SP pallet (hooks): pre-commit hook

[aidan46]

Pre-commit hook

This hook checks pre-committed sectors for all SPs and checks if the sector is expired (no proof submitted for X amount of time). If the sector is expired, it should remove that sector and burn the SPs initial deposit.

Reference:

• https://github.com/filecoin-project/builtin-actors/blob/17ede2b256bc819dc309edf38e031e246a516486/actors/miner/src/lib.rs#L3293-L3301

[th7nder]

It's handled in handle_proving_deadline, which is executed in on_deferred_cron_event .

Pre-commits are stored in state.pre_committed_sectors_cleanup, and are modified by two functions:

• cleanup_expired_pre_commits -> https://github.com/filecoin-project/builtin-actors/blob/82d02e58f9ef456aeaf2a6c737562ac97b22b244/actors/miner/src/state.rs#L1071
• add_pre_commit_cleanup -> https://github.com/filecoin-project/builtin-actors/blob/82d02e58f9ef456aeaf2a6c737562ac97b22b244/actors/miner/src/state.rs#L1054

They are added to add_precommit on pre_commit in storage provider.

I'm yet to figure out when CRON_EVENT_PROVING_DEADLINE is scheduled and executed.

[th7nder]

Cron Events are scheduled for a Miner Actor on a deadline for a pre-commit.

https://github.com/filecoin-project/builtin-actors/blob/17ede2b256bc819dc309edf38e031e246a516486/actors/miner/src/lib.rs#L1737

[th7nder]

As we don't have neither Miner Actor, nor 'scheduled crons', only have hooks, we need to 'schedule' it differently.
We need a data structure, that'll hold Pre-commits, indexed by BlockNumber, that were supposed to be activated by a given time.
The hook, which will be executed on every block, will check whether we got something to slash. If we have it, then we will.
Prove Commit will delete it from this data structure.

[th7nder]

The first (#125) proposed implementation works like this:

• On every block (in on_finalize), go through every Storage Provider and every Pre-Commit Sector (no matter the order) and see whether they were expired or not. It's O(StorageProviders * Sectors * Blocks) algorithm and at some point it maybe to heavy to be executed in on_finalize. Because it's supposed to be very short and it's not, it can affect block production.

I propose to make a O(Sectors * Blocks) algorithm (as in FileCoin), so -> indexing pre-commits by their expiration (BlockNumber). Then the work done in on_finalize is optimal, as each blocks only accesses data relevant to the block, without further scans.

[jmg-duarte]

Go with bad complexity impl and document the good approach for refactoring