feat(dot/parachain/backing): handle second incoming overseer message (

#3682)

The `Second` message is a message received from the overseer.
Candidate Backing subsystem should second the given candidate in the context of the given relay parent.
This candidate must be validated.

Sign and dispatch a `Seconded` statement only if we have not signed a Valid statement for the requested candidate.

dot/parachain/backing/candidate_backing.go

@@ -224,7 +224,7 @@ func (cb *CandidateBacking) processMessage(msg any, chRelayParentAndCommand chan
 			logger.Debug(fmt.Sprintf("can't second the candidate: %s", err))
 		}
 	case SecondMessage:
-		cb.handleSecondMessage()
+		return cb.handleSecondMessage(msg.CandidateReceipt, msg.PersistedValidationData, msg.PoV, chRelayParentAndCommand)
 	case StatementMessage:
 		return cb.handleStatementMessage(msg.RelayParent, msg.SignedFullStatement, chRelayParentAndCommand)
 	case parachaintypes.ActiveLeavesUpdateSignal:
@@ -249,10 +249,6 @@ func (cb *CandidateBacking) handleGetBackedCandidatesMessage() {
 	// TODO: Implement this #3504
 }
 
-func (cb *CandidateBacking) handleSecondMessage() {
-	// TODO: Implement this #3506
-}
-
 // Import the statement and kick off validation work if it is a part of our assignment.
 func (cb *CandidateBacking) handleStatementMessage(
 	relayParent common.Hash,

dot/parachain/backing/second.go

@@ -0,0 +1,88 @@
+// Copyright 2024 ChainSafe Systems (ON)
+// SPDX-License-Identifier: LGPL-3.0-only
+
+package backing
+
+import (
+	"errors"
+	"fmt"
+
+	parachaintypes "github.com/ChainSafe/gossamer/dot/parachain/types"
+)
+
+var (
+	errWrongPVDForSecondingCandidate = errors.New(
+		"incorrect persisted validation data provided for seconding candidate")
+	errUnknownRelayParentForSecondingCandidate = errors.New(
+		"attempted to second a candidate with an unknown relay parent")
+	errParaOutsideAssignmentForSeconding = errors.New(
+		"subsystem requested to second for parachain beyond our assignment scope")
+	errAlreadySignedValidStatement = errors.New("already signed a valid statement for this candidate")
+)
+
+func (cb *CandidateBacking) handleSecondMessage(
+	candidateReceipt parachaintypes.CandidateReceipt,
+	pvd parachaintypes.PersistedValidationData,
+	pov parachaintypes.PoV,
+	chRelayParentAndCommand chan relayParentAndCommand,
+) error {
+	hash, err := candidateReceipt.Hash()
+	if err != nil {
+		return fmt.Errorf("hashing candidate receipt: %w", err)
+	}
+	candidateHash := parachaintypes.CandidateHash{Value: hash}
+
+	pvdHash, err := pvd.Hash()
+	if err != nil {
+		return fmt.Errorf("hashing persisted validation data: %w", err)
+	}
+
+	if candidateReceipt.Descriptor.PersistedValidationDataHash != pvdHash {
+		return fmt.Errorf("%w; mismatch between persisted validation data hash in candidate descriptor: %s "+
+			"and calculated hash of given persisted validation data: %s",
+			errWrongPVDForSecondingCandidate,
+			candidateReceipt.Descriptor.PersistedValidationDataHash,
+			pvdHash,
+		)
+	}
+
+	rpState, ok := cb.perRelayParent[candidateReceipt.Descriptor.RelayParent]
+	if !ok {
+		return fmt.Errorf("%w; candidate hash: %s; relay parent: %s",
+			errUnknownRelayParentForSecondingCandidate,
+			candidateHash,
+			candidateReceipt.Descriptor.RelayParent,
+		)
+	}
+
+	// Sanity check that candidate is from our assignment.
+	if candidateReceipt.Descriptor.ParaID != uint32(rpState.assignment) {
+		return fmt.Errorf("%w: candidate hash: %s; candidate paraID: %d; assignment: %d",
+			errParaOutsideAssignmentForSeconding,
+			candidateHash,
+			candidateReceipt.Descriptor.ParaID,
+			rpState.assignment,
+		)
+	}
+
+	// If the message is a `CandidateBackingMessage::Second`, sign and dispatch a
+	// Seconded statement only if we have not signed a Valid statement for the requested candidate.
+	if rpState.issuedStatements[candidateHash] {
+		return fmt.Errorf("%w: candidate hash: %s", errAlreadySignedValidStatement, candidateHash)
+	}
+
+	// Kick off background validation with intent to second.
+	logger.Debugf("validate and second candidate: %s", candidateHash)
+	return rpState.validateAndMakeAvailable(
+		executorParamsAtRelayParent,
+		cb.SubSystemToOverseer,
+		chRelayParentAndCommand,
+		candidateReceipt,
+		rpState.relayParent,
+		pvd,
+		pov,
+		uint32(len(rpState.tableContext.validators)),
+		second,
+		candidateHash,
+	)
+}

dot/parachain/backing/second_test.go

@@ -0,0 +1,133 @@
+// Copyright 2024 ChainSafe Systems (ON)
+// SPDX-License-Identifier: LGPL-3.0-only
+
+package backing
+
+import (
+	"testing"
+
+	parachaintypes "github.com/ChainSafe/gossamer/dot/parachain/types"
+	"github.com/ChainSafe/gossamer/lib/common"
+	"github.com/stretchr/testify/require"
+)
+
+func TestHandleSecondMessage(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		description      string
+		cb               *CandidateBacking
+		candidateReceipt parachaintypes.CandidateReceipt
+		pvd              parachaintypes.PersistedValidationData
+		err              error
+	}{
+		{
+			description:      "wrong_persisted_validation_data_for_seconding_candidate",
+			cb:               &CandidateBacking{},
+			candidateReceipt: parachaintypes.CandidateReceipt{},
+			pvd:              parachaintypes.PersistedValidationData{},
+			err:              errWrongPVDForSecondingCandidate,
+		},
+		{
+			description:      "unknown_relay_parent_for_seconding_candidate",
+			cb:               &CandidateBacking{},
+			candidateReceipt: dummyCandidateReceipt(t),
+			pvd:              dummyPVD(t),
+			err:              errUnknownRelayParentForSecondingCandidate,
+		},
+		{
+			description: "parachain_outside_assignment_for_seconding",
+			cb: &CandidateBacking{
+				perRelayParent: map[common.Hash]*perRelayParentState{
+					getDummyHash(t, 6): {
+						assignment: 10,
+					},
+				},
+			},
+			candidateReceipt: dummyCandidateReceipt(t),
+			pvd:              dummyPVD(t),
+			err:              errParaOutsideAssignmentForSeconding,
+		},
+		{
+			description: "already_signed_valid_statement_for_candidate",
+			cb: &CandidateBacking{
+				perRelayParent: map[common.Hash]*perRelayParentState{
+					getDummyHash(t, 6): {
+						assignment: 1,
+						issuedStatements: map[parachaintypes.CandidateHash]bool{
+							dummyCandidateHash(t): true,
+						},
+					},
+				},
+			},
+			candidateReceipt: dummyCandidateReceipt(t),
+			pvd:              dummyPVD(t),
+			err:              errAlreadySignedValidStatement,
+		},
+		{
+			description: "kick_off_background_validation_with_intent_to_second",
+			cb: &CandidateBacking{
+				perRelayParent: map[common.Hash]*perRelayParentState{
+					getDummyHash(t, 6): {
+						assignment: 1,
+						awaitingValidation: map[parachaintypes.CandidateHash]bool{
+							dummyCandidateHash(t): true,
+						},
+					},
+				},
+			},
+			candidateReceipt: dummyCandidateReceipt(t),
+			pvd:              dummyPVD(t),
+			err:              nil,
+		},
+	}
+
+	for _, tc := range testCases {
+		tc := tc
+		t.Run(tc.description, func(t *testing.T) {
+			t.Parallel()
+
+			err := tc.cb.handleSecondMessage(tc.candidateReceipt, tc.pvd, parachaintypes.PoV{}, nil)
+			if err == nil {
+				require.NoError(t, err)
+			} else {
+				require.ErrorIs(t, err, tc.err)
+			}
+		})
+	}
+}
+
+func dummyPVD(t *testing.T) parachaintypes.PersistedValidationData {
+	t.Helper()
+
+	return parachaintypes.PersistedValidationData{
+		ParentHead: parachaintypes.HeadData{
+			Data: []byte{1, 2, 3},
+		},
+		RelayParentNumber:      5,
+		RelayParentStorageRoot: getDummyHash(t, 5),
+		MaxPovSize:             3,
+	}
+}
+
+func dummyCandidateReceipt(t *testing.T) parachaintypes.CandidateReceipt {
+	t.Helper()
+
+	cr := getDummyCommittedCandidateReceipt(t).ToPlain()
+
+	// blake2bhash of PVD in dummyPVD(t *testing.T) function
+	cr.Descriptor.PersistedValidationDataHash =
+		common.MustHexToHash("0x3544fbcdcb094751a5e044a30b994b2586ffc0b50e8b88c381461fe023a7242f")
+
+	return cr
+}
+
+func dummyCandidateHash(t *testing.T) parachaintypes.CandidateHash {
+	t.Helper()
+
+	cr := dummyCandidateReceipt(t)
+	hash, err := cr.Hash()
+	require.NoError(t, err)
+
+	return parachaintypes.CandidateHash{Value: hash}
+}

dot/parachain/backing/validated_candidate_command.go

@@ -36,7 +36,7 @@ type validatedCandidateCommand byte
 
 const (
 	// We were instructed to second the candidate that has been already validated.
-	second = validatedCandidateCommand(iota) //nolint:unused
+	second = validatedCandidateCommand(iota)
 	// We were instructed to validate the candidate.
 	attest
 	// We were not able to `Attest` because backing validator did not send us the PoV.

dot/parachain/types/types.go

@@ -471,6 +471,15 @@ type PersistedValidationData struct {
 	MaxPovSize             uint32      `scale:"4"`
 }
 
+func (pvd PersistedValidationData) Hash() (common.Hash, error) {
+	bytes, err := scale.Marshal(pvd)
+	if err != nil {
+		return common.Hash{}, fmt.Errorf("marshalling PersistedValidationData: %w", err)
+	}
+
+	return common.Blake2bHash(bytes)
+}
+
 type OccupiedCoreAssumptionValues interface {
 	IncludedOccupiedCoreAssumption | TimedOutOccupiedCoreAssumption | FreeOccupiedCoreAssumption
 }
@@ -571,6 +580,10 @@ type CandidateHash struct {
 	Value common.Hash `scale:"1"`
 }
 
+func (ch CandidateHash) String() string {
+	return ch.Value.String()
+}
+
 // PoV represents a Proof-of-Validity block (PoV block) or a parachain block.
 // It contains the necessary data for the parachain specific state transition logic.
 type PoV struct {