multi: fix linter

diff --git a/channeldb/codec.go b/channeldb/codec.go
index b82a258..e5bab3d 100644
--- a/channeldb/codec.go
+++ b/channeldb/codec.go
@@ -488,5 +488,6 @@ func serializeTime(w io.Writer, t time.Time) error {
 
 	byteOrder.PutUint64(scratch[:], uint64(unixNano))
 	_, err := w.Write(scratch[:])
+
 	return err
 }
diff --git a/lnrpc/routerrpc/router_server.go b/lnrpc/routerrpc/router_server.go
index 10967d9..894edf2 100644
--- a/lnrpc/routerrpc/router_server.go
+++ b/lnrpc/routerrpc/router_server.go
@@ -1448,7 +1448,11 @@ func (s *Server) trackPaymentStream(context context.Context,
 				// No more payment updates.
 				return nil
 			}
-			result := item.(*paymentsdb.MPPayment)
+			result, ok := item.(*paymentsdb.MPPayment)
+			if !ok {
+				return fmt.Errorf("unexpected payment type: %T",
+					item)
+			}
 
 			log.Tracef("Payment %v updated to state %v",
 				result.Info.PaymentIdentifier, result.Status)
diff --git a/payments/db/codec.go b/payments/db/codec.go
index 997dab0..cd9c0c7 100644
--- a/payments/db/codec.go
+++ b/payments/db/codec.go
@@ -2,6 +2,7 @@ package paymentsdb
 
 import (
 	"encoding/binary"
+	"errors"
 	"io"
 	"time"
 
@@ -19,21 +20,20 @@ type UnknownElementType = channeldb.UnknownElementType
 func ReadElement(r io.Reader, element interface{}) error {
 	err := channeldb.ReadElement(r, element)
 	switch {
-
 	// Known to channeldb codec.
 	case err == nil:
 		return nil
 
 	// Fail if error is not UnknownElementType.
 	default:
-		if _, ok := err.(UnknownElementType); !ok {
+		var unknownElementType UnknownElementType
+		if !errors.As(err, &unknownElementType) {
 			return err
 		}
 	}
 
 	// Process any paymentsdb-specific extensions to the codec.
 	switch e := element.(type) {
-
 	case *paymentIndexType:
 		if err := binary.Read(r, byteOrder, e); err != nil {
 			return err
@@ -53,21 +53,20 @@ func ReadElement(r io.Reader, element interface{}) error {
 func WriteElement(w io.Writer, element interface{}) error {
 	err := channeldb.WriteElement(w, element)
 	switch {
-
 	// Known to channeldb codec.
 	case err == nil:
 		return nil
 
 	// Fail if error is not UnknownElementType.
 	default:
-		if _, ok := err.(UnknownElementType); !ok {
+		var unknownElementType UnknownElementType
+		if !errors.As(err, &unknownElementType) {
 			return err
 		}
 	}
 
 	// Process any paymentsdb-specific extensions to the codec.
 	switch e := element.(type) {
-
 	case paymentIndexType:
 		if err := binary.Write(w, byteOrder, e); err != nil {
 			return err
@@ -137,5 +136,6 @@ func serializeTime(w io.Writer, t time.Time) error {
 
 	byteOrder.PutUint64(scratch[:], uint64(unixNano))
 	_, err := w.Write(scratch[:])
+
 	return err
 }
diff --git a/payments/db/kv_store_test.go b/payments/db/kv_store_test.go
index cc64776..49e80df 100644
--- a/payments/db/kv_store_test.go
+++ b/payments/db/kv_store_test.go
@@ -954,7 +954,6 @@ func TestKVPaymentsDBMultiShard(t *testing.T) {
 	}
 
 	for _, test := range tests {
-		test := test
 		subTest := fmt.Sprintf("first=%v, second=%v",
 			test.settleFirst, test.settleLast)
 
@@ -1643,9 +1642,8 @@ func TestFetchPaymentWithSequenceNumber(t *testing.T) {
 	}
 
 	for _, test := range tests {
-		test := test
-
 		t.Run(test.name, func(t *testing.T) {
+			//nolint:ll
 			err := kvdb.Update(
 				paymentDB.db, func(tx walletdb.ReadWriteTx) error {
 					var seqNrBytes [8]byte
diff --git a/payments/db/log.go b/payments/db/log.go
index 08994ef..8a77dbc 100644
--- a/payments/db/log.go
+++ b/payments/db/log.go
@@ -8,8 +8,6 @@ import (
 // log is a logger that is initialized with no output filters.  This
 // means the package will not perform any logging by default until the caller
 // requests it.
-//
-//nolint:unused
 var log btclog.Logger
 
 // Subsystem defines the logging identifier for this subsystem.
diff --git a/payments/db/payment.go b/payments/db/payment.go
index a5ad753..72338a4 100644
--- a/payments/db/payment.go
+++ b/payments/db/payment.go
@@ -255,8 +255,8 @@ const (
 	// reason.
 	HTLCFailUnknown HTLCFailReason = 0
 
-	// HTLCFailUnreadable is recorded for htlcs that had a failure message that
-	// couldn't be decrypted.
+	// HTLCFailUnreadable is recorded for htlcs that had a failure message
+	// that couldn't be decrypted.
 	HTLCFailUnreadable HTLCFailReason = 1
 
 	// HTLCFailInternal is recorded for htlcs that failed because of an
diff --git a/payments/db/payment_test.go b/payments/db/payment_test.go
index 2f6bf40..9889f9d 100644
--- a/payments/db/payment_test.go
+++ b/payments/db/payment_test.go
@@ -60,19 +60,6 @@ var (
 		LegacyPayload:    true,
 	}
 
-	testHop3 = &route.Hop{
-		PubKeyBytes:      route.NewVertex(pub),
-		ChannelID:        12345,
-		OutgoingTimeLock: 111,
-		AmtToForward:     555,
-		CustomRecords: record.CustomSet{
-			65536: []byte{},
-			80001: []byte{},
-		},
-		AMP:      record.NewAMP([32]byte{0x69}, [32]byte{0x42}, 1),
-		Metadata: []byte{1, 2, 3},
-	}
-
 	testRoute = route.Route{
 		TotalTimeLock: 123,
 		TotalAmount:   1234567,
diff --git a/routing/control_tower_test.go b/routing/control_tower_test.go
index 9a49377..3e01065 100644
--- a/routing/control_tower_test.go
+++ b/routing/control_tower_test.go
@@ -130,15 +130,23 @@ func TestControlTowerSubscribeSuccess(t *testing.T) {
 		for result == nil || !result.Terminated() {
 			select {
 			case item := <-s.Updates():
-				result = item.(*paymentsdb.MPPayment)
+				payment, ok := item.(*paymentsdb.MPPayment)
+				require.True(
+					t, ok, "unexpected payment type: %T",
+					item)
+
+				result = payment
+
 			case <-time.After(testTimeout):
 				t.Fatal("timeout waiting for payment result")
 			}
 		}
 
-		require.Equalf(t, paymentsdb.StatusSucceeded, result.GetStatus(),
-			"subscriber %v failed, want %s, got %s", i,
-			paymentsdb.StatusSucceeded, result.GetStatus())
+		require.Equalf(t, paymentsdb.StatusSucceeded,
+			result.GetStatus(), "subscriber %v failed, want %s, "+
+				"got %s", i, paymentsdb.StatusSucceeded,
+			result.GetStatus(),
+		)
 
 		attempt, _ := result.TerminalInfo()
 		if attempt.Settle.Preimage != preimg {
@@ -261,8 +269,14 @@ func TestKVPaymentsDBSubscribeAllSuccess(t *testing.T) {
 	for i := 0; i < 6; i++ {
 		select {
 		case item := <-subscription.Updates():
-			id := item.(*paymentsdb.MPPayment).Info.PaymentIdentifier
-			results[id] = item.(*paymentsdb.MPPayment)
+			payment, ok := item.(*paymentsdb.MPPayment)
+			require.True(
+				t, ok, "unexpected payment type: %T",
+				item)
+
+			id := payment.Info.PaymentIdentifier
+			results[id] = payment
+
 		case <-time.After(testTimeout):
 			require.Fail(t, "timeout waiting for payment result")
 		}
@@ -337,11 +351,17 @@ func TestKVPaymentsDBSubscribeAllImmediate(t *testing.T) {
 	select {
 	case update := <-subscription.Updates():
 		require.NotNil(t, update)
+		payment, ok := update.(*paymentsdb.MPPayment)
+		if !ok {
+			t.Fatalf("unexpected payment type: %T", update)
+		}
+
 		require.Equal(
 			t, info.PaymentIdentifier,
-			update.(*paymentsdb.MPPayment).Info.PaymentIdentifier,
+			payment.Info.PaymentIdentifier,
 		)
 		require.Len(t, subscription.Updates(), 0)
+
 	case <-time.After(testTimeout):
 		require.Fail(t, "timeout waiting for payment result")
 	}
@@ -502,7 +522,12 @@ func testKVPaymentsDBSubscribeFail(t *testing.T, registerAttempt,
 		for result == nil || !result.Terminated() {
 			select {
 			case item := <-s.Updates():
-				result = item.(*paymentsdb.MPPayment)
+				payment, ok := item.(*paymentsdb.MPPayment)
+				require.True(
+					t, ok, "unexpected payment type: %T",
+					item)
+
+				result = payment
 			case <-time.After(testTimeout):
 				t.Fatal("timeout waiting for payment result")
 			}
diff --git a/routing/mock_test.go b/routing/mock_test.go
index 596b511..9cb938f 100644
--- a/routing/mock_test.go
+++ b/routing/mock_test.go
@@ -441,6 +441,7 @@ func (m *mockControlTowerOld) SettleAttempt(phash lntypes.Hash,
 
 		// Mark the payment successful on first settled attempt.
 		m.successful[phash] = struct{}{}
+
 		return &paymentsdb.HTLCAttempt{
 			Settle: settleInfo,
 		}, nil
@@ -479,6 +480,7 @@ func (m *mockControlTowerOld) FailAttempt(phash lntypes.Hash, pid uint64,
 		}
 
 		p.attempts[i].Failure = failInfo
+
 		return &paymentsdb.HTLCAttempt{
 			Failure: failInfo,
 		}, nil