fix validator set proposer priorities in light client provider (#5307)

CHANGELOG_PENDING.md

@@ -22,3 +22,5 @@ Friendly reminder, we have a [bug bounty program](https://hackerone.com/tendermi
 - [blockchain] \#5249 Fix fast sync halt with initial height > 1 (@erikgrinaker)
 
 - [statesync] \#5302 Fix genesis state propagation to state sync routine (@erikgrinaker)
+
+- [light] [\#5307](https://github.com/tendermint/tendermint/pull/5307) persist correct proposer priority in light client validator sets (@cmwaters)

light/provider/http/http.go

@@ -119,7 +119,7 @@ func (p *http) ValidatorSet(height int64) (*types.ValidatorSet, error) {
 		page++
 	}
 
-	return types.NewValidatorSet(vals), nil
+	return types.ValidatorSetFromExistingValidators(vals)
 }
 
 // ReportEvidence calls `/broadcast_evidence` endpoint.

types/validator_set.go

@@ -822,6 +822,24 @@ func (vals *ValidatorSet) VerifyCommitLightTrusting(chainID string, commit *Comm
 	return ErrNotEnoughVotingPowerSigned{Got: talliedVotingPower, Needed: votingPowerNeeded}
 }
 
+// findPreviousProposer reverses the compare proposer priority function to find the validator
+// with the lowest proposer priority which would have been the previous proposer.
+//
+// Is used when recreating a validator set from an existing array of validators.
+func (vals *ValidatorSet) findPreviousProposer() *Validator {
+	var previousProposer *Validator
+	for _, val := range vals.Validators {
+		if previousProposer == nil {
+			previousProposer = val
+			continue
+		}
+		if previousProposer == previousProposer.CompareProposerPriority(val) {
+			previousProposer = val
+		}
+	}
+	return previousProposer
+}
+
 //-----------------
 
 // IsErrNotEnoughVotingPowerSigned returns true if err is
@@ -966,6 +984,21 @@ func ValidatorSetFromProto(vp *tmproto.ValidatorSet) (*ValidatorSet, error) {
 	return vals, vals.ValidateBasic()
 }
 
+// ValidatorSetFromExistingValidators takes an existing array of validators and rebuilds
+// the exact same validator set that corresponds to it without changing the proposer priority or power
+func ValidatorSetFromExistingValidators(valz []*Validator) (*ValidatorSet, error) {
+	if len(valz) == 0 {
+		return nil, errors.New("no validators given")
+	}
+	vals := &ValidatorSet{
+		Validators: valz,
+	}
+	vals.Proposer = vals.findPreviousProposer()
+	vals.updateTotalVotingPower()
+	sort.Sort(ValidatorsByVotingPower(vals.Validators))
+	return vals, nil
+}
+
 //----------------------------------------
 
 // RandValidatorSet returns a randomized validator set (size: +numValidators+),

types/validator_set_test.go

@@ -1425,6 +1425,24 @@ func verifyValSetUpdatePriorityOrder(t *testing.T, valSet *ValidatorSet, cfg tes
 	}
 }
 
+func TestNewValidatorSetFromExistingValidators(t *testing.T) {
+	size := 5
+	vals := make([]*Validator, size)
+	for i := 0; i < size; i++ {
+		pv := NewMockPV()
+		vals[i] = pv.ExtractIntoValidator(int64(i + 1))
+	}
+	valSet := NewValidatorSet(vals)
+	valSet.IncrementProposerPriority(5)
+
+	newValSet := NewValidatorSet(valSet.Validators)
+	existingValSet, err := ValidatorSetFromExistingValidators(valSet.Validators)
+	require.NoError(t, err)
+	assert.NotEqual(t, valSet, newValSet)
+	assert.Equal(t, valSet, existingValSet)
+	assert.Equal(t, valSet.CopyIncrementProposerPriority(3), existingValSet.CopyIncrementProposerPriority(3))
+}
+
 func TestValSetUpdateOverflowRelated(t *testing.T) {
 	testCases := []testVSetCfg{
 		{