feat/2841 - compute collateral as a function of sector size + track collateral as actor balance + collateral check before adding new commitments

[laser]

Makes incremental progress towards #2530.

Fixes #2841.

Why does this PR exist?

The spec demonstrates that a miner's collateral is its balance, and the collateral which is has locked up for sectors which it is actively proving must be tracked via the ActiveCollateral field. This field's value must be incremented when a sector is committed, and commitments should be rejected if the miner doesn't have enough collateral to cover a new commitment.

The spec also demonstrates that per-sector collateral is a function of sector size. We believe that this will change soon (see Pledge Collateral Design Doc (Baseline (Pro-rata) Construction)), but this moves the codebase in the right direction.

What's in this PR?

• fix the broken retrieval script (as of init creates a symlink to actual repo directory. #2702, repo dir must not exist when init is called)
• track miner collateral via its balance
• add a method to VM context called MyBalance which does what you'd expect (this is a specified function)
• verify miner has enough collateral to cover commitments

[acruikshank]

I have concerns about adding a constructor for miner actors that takes a balance. Other than that, this PR looks solid.

[acruikshank]

This is a little sketchy. This only gets used in tests an genesis, but it suggests you can create a miner actor in a state transition with a non-zero balance. This would be really bad, since it would be generating FIL from nowhere. For clarity's sake, I'd rather you not add balance to the constructor and set it manually after the actor is constructed.

[laser]

For clarity's sake, I'd rather you not add balance to the constructor and set it manually after the actor is constructed.

OK. This change is easy to roll back. I introduced it to be consistent with the account actor-creation constructor. Do you think that the same logic applies to the account actor?

[acruikshank]

🙏

[ZenGround0]

LGTM. It's worth checking the wiki and potentially making an update explaining that there is a minimum collateral during the miner setup workflow (though it might not affect anything if users run with the default FIL values).

[laser]

@ZenGround0

It's worth checking the wiki and potentially making an update explaining that there is a minimum collateral during the miner setup workflow (though it might not affect anything if users run with the default FIL values).

Sounds good. I'm about to delete the pledge concept from the storage miner (and corresponding miner create command). I'll update the wiki to reflect both changes simultaneously.