feat: sync: harden chain sync

[Stebalien]

Proposed Changes

This makes two changes:

1. Write headers in chain order, not reverse order. This shouldn't matter, but it may help if badger crashes.
2. Force collectChain to collect the correct chain. This shouldn't matter, but we might as well (as long as it's safe).

WRT 1, I'm worried about the following:

1. Attempt to sync a chain from epochs 3-7.
2. Write tipsets 7, 6, 5.
3. Badger realizes that the current vlog is "full", so it creates a new one.
4. Write tipsets 4, 3.

My concern is that, while badger will write synchronously, it doesn't call fsync on the file itself, and the directory containing the file. That means, if the machine crashes, the new vlog might just disappear.

Of course, the right solution would be to fix badger to sync the file/directory metadata, but that's not likely to happen, ever.

Checklist

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• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
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• Tests exist for new functionality or change in behavior
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[arajasek]

@Stebalien I think the 2 changes you describe in the PR description make sense and are good to go. I don't understand why we're no longer putting the maybeHead, though.

[Stebalien]

I don't understand why we're no longer putting the maybeHead, though.

We should already be doing that here:

lotus/chain/sync.go

Lines 1191 to 1198 in 3f03e63

	for i := len(headers) - 1; i >= 0; i++ {
	ts := headers[i]
	if err := syncer.store.PersistTipset(ctx, ts); err != nil {
	err = xerrors.Errorf("failed to persist synced tipset to the chainstore: %w", err)
	ss.Error(err)
	return err
	}
	}

We check that headers[0] is the correct head here:

lotus/chain/sync.go

Lines 1184 to 1186 in 3f03e63

	if !headers[0].Equals(ts) {
	return xerrors.Errorf("collectChain synced %s, wanted to sync %s", headers[0].Cids(), ts.Cids())
	}

.

[arajasek]

@Stebalien I do not think you're right here -- this is PutTipsetting, which is tremendously unfortunately named, but is what actually:

• expands tipsets where possible
• actually updates the chain head (if necessary).

I think dropping that code will break everything, though I'm admittedly not 100% sure.

[Stebalien]

Oh! Yes, you're right. I saw P...TipSet and didn't even register that they were different functions.