fix(tests): correct committee lookback index calculation in GetCommittee (#995)

* fix: Correct committee lookback index calculation in GetCommittee

Signed-off-by: Jakub Sztandera <[email protected]>

* add extra check

Signed-off-by: Jakub Sztandera <[email protected]>

* add note that certchain is testing utility to avoid further reports

Signed-off-by: Jakub Sztandera <[email protected]>

---------

Signed-off-by: Jakub Sztandera <[email protected]>

Author: Kubuxu

certchain/certchain.go

@@ -1,3 +1,6 @@
+// certchain package is a testing utility for generating and validating
+// finality certificates.
+// It has no effect on the consensus protocol.
 package certchain
 
 import (
@@ -50,7 +53,10 @@ func (cc *CertChain) GetCommittee(ctx context.Context, instance uint64) (*gpbft.
 	if instance < cc.m.InitialInstance+cc.m.CommitteeLookback {
 		committeeEpoch = cc.m.BootstrapEpoch - cc.m.EC.Finality
 	} else {
-		lookbackIndex := instance - cc.m.CommitteeLookback + 1
+		lookbackIndex := instance - cc.m.CommitteeLookback - cc.m.InitialInstance + 1
+		if lookbackIndex >= uint64(len(cc.certificates)) {
+			return nil, fmt.Errorf("no prior finality certificate to get committee at instance %d", instance)
+		}
 		certAtLookback := cc.certificates[lookbackIndex]
 		committeeEpoch = certAtLookback.ECChain.Head().Epoch
 	}

certchain/certchain_test.go

@@ -24,6 +24,7 @@ func TestCertChain_GenerateAndVerify(t *testing.T) {
 
 	ctx, clk := clock.WithMockClock(context.Background())
 	m := manifest.LocalDevnetManifest()
+	m.InitialInstance = 100
 	signVerifier := signing.NewFakeBackend()
 	rng := rand.New(rand.NewSource(seed * 23))
 	generatePublicKey := func(id gpbft.ActorID) gpbft.PubKey {