## Issue Addressed
Closes#1488
## Proposed Changes
* Prevent the pruning algorithm from over-eagerly deleting states at skipped slots when they are shared with the canonical chain.
* Add `debug` logging to the pruning algorithm so we have so better chance of debugging future issues from logs.
* Modify the handling of the "finalized state" in the beacon chain, so that it's always the state at the first slot of the finalized epoch (previously it was the state at the finalized block). This gives database pruning a clearer and cleaner view of things, and will marginally impact the pruning of the op pool, observed proposers, etc (in ways that are safe as far as I can tell).
* Remove duplicated `RevertedFinalizedEpoch` check from `after_finalization`
* Delete useless and unused `max_finality_distance`
* Add tests that exercise pruning with shared states at skip slots
* Delete unnecessary `block_strategy` argument from `add_blocks` and friends in the test harness (will likely conflict with #1380 slightly, sorry @adaszko -- but we can fix that)
* Bonus: add a `BeaconChain::with_head` method. I didn't end up needing it, but it turned out quite nice, so I figured we could keep it?
## Additional Info
Any users who have experienced pruning errors on Medalla will need to resync after upgrading to a release including this change. This should end unbounded `chain_db` growth! 🎉
* 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.
* Address compiler warning
* Prune abandoned fork choice forks
* New approach to pruning
* Wrap some block hashes in a newtype pattern
For increased type safety.
* Add Graphviz chain dump emitter for debugging
* Fix broken test case
* Make prunes_abandoned_forks use real DiskStore
* Mark finalized blocks in the GraphViz output
* Refine debug stringification of Slot and Epoch
Before this commit: print!("{:?}", Slot(123)) == "Slot(\n123\n)".
After this commit: print!("{:?", Slot(123)) == "Slot(123)".
* Simplify build_block()
* Rewrite test case using more composable test primitives
* Working rewritten test case
* Tighten fork prunning test checks
* Add another pruning test case
* Bugfix: Finalized blocks weren't always properly detected
* Pruning: Add pruning_does_not_touch_blocks_prior_to_finalization test case
* Tighten pruning tests: check if heads are tracked properly
* Add a failing test case for a buggy scenario
* Change name of function to a more accurate one
* Fix failing test case
* Test case: Were skipped slots' states pruned?
* Style fix: Simplify dereferencing
* Tighten pruning tests: check if abandoned states are deleted
* Towards atomicity of db ops
* Correct typo
* Prune also skipped slots' states
* New logic for handling skipped states
* Make skipped slots test pass
* Post conflict resolution fixes
* Formatting fixes
* Tests passing
* Block hashes in Graphviz node labels
* Removed unused changes
* Fix bug with states having < SlotsPerHistoricalRoot roots
* Consolidate State/BlockRootsIterator for pruning
* Address review feedback
* Fix a bug in pruning tests
* Detach prune_abandoned_forks() from its object
* Move migrate.rs from store to beacon_chain
* Move forks pruning onto a background thread
* Bugfix: Heads weren't pruned when prune set contained only the head
* Rename: freeze_to_state() -> process_finalization()
* Eliminate redundant function parameter
Co-authored-by: Michael Sproul <michael@sigmaprime.io>
