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 must be a
+	// descendant of the latest finalized tipset.
+	//
+	// See Finalize.
 	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)
+	// Finalize marks the tipset that corresponds to the given key 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.
+	Finalize(context.Context, gpbft.TipSetKey) error
 }
 
 type TipSet interface {

host.go

@@ -15,10 +15,9 @@ import (
 	"github.com/filecoin-project/go-f3/internal/clock"
 	"github.com/filecoin-project/go-f3/internal/psutil"
 	"github.com/filecoin-project/go-f3/manifest"
-	"go.opentelemetry.io/otel/metric"
-
 	pubsub "github.com/libp2p/go-libp2p-pubsub"
 	peer "github.com/libp2p/go-libp2p/core/peer"
+	"go.opentelemetry.io/otel/metric"
 	"go.uber.org/multierr"
 	"golang.org/x/sync/errgroup"
 )
@@ -188,6 +187,18 @@ func (h *gpbftRunner) Start(ctx context.Context) (_err error) {
 }
 
 func (h *gpbftRunner) receiveCertificate(c *certs.FinalityCertificate) error {
+	// Call to Finalize checkpoints the tipset in Lotus chain store. This may cause a
+	// sync in a case where the target tipset is not already stored by the chain
+	// store, which is a blocking operation.
+	//
+	// Triggering the checkpointing here means that certstore remains the sole source
+	// of truth in terms of tipset has been finalised.
+	if err := h.ec.Finalize(h.runningCtx, c.ECChain.Head().Key); err != nil {
+		err := fmt.Errorf("error while finalizing decision at EC: %w", err)
+		log.Error(err)
+		return err
+	}
+
 	nextInstance := c.GPBFTInstance + 1
 	currentInstance := h.participant.CurrentInstance()
 	if h.participant.CurrentInstance() >= nextInstance {
@@ -428,7 +439,7 @@ func (h *gpbftHost) collectChain(base ec.TipSet, head ec.TipSet) ([]ec.TipSet, e
 	return res[1:], nil
 }
 
-func (h *gpbftRunner) Stop(_ctx context.Context) error {
+func (h *gpbftRunner) Stop(context.Context) error {
 	h.ctxCancel()
 	return multierr.Combine(
 		h.errgrp.Wait(),
@@ -624,7 +635,7 @@ func (h *gpbftHost) SetAlarm(at time.Time) {
 	// we cannot reuse the timer because we don't know if it was read or not
 	h.alertTimer.Stop()
 	if at.IsZero() {
-		// It "at" is zero, we cancel the timer entirely. Unfortunately, we still have to
+		// If "at" is zero, we cancel the timer entirely. Unfortunately, we still have to
 		// replace it for the reason stated above.
 		h.alertTimer = h.clock.Timer(0)
 		if !h.alertTimer.Stop() {

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) Finalize(context.Context, gpbft.TipSetKey) error { return nil }