Refactor FundManager

[dirkmc]

Currently in markets we have

• a mechanism to reserve and release funds needed for a particular deal (no on-chain messages)
• a mechanism to withdraw funds that must respect the reserved amount (calls withdraw on market actor)
• a mechanism to ensure reserved funds are available when starting a deal (calls add funds on market actor)

This PR unifies the logic for these mechanisms into a single interface:

// Reserve adds amt to `reserved`. If there are not enough available funds for
// the address, submits a message on chain to top up available funds.
Reserve(ctx context.Context, wallet, addr address.Address, amt abi.TokenAmount) (cid.Cid, error)

// Subtract from `reserved`.
Release(addr address.Address, amt abi.TokenAmount)

// Withdraw unreserved funds. Only succeeds if there are enough unreserved
// funds for the address.
Withdraw(ctx context.Context, wallet, addr address.Address, amt abi.TokenAmount) (cid.Cid, error)

TODO:

• Implement the logic change in go-fil-markets Integrate new FundManager go-fil-markets#445
• Implement data migration from markets DealFunds to lotus FundManager

[hannahhoward]

Early comments:

Overall, this is great. The code for the loop to handle reserves/releases/withdrawals is really good.

I have two changes, one fairly simple and the other... unfortunately not.

The first is your WithdrawFunds won't work as written. WithdrawBalance is a seperate method in the market actor.

The second is, we have to account for multiple wallets, and I'm not sure the best way to do this. Right now, at least on the client side, we assume that any wallet the client owns can initiate the deal. It's baked into the call to StorageClient.ProposeStorageDeal && all the way up to the Lotus full node API for ClientStartDeal. So, assuming a universal source wallet probably doesn't work. On the provider side, this is actually true -- the miner owner/worker address is always the source. However, there's no getting around it on the client. I'm sorry if my method signatures I provided obscured that fact.

I don't think the change is particularly hard, except your current code combines reserve requests & withdraw requests on the assumption of the same wallet. I believe the only way to ultimately address this is to add a source wallet parameter to Reserve and Withdraw, and to only combine messages for the same wallet address at the same time (deferring the messages for other wallets for the next message submission for the given destination address)

[arajasek]

Fixes #2685

[hannahhoward]

LGTM, pending integration. Remaining suggestions are non-blocking, up to you.

[hannahhoward]

I am marking this ready for review because I believe it can be merged without having any significant effect -- while there is new code, it is not actually active in Lotus yet as the integration aspects of this PR are not complete, and covered seperately in #4787 & filecoin-project/go-fil-markets#445

[hannahhoward]

IOW, there is no harm in code reviewing this code, merging to master, in order to limit the challenge of reviewing #4787

[arajasek]

LGTM