go/keymanager/churp: Fix committee size verification

go/consensus/cometbft/apps/keymanager/churp/epoch.go

@@ -31,11 +31,11 @@ func (ext *churpExt) onEpochChange(ctx *tmapi.Context, epoch beacon.EpochTime) e
 
 			switch epoch {
 			case status.NextHandoff:
-				// The epoch for the handoff just started, meaning that registrations
-				// are now closed. If not enough nodes applied for the next committee,
-				// we need to reset applications and start collecting again.
-				minCommitteeSize := int(status.Threshold)*2 + 1
-				if len(status.Applications) >= minCommitteeSize {
+				// The epoch for the handoff just started, meaning that
+				// application submissions are now closed. If not enough
+				// nodes applied for the next committee, we need to reset
+				// applications and start collecting again.
+				if len(status.Applications) >= status.MinApplicants() {
 					continue
 				}
 			case status.NextHandoff + 1:

go/consensus/cometbft/apps/keymanager/churp/txs.go

@@ -454,20 +454,14 @@ func tryFinalizeHandoff(status *churp.Status, epochChange bool) bool {
 		}
 	}
 
-	// Verify the size of the committee.
-	switch epochChange {
-	case true:
-		// At the end of the handoff epoch, a threshold number of applicants
-		// will suffice.
-		minCommitteeSize := status.Threshold + 1 + status.ExtraShares
-		for len(committee) < int(minCommitteeSize) {
-			return false
-		}
-	case false:
-		// During the handoff epoch, all applicants must send confirmation.
-		for len(committee) != len(status.Applications) {
-			return false
-		}
+	// Verify the committee size if the number of extra shares has changed.
+	if len(committee) < status.MinCommitteeSize() {
+		return false
+	}
+
+	// During the handoff epoch, all applicants must send confirmation.
+	if !epochChange && len(committee) != len(status.Applications) {
+		return false
 	}
 
 	// Sort the committee to ensure a deterministic order.

go/keymanager/churp/status.go

@@ -1,6 +1,8 @@
 package churp
 
 import (
+	"math"
+
 	beacon "github.com/oasisprotocol/oasis-core/go/beacon/api"
 	"github.com/oasisprotocol/oasis-core/go/common"
 	"github.com/oasisprotocol/oasis-core/go/common/crypto/hash"
@@ -18,6 +20,34 @@ const (
 	EccNistP384 uint8 = iota
 )
 
+// HandoffKind represents the kind of a handoff.
+type HandoffKind int
+
+const (
+	// HandoffKindDealingPhase represents the initial setup phase.
+	HandoffKindDealingPhase HandoffKind = iota
+	// HandoffKindCommitteeUnchanged represents a handoff where the committee
+	// doesn't change.
+	HandoffKindCommitteeUnchanged
+	// HandoffKindCommitteeChanged represents a handoff where the committee
+	// changes.
+	HandoffKindCommitteeChanged
+)
+
+// String returns the string representation of the HandoffKind.
+func (h HandoffKind) String() string {
+	switch h {
+	case HandoffKindDealingPhase:
+		return "dealing phase"
+	case HandoffKindCommitteeUnchanged:
+		return "committee unchanged"
+	case HandoffKindCommitteeChanged:
+		return "committee changed"
+	default:
+		return "unknown"
+	}
+}
+
 // ConsensusParameters are the key manager CHURP consensus parameters.
 type ConsensusParameters struct {
 	GasCosts transaction.Costs `json:"gas_costs,omitempty"`
@@ -107,6 +137,50 @@ type Status struct {
 	Applications map[signature.PublicKey]Application `json:"applications,omitempty"`
 }
 
+// HandoffKind returns the type of the next handoff depending on which nodes
+// submitted an application to form the next committee.
+func (s *Status) HandoffKind() HandoffKind {
+	if len(s.Committee) == 0 {
+		return HandoffKindDealingPhase
+	}
+
+	if len(s.Committee) != len(s.Applications) {
+		return HandoffKindCommitteeChanged
+	}
+
+	for _, id := range s.Committee {
+		if _, ok := s.Applications[id]; !ok {
+			return HandoffKindCommitteeChanged
+		}
+	}
+
+	return HandoffKindCommitteeUnchanged
+}
+
+// MinCommitteeSize returns the minimum number of nodes in the committee.
+func (s *Status) MinCommitteeSize() int {
+	t := int(s.Threshold)
+	e := int(s.ExtraShares)
+	return t + e + 1
+}
+
+// MinApplicants returns the minimum number of nodes that must participate
+// in a handoff.
+func (s *Status) MinApplicants() int {
+	t := int(s.Threshold)
+	e := int(s.ExtraShares)
+
+	switch s.HandoffKind() {
+	case HandoffKindDealingPhase, HandoffKindCommitteeUnchanged:
+		return t + e + 1
+	case HandoffKindCommitteeChanged:
+		return max(t+e+1, 2*t+1)
+	default:
+		// Dead code.
+		return math.MaxInt
+	}
+}
+
 // HandoffsDisabled returns true if and only if handoffs are disabled.
 func (s *Status) HandoffsDisabled() bool {
 	return s.HandoffInterval == HandoffsDisabled