feat(lib/babe): implement secondary slot block production

lib/babe/crypto.go

@@ -4,11 +4,13 @@
 package babe
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"math"
 	"math/big"
 
+	"github.com/ChainSafe/gossamer/dot/types"
 	"github.com/ChainSafe/gossamer/lib/crypto"
 	"github.com/ChainSafe/gossamer/lib/crypto/sr25519"
 	"github.com/ChainSafe/gossamer/pkg/scale"
@@ -83,6 +85,45 @@ func checkPrimaryThreshold(randomness Randomness,
 	return inoutUint.Compare(threshold) < 0, nil
 }
 
+func i32toBytes(val uint32) []byte {
+	r := make([]byte, 4)
+	for i := uint32(0); i < 4; i++ {
+		r[i] = byte((val >> (8 * i)) & 0xff)
+	}
+	return r
+}
+
+func claimSecondarySlot(randomness Randomness,
+	slot, epoch uint64,
+	authorities []types.Authority,
+	threshold *scale.Uint128,
+	keypair *sr25519.Keypair,
+) (*VrfOutputAndProof, error) {
+
+	secondarySlotAuthor, err := getSecondarySlotAuthor(slot, len(authorities), randomness)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, authority := range authorities {
+		if bytes.Equal(authority.ToRaw().Key[:], i32toBytes(secondarySlotAuthor)) {
+			transcript := makeTranscript(randomness, slot, epoch)
+
+			out, proof, err := keypair.VrfSign(transcript)
+			if err != nil {
+				return nil, err
+			}
+
+			return &VrfOutputAndProof{
+				output: out,
+				proof:  proof,
+			}, nil
+		}
+	}
+
+	return nil, nil
+}
+
 // CalculateThreshold calculates the slot lottery threshold
 // equation: threshold = 2^128 * (1 - (1-c)^(1/len(authorities))
 // see https://github.com/paritytech/substrate/blob/master/client/consensus/babe/src/authorship.rs#L44

lib/babe/epoch.go

@@ -65,6 +65,7 @@ func (b *Service) initiateEpoch(epoch uint64) error {
 				authorities:    data.Authorities,
 				authorityIndex: idx,
 				threshold:      threshold,
+				secondary:      cfgData.SecondarySlots,
 			}
 		} else {
 			b.epochData = &epochData{
@@ -174,11 +175,21 @@ func (b *Service) incrementEpoch() (uint64, error) {
 // if it is not authorised.
 // output = return[0:32]; proof = return[32:96]
 func (b *Service) runLottery(slot, epoch uint64) (*VrfOutputAndProof, error) {
-	return claimPrimarySlot(
+	// here: If we can't claim primary slot, claim secondary slot
+	proof, err := claimPrimarySlot(
 		b.epochData.randomness,
 		slot,
 		epoch,
 		b.epochData.threshold,
 		b.keypair,
 	)
+	if err == nil {
+		return proof, nil
+	}
+
+	if b.epochData.secondary != 1 && b.epochData.secondary != 2 {
+		return nil, err
+	}
+
+	return claimSecondarySlot(b.epochData.randomness, slot, epoch, b.epochData.authorities, b.epochData.threshold, b.keypair)
 }

lib/babe/secondary.go

@@ -12,6 +12,7 @@ import (
 	"github.com/ChainSafe/gossamer/lib/crypto/sr25519"
 )
 
+// https://github.com/paritytech/substrate/blob/b07765f9ecc4c508fb4d0fba930b476c8509f5c2/client/consensus/babe/src/authorship.rs#L98
 func getSecondarySlotAuthor(slot uint64, numAuths int, randomness Randomness) (uint32, error) {
 	s := make([]byte, 8)
 	binary.LittleEndian.PutUint64(s, slot)

lib/babe/types.go

@@ -60,6 +60,7 @@ type epochData struct {
 	authorityIndex uint32
 	authorities    []types.Authority
 	threshold      *scale.Uint128
+	secondary      byte
 }
 
 func (ed *epochData) String() string {