FIP-0076 Add Market::GetDealSector method

[anorth]

Implementation is filecoin-project/builtin-actors#1443

[jennijuju]

Peer q: How useful is this api without a GetSectorInfo like api? One use case I can think of is using this to figure out when the data will actually mo longer be stored in a sector and trigger a renew deal when it’s close to that time, after finding out the sector i will need to further getting the sector start epoch & duration to get the expiration of the sector.

(would it be useful to add getsectorinfo api as well?

[anorth]

How useful is this api without a GetSectorInfo like api? One use case I can think of is using this to figure out when the data will actually mo longer be stored in a sector and trigger a renew deal when it’s close to that time

The only thing a client should rely on to understand when data will no longer be stored is the deal's duration. The fact that today one can, externally, tell how much longer the data is committed is an artefact of our very limited system that prevents replacing stale data.

I don't know exactly how useful this API will be. Most likely, today, it could be useful in concert with direct state inspection from off-chain. This is one step towards reducing the need for direct state inspection. I don't really think this step should be blocked on having not taken all the other ones at the same time, but I'll just as happily retract this change.