fix(sync): enforce ForkLengthThreshold for synced chain (filecoin-pro…

…ject#5182)

chain/sync.go

@@ -1432,7 +1432,7 @@ func (syncer *Syncer) syncFork(ctx context.Context, incoming *types.TipSet, know
 		return nil, ErrForkCheckpoint
 	}
 
-	// TODO: Does this mean we always ask for ForkLengthThreshold blocks from the network, even if we just need, like, 2?
+	// TODO: Does this mean we always ask for ForkLengthThreshold blocks from the network, even if we just need, like, 2? Yes.
 	// Would it not be better to ask in smaller chunks, given that an ~ForkLengthThreshold is very rare?
 	tips, err := syncer.Exchange.GetBlocks(ctx, incoming.Parents(), int(build.ForkLengthThreshold))
 	if err != nil {
@@ -1443,6 +1443,10 @@ func (syncer *Syncer) syncFork(ctx context.Context, incoming *types.TipSet, know
 	if err != nil {
 		return nil, xerrors.Errorf("failed to load next local tipset: %w", err)
 	}
+	// Track the fork length on our side of the synced chain to enforce
+	// `ForkLengthThreshold`. Initialized to 1 because we already walked back
+	// one tipset from `known` (our synced head).
+	forkLengthInHead := 1
 
 	for cur := 0; cur < len(tips); {
 		if nts.Height() == 0 {
@@ -1459,6 +1463,13 @@ func (syncer *Syncer) syncFork(ctx context.Context, incoming *types.TipSet, know
 		if nts.Height() < tips[cur].Height() {
 			cur++
 		} else {
+			// Walk back one block in our synced chain to try to meet the fork's
+			// height.
+			forkLengthInHead++
+			if forkLengthInHead > int(build.ForkLengthThreshold) {
+				return nil, ErrForkTooLong
+			}
+
 			// We will be forking away from nts, check that it isn't checkpointed
 			if nts.Key() == chkpt {
 				return nil, ErrForkCheckpoint