In the ChainDAG, 3 block pointers are kept: genesis, tail and head. This
PR adds one more block pointer: the backfill block which represents the
block that has been backfilled so far.
When doing a checkpoint sync, a random block is given as starting point
- this is the tail block, and we require that the tail block has a
corresponding state.
When backfilling, we end up with blocks without corresponding states,
hence we cannot use `tail` as a backfill pointer - there is no state.
Nonetheless, we need to keep track of where we are in the backfill
process between restarts, such that we can answer GetBeaconBlocksByRange
requests.
This PR adds the basic support for backfill handling - it needs to be
integrated with backfill sync, and the REST API needs to be adjusted to
take advantage of the new backfilled blocks when responding to certain
requests.
Future work will also enable moving the tail in either direction:
* pruning means moving the tail forward in time and removing states
* backwards means recreating past states from genesis, such that
intermediate states are recreated step by step all the way to the tail -
at that point, tail, genesis and backfill will match up.
* backfilling is done when backfill != genesis - later, this will be the
WSS checkpoint instead
* BlockId reform
Introduce `BlockId` that helps track a root/slot pair - this prepares
the codebase for backfilling and handling out-of-dag blocks
* move block dag code to separate module
* fix finalised state root in REST event stream
* fix finalised head computation on head update, when starting from
checkpoint
* clean up chaindag init
* revert `epochAncestor` change in introduced in #3144 that would return
an epoch ancestor from the canoncial history instead of the given
history, causing `EpochRef` keys to point to the wrong block
* Introduce slot->BlockRef mapping for finalized chain
The finalized chain is linear, thus we can use a seq to lookup blocks by
slot number.
Here, we introduce such a seq, even though in the future, it should
likely be backed by a database structure instead, or, more likely, a
flat era file with a flat lookup index.
This dramatically speeds up requests by slot, such as those coming from
the REST interface or GetBlocksByRange, as these are currently served by
a linear iteration from head.
* fix REST block requests to not return blocks from an earlier slot when
the given slot is empty
* fix StateId interpretation such that it doesn't treat state roots as
block roots
* don't load full block from database just to return its root
* move quarantine outside of chaindag
The quarantine has been part of the ChainDAG for the longest time, but
this design has a few issues:
* the function in which blocks are verified and added to the dag becomes
reentrant and therefore difficult to reason about - we're currently
using a stateful flag to work around it
* quarantined blocks bypass the processing queue leading to a processing
stampede
* the quarantine flow is unsuitable for orphaned attestations - these
should also should be quarantined eventually
Instead of processing the quarantine inside ChainDAG, this PR moves
re-queueing to `block_processor` which already is responsible for
dealing with follow-up work when a block is added to the dag
This sets the stage for keeping attestations in the quarantine as well.
Also:
* make `BlockError` `{.pure.}`
* avoid use of `ValidationResult` in block clearance (that's for gossip)
Jacek Sieka