of SimpleDiskStore type alias
* Correction: A method took both self and a ref to Self
* Improve error handling in block iteration
* Introduce atomic DB operations
* Fix race condition
An invariant was violated: For every block hash in head_tracker, that
block is accessible from the store.
* Add LRU caches to store
* Improvements to LRU caches
* Take state by value in `Store::put_state`
* Store blocks by value, configurable cache sizes
* Use a StateBatch to efficiently store skip states
* Fix store tests
* Add CloneConfig test, remove unused metrics
* Use Mutexes instead of RwLocks for LRU caches
* Implement freezer database for state vectors
* Improve BeaconState safe accessors
And fix a bug in the compact committees accessor.
* Banish dodgy type bounds back to gRPC
* Clean up
* Switch to exclusive end points in chunked vec
* Cleaning up and start of tests
* Randao fix, more tests
* Fix unsightly hack
* Resolve test FIXMEs
* Config file support
* More clean-ups, migrator beginnings
* Finish migrator, integrate into BeaconChain
* Fixups
* Fix store tests
* Fix BeaconChain tests
* Fix LMD GHOST tests
* Address review comments, delete 'static bounds
* Cargo format
* Address review comments
* Fix LMD ghost tests
* Update to spec v0.9.0
* Update to v0.9.1
* Bump spec tags for v0.9.1
* Formatting, fix CI failures
* Resolve accidental KeyPair merge conflict
* Document new BeaconState functions
* Fix incorrect cache drops in `advance_caches`
* Update fork choice for v0.9.1
* Clean up some FIXMEs
* Fix a few docs/logs
* Update for new builder paradigm, spec changes
* Freezer DB integration into BeaconNode
* Cleaning up
* This works, clean it up
* Cleanups
* Fix and improve store tests
* Refine store test
* Delete unused beacon_chain_builder.rs
* Fix CLI
* Store state at split slot in hot database
* Make fork choice lookup fast again
* Store freezer DB split slot in the database
* Handle potential div by 0 in chunked_vector
* Exclude committee caches from freezer DB
* Remove FIXME about long-running test
