Add HeadLookback parameter (#513)

* Add HeadLookback parameter

Signed-off-by: Jakub Sztandera <oss@kubuxu.com>

* use head lookback

* max

* check head lookback for equality

* flip things

* and fix head check

---------

Signed-off-by: Jakub Sztandera <oss@kubuxu.com>
Co-authored-by: Steven Allen <steven@stebalien.com>

host.go

@@ -14,6 +14,7 @@ import (
 	"github.com/filecoin-project/go-f3/gpbft"
 	"github.com/filecoin-project/go-f3/internal/clock"
 	"github.com/filecoin-project/go-f3/manifest"
+
 	pubsub "github.com/libp2p/go-libp2p-pubsub"
 	peer "github.com/libp2p/go-libp2p/core/peer"
 	"go.uber.org/multierr"
@@ -436,20 +437,22 @@ func (h *gpbftHost) GetProposalForInstance(instance uint64) (*gpbft.Supplemental
 	if err != nil {
 		return nil, nil, fmt.Errorf("getting head TS: %w", err)
 	}
-	if h.clock.Since(headTs.Timestamp()) < h.manifest.EC.Period {
-		// less than ECPeriod since production of the head
-		// agreement is unlikely
-		headTs, err = h.ec.GetParent(h.runningCtx, headTs)
-		if err != nil {
-			return nil, nil, fmt.Errorf("getting the parent of head TS: %w", err)
-		}
-	}
 
 	collectedChain, err := h.collectChain(baseTs, headTs)
 	if err != nil {
 		return nil, nil, fmt.Errorf("collecting chain: %w", err)
 	}
 
+	// If we have an explicit head-lookback, trim the chain.
+	if h.manifest.EC.HeadLookback > 0 {
+		collectedChain = collectedChain[:max(0, len(collectedChain)-h.manifest.EC.HeadLookback)]
+	}
+
+	// less than ECPeriod since production of the head agreement is unlikely, trim the chain.
+	if len(collectedChain) > 0 && h.clock.Since(collectedChain[len(collectedChain)-1].Timestamp()) < h.manifest.EC.Period {
+		collectedChain = collectedChain[:len(collectedChain)-1]
+	}
+
 	base := gpbft.TipSet{
 		Epoch: baseTs.Epoch(),
 		Key:   baseTs.Key(),

manifest/manifest.go

@@ -26,6 +26,7 @@ var (
 		DelayMultiplier: 2.,
 		// MaxBackoff is 15min given default params
 		BaseDecisionBackoffTable: []float64{1.3, 1.69, 2.2, 2.86, 3.71, 4.83, 6.27, 7.5},
+		HeadLookback:             0,
 	}
 
 	DefaultGpbftConfig = GpbftConfig{
@@ -113,17 +114,22 @@ type EcConfig struct {
 	DelayMultiplier float64
 	// Table of incremental multipliers to backoff in units of Delay in case of base decisions
 	BaseDecisionBackoffTable []float64
+	// HeadLookback number of unfinalized tipsets to remove from the head
+	HeadLookback int
 }
 
 func (e *EcConfig) Equal(o *EcConfig) bool {
 	return e.Period == o.Period &&
 		e.Finality == o.Finality &&
 		e.DelayMultiplier == o.DelayMultiplier &&
+		e.HeadLookback == o.HeadLookback &&
 		slices.Equal(e.BaseDecisionBackoffTable, o.BaseDecisionBackoffTable)
 }
 
 func (e *EcConfig) Validate() error {
 	switch {
+	case e.HeadLookback < 0:
+		return fmt.Errorf("EC head lookback must be non-negative, was %d", e.HeadLookback)
 	case e.Period <= 0:
 		return fmt.Errorf("EC period must be positive, was %s", e.Period)
 	case e.Finality < 0:

manifest/manifest_test.go

@@ -38,6 +38,7 @@ var base = manifest.Manifest{
 		DelayMultiplier:          2.0,
 		Period:                   30 * time.Second,
 		BaseDecisionBackoffTable: []float64{1.3, 1.69, 2.2, 2.86, 3.71, 4.83, 6.27, 8.16, 10.6, 13.79, 15.},
+		HeadLookback:             0,
 	},
 	CertificateExchange: manifest.CxConfig{
 		ClientRequestTimeout: 10 * time.Second,