Change host API to separate fetching chain from fetching power tables

gpbft/api.go

@@ -23,15 +23,17 @@ type Receiver interface {
 }
 
 type Chain interface {
-	// Returns inputs to the next GPBFT instance.
-	// These are:
-	// - the EC chain to propose,
-	// - the power table specifying the participants,
-	// - the beacon value for generating tickets.
-	// These will be used as input to a subsequent instance of the protocol.
-	// The chain should be a suffix of the last chain notified to the host via
-	// ReceiveDecision (or known to be final via some other channel).
-	GetCanonicalChain() (chain ECChain, power PowerTable, beacon []byte)
+	// Returns the chain to propose for a new GPBFT instance.
+	// This should be a suffix of the chain finalised by the immediately prior instance.
+	// Returns an error if the chain for the instance is not available.
+	GetChainForInstance(instance uint64) (chain ECChain, err error)
+
+	// Returns the power table and beacon value to be used for a GPBFT instance.
+	// These values should be derived from a chain previously received as final by the host,
+	// or known to be final via some other channel (e.g. when bootstrapping the protocol).
+	// The offset (how many instances to look back) is determined by the host.
+	// Returns an error if the committee for the instance is not available.
+	GetCommitteeForInstance(instance uint64) (power *PowerTable, beacon []byte, err error)
 }
 
 // Endpoint to which participants can send messages.

gpbft/participant.go

@@ -144,7 +144,10 @@ func (p *Participant) ReceiveAlarm() (err error) {
 }
 
 func (p *Participant) beginInstance() error {
-	chain, power, beacon := p.host.GetCanonicalChain()
+	chain, err := p.host.GetChainForInstance(p.nextInstance)
+	if err != nil {
+		return fmt.Errorf("failed fetching chain for instance %d: %w", p.nextInstance, err)
+	}
 	// Limit length of the chain to be proposed.
 	if chain.IsZero() {
 		return errors.New("canonical chain cannot be zero-valued")
@@ -153,11 +156,15 @@ func (p *Participant) beginInstance() error {
 	if err := chain.Validate(); err != nil {
 		return fmt.Errorf("invalid canonical chain: %w", err)
 	}
+
+	power, beacon, err := p.host.GetCommitteeForInstance(p.nextInstance)
+	if err != nil {
+		return fmt.Errorf("failed fetching power table for instance %d: %w", p.nextInstance, err)
+	}
 	if err := power.Validate(); err != nil {
 		return fmt.Errorf("invalid power table: %w", err)
 	}
-	var err error
-	if p.granite, err = newInstance(p, p.nextInstance, chain, power, beacon); err != nil {
+	if p.granite, err = newInstance(p, p.nextInstance, chain, *power, beacon); err != nil {
 		return fmt.Errorf("failed creating new granite instance: %w", err)
 	}
 	p.nextInstance += 1

gpbft/participant_test.go

@@ -82,14 +82,16 @@ func newParticipantTestSubject(t *testing.T, seed int64, instance uint64) *parti
 
 func (pt *participantTestSubject) expectBeginInstance() {
 	// Prepare the test host.
-	pt.host.On("GetCanonicalChain").Return(pt.canonicalChain, *pt.powerTable, pt.beacon)
+	pt.host.On("GetChainForInstance", pt.instance).Return(pt.canonicalChain, nil)
+	pt.host.On("GetCommitteeForInstance", pt.instance).Return(pt.powerTable, pt.beacon, nil)
 	pt.host.On("Time").Return(pt.time)
 	pt.host.On("NetworkName").Return(pt.networkName).Maybe()
 	pt.host.On("MarshalPayloadForSigning", mock.AnythingOfType("*gpbft.Payload")).
 		Return([]byte(gpbft.DOMAIN_SEPARATION_TAG + ":" + pt.networkName))
 
 	// Expect calls to get the host state prior to beginning of an instance.
-	pt.host.EXPECT().GetCanonicalChain()
+	pt.host.EXPECT().GetChainForInstance(pt.instance)
+	pt.host.EXPECT().GetCommitteeForInstance(pt.instance)
 	pt.host.EXPECT().Time()
 
 	// Expect alarm is set to 2X of configured delta.
@@ -189,14 +191,14 @@ func TestParticipant(t *testing.T) {
 	t.Run("panic is recovered", func(t *testing.T) {
 		t.Run("on Start", func(t *testing.T) {
 			subject := newParticipantTestSubject(t, seed, 0)
-			subject.host.On("GetCanonicalChain").Panic("saw me no chain")
+			subject.host.On("GetChainForInstance", subject.instance).Panic("saw me no chain")
 			require.NotPanics(t, func() {
 				require.ErrorContains(t, subject.Start(), "saw me no chain")
 			})
 		})
 		t.Run("on ReceiveAlarm", func(t *testing.T) {
 			subject := newParticipantTestSubject(t, seed, 0)
-			subject.host.On("GetCanonicalChain").Panic("saw me no chain")
+			subject.host.On("GetChainForInstance", subject.instance).Panic("saw me no chain")
 			require.NotPanics(t, func() {
 				require.ErrorContains(t, subject.ReceiveAlarm(), "saw me no chain")
 			})
@@ -249,23 +251,45 @@ func TestParticipant(t *testing.T) {
 				subject.requireInstanceRoundPhase(47, 0, gpbft.QUALITY_PHASE)
 			})
 		})
-		t.Run("instance is no begun", func(t *testing.T) {
+		t.Run("instance is not begun", func(t *testing.T) {
 			t.Run("on zero canonical chain", func(t *testing.T) {
 				subject := newParticipantTestSubject(t, seed, 0)
 				var zeroChain gpbft.ECChain
-				subject.host.On("GetCanonicalChain").Return(zeroChain, *subject.powerTable, subject.beacon)
+				subject.host.On("GetChainForInstance", subject.instance).Return(zeroChain, nil)
 				require.ErrorContains(t, subject.Start(), "cannot be zero-valued")
 				subject.assertHostExpectations()
 				subject.requireNotStarted()
 			})
 			t.Run("on invalid canonical chain", func(t *testing.T) {
 				subject := newParticipantTestSubject(t, seed, 0)
 				invalidChain := gpbft.ECChain{gpbft.TipSet{}}
-				subject.host.On("GetCanonicalChain").Return(invalidChain, *subject.powerTable, subject.beacon)
+				subject.host.On("GetChainForInstance", subject.instance).Return(invalidChain, nil)
 				require.ErrorContains(t, subject.Start(), "invalid canonical chain")
 				subject.assertHostExpectations()
 				subject.requireNotStarted()
 			})
+			t.Run("on failure to fetch chain", func(t *testing.T) {
+				subject := newParticipantTestSubject(t, seed, 0)
+				invalidChain := gpbft.ECChain{gpbft.TipSet{}}
+				subject.host.On("GetChainForInstance", subject.instance).Return(invalidChain, errors.New("fish"))
+				require.ErrorContains(t, subject.Start(), "fish")
+				subject.assertHostExpectations()
+				subject.requireNotStarted()
+			})
+			t.Run("on failure to fetch committee", func(t *testing.T) {
+				subject := newParticipantTestSubject(t, seed, 0)
+				chain := gpbft.ECChain{gpbft.TipSet{
+					Epoch:       0,
+					Key:         []byte("key"),
+					PowerTable:  []byte("pt"),
+					Commitments: [32]byte{},
+				}}
+				subject.host.On("GetChainForInstance", subject.instance).Return(chain, nil)
+				subject.host.On("GetCommitteeForInstance", subject.instance).Return(nil, nil, errors.New("fish"))
+				require.ErrorContains(t, subject.Start(), "fish")
+				subject.assertHostExpectations()
+				subject.requireNotStarted()
+			})
 		})
 	})
 	t.Run("when started", func(t *testing.T) {

sim/adversary/decide.go

@@ -36,7 +36,7 @@ func (i *ImmediateDecide) ID() gpbft.ActorID {
 }
 
 func (i *ImmediateDecide) Start() error {
-	_, powertable, _ := i.host.GetCanonicalChain()
+	powertable, _, _ := i.host.GetCommitteeForInstance(0)
 	// Immediately send a DECIDE message
 	payload := gpbft.Payload{
 		Instance: 0,
@@ -91,7 +91,7 @@ func (i *ImmediateDecide) AllowMessage(_ gpbft.ActorID, _ gpbft.ActorID, _ gpbft
 	return true
 }
 
-func (i *ImmediateDecide) broadcast(payload gpbft.Payload, justification *gpbft.Justification, powertable gpbft.PowerTable) {
+func (i *ImmediateDecide) broadcast(payload gpbft.Payload, justification *gpbft.Justification, powertable *gpbft.PowerTable) {
 
 	pS := i.host.MarshalPayloadForSigning(&payload)
 	_, pubkey := powertable.Get(i.id)

sim/adversary/repeat.go

@@ -76,7 +76,7 @@ func (r *Repeat) ReceiveMessage(msg *gpbft.GMessage, _ bool) (bool, error) {
 	}
 
 	sigPayload := r.host.MarshalPayloadForSigning(&msg.Vote)
-	_, power, beacon := r.host.GetCanonicalChain()
+	power, beacon, _ := r.host.GetCommitteeForInstance(0)
 	_, pubkey := power.Get(r.id)
 
 	sig, err := r.host.Sign(pubkey, sigPayload)

sim/adversary/withhold.go

@@ -55,7 +55,7 @@ func (w *WithholdCommit) Start() error {
 		return errors.New("victims must be set")
 	}
 
-	_, powertable, _ := w.host.GetCanonicalChain()
+	powertable, _, _ := w.host.GetCommitteeForInstance(0)
 	broadcast := w.broadcastHelper(w.id, powertable)
 	// All victims need to see QUALITY and PREPARE in order to send their COMMIT,
 	// but only the one victim will see our COMMIT.
@@ -165,7 +165,7 @@ func (w *WithholdCommit) sign(pubkey gpbft.PubKey, msg []byte) []byte {
 	return sig
 }
 
-func (w *WithholdCommit) broadcastHelper(sender gpbft.ActorID, powertable gpbft.PowerTable) func(gpbft.Payload, *gpbft.Justification) {
+func (w *WithholdCommit) broadcastHelper(sender gpbft.ActorID, powertable *gpbft.PowerTable) func(gpbft.Payload, *gpbft.Justification) {
 	return func(payload gpbft.Payload, justification *gpbft.Justification) {
 		pS := w.host.MarshalPayloadForSigning(&payload)
 		_, pubkey := powertable.Get(sender)

sim/ec.go

@@ -28,9 +28,9 @@ type ECInstance struct {
 	Instance uint64
 	// The base of all chains, which participants must agree on.
 	BaseChain gpbft.ECChain
-	// The power table at the base chain head.
+	// The power table to be used for this instance.
 	PowerTable *gpbft.PowerTable
-	// The beacon value of the base chain head.
+	// The beacon value to use for this instance.
 	Beacon []byte
 
 	ec        *simEC
@@ -56,7 +56,10 @@ func newEC(opts *options) *simEC {
 	}
 }
 
-func (ec *simEC) BeginInstance(baseChain gpbft.ECChain, pt *gpbft.PowerTable, beacon []byte) *ECInstance {
+func (ec *simEC) BeginInstance(baseChain gpbft.ECChain, pt *gpbft.PowerTable) *ECInstance {
+	// Take beacon value from the head of the base chain.
+	// Note a real beacon value will come from a finalised chain with some lookback.
+	beacon := baseChain.Head().Key
 	instance := &ECInstance{
 		Instance:   uint64(ec.Len()),
 		BaseChain:  baseChain,