Miner: Simplify sector number allocation during precommit

[wadealexc]

Miner.PreCommitSector requires that params.SectorNumber has not been previously allocated, as sector numbers should not be reused:

specs-actors/actors/builtin/miner/miner_actor.go

Lines 445 to 461 in 9d42fb1

	err := st.AllocateSectorNumber(store, params.SectorNumber)
	builtin.RequireNoErr(rt, err, exitcode.ErrIllegalState, "failed to allocate sector id %d", params.SectorNumber)
	// The following two checks shouldn't be necessary, but it can't
	// hurt to double-check (unless it's really just too
	// expensive?).
	_, preCommitFound, err := st.GetPrecommittedSector(store, params.SectorNumber)
	builtin.RequireNoErr(rt, err, exitcode.ErrIllegalState, "failed to check pre-commit %v", params.SectorNumber)
	if preCommitFound {
	rt.Abortf(exitcode.ErrIllegalState, "sector %v already pre-committed", params.SectorNumber)
	}
	sectorFound, err := st.HasSectorNo(store, params.SectorNumber)
	builtin.RequireNoErr(rt, err, exitcode.ErrIllegalState, "failed to check sector %v", params.SectorNumber)
	if sectorFound {
	rt.Abortf(exitcode.ErrIllegalState, "sector %v already committed", params.SectorNumber)
	}

As the comment states, the checks following the call to AllocateSectorNumber are not entirely necessary, but they may be helpful sanity checks.

Assuming they're being kept, I propose moving the existence checks to the AllocateSectorNumber method - they're functionally similar, and this would make PreCommitSector easier to parse.

[ZenGround0]

These duplicate checks have been removed

[ZenGround0]

• In v3 we modified HAMT to have PutIfAbsent which delegates a more efficient check to PutPrecommittedSectors
• At some point (I think also v3) we stopped the HasSectorNo check