Add finalize API to EC backend

Expand the F3 EC backend API to mark a given tipset as final. This
translates to checkpointing the tipset in Lotus beyond which no forks
are allowed.

Part of #603

Author: masih

ec/ec.go

@@ -14,12 +14,21 @@ type Backend interface {
 	GetTipsetByEpoch(ctx context.Context, epoch int64) (TipSet, error)
 	// GetTipset returns the tipset with the given key.
 	GetTipset(context.Context, gpbft.TipSetKey) (TipSet, error)
-	// GetHead returns the current head tipset of the chain
+	// GetHead returns the current head tipset of the chain, which is guaranteed to
+	// be a descendant of the latest finalized tipset.
+	//
+	// See FinalizeTipset.
 	GetHead(context.Context) (TipSet, error)
 	// GetParent returns the parent of the current tipset.
 	GetParent(context.Context, TipSet) (TipSet, error)
 	// GetPowerTable returns the power table at the tipset given as an argument.
 	GetPowerTable(context.Context, gpbft.TipSetKey) (gpbft.PowerEntries, error)
+	// FinalizeTipset marks the given tipset as finalised beyond which no forks are
+	// allowed to occur. The finalised tipset overrides the head tipset if it is not
+	// an ancestor of the current head.
+	//
+	// See GetHead.
+	FinalizeTipset(context.Context, TipSet) error
 }
 
 type TipSet interface {

internal/consensus/fake_ec.go

@@ -183,3 +183,5 @@ func (ec *FakeEC) GetTipset(_ context.Context, tsk gpbft.TipSetKey) (ec.TipSet,
 	epoch := binary.BigEndian.Uint64(tsk[6+32-8 : 6+32])
 	return ec.genTipset(int64(epoch)), nil
 }
+
+func (ec *FakeEC) FinalizeTipset(_ context.Context, ts ec.TipSet) error { return nil }