multi: introduce new paymentsDB package

diff --git a/channeldb/duplicate_payments.go b/channeldb/duplicate_payments.go
index cae3e95..004722f 100644
--- a/channeldb/duplicate_payments.go
+++ b/channeldb/duplicate_payments.go
@@ -11,6 +11,7 @@ import (
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
+	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
 	"github.com/lightningnetwork/lnd/routing/route"
 )
 
@@ -74,7 +75,7 @@ func fetchDuplicatePaymentStatus(bucket kvdb.RBucket) (PaymentStatus, error) {
 		return StatusInFlight, nil
 	}
 
-	return 0, ErrPaymentNotInitiated
+	return 0, paymentsdb.ErrPaymentNotInitiated
 }
 
 func deserializeDuplicateHTLCAttemptInfo(r io.Reader) (
diff --git a/channeldb/mp_payment.go b/channeldb/mp_payment.go
index b94bfd4..f75357a 100644
--- a/channeldb/mp_payment.go
+++ b/channeldb/mp_payment.go
@@ -15,6 +15,7 @@ import (
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnutils"
 	"github.com/lightningnetwork/lnd/lnwire"
+	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
 	"github.com/lightningnetwork/lnd/routing/route"
 )
 
@@ -349,12 +350,12 @@ func (m *MPPayment) Registrable() error {
 	// are settled HTLCs or the payment is failed. If we already have
 	// settled HTLCs, we won't allow adding more HTLCs.
 	if m.State.HasSettledHTLC {
-		return ErrPaymentPendingSettled
+		return paymentsdb.ErrPaymentPendingSettled
 	}
 
 	// If the payment is already failed, we won't allow adding more HTLCs.
 	if m.State.PaymentFailed {
-		return ErrPaymentPendingFailed
+		return paymentsdb.ErrPaymentPendingFailed
 	}
 
 	// Otherwise we can add more HTLCs.
@@ -371,8 +372,8 @@ func (m *MPPayment) setState() error {
 	// Sanity check we haven't sent a value larger than the payment amount.
 	totalAmt := m.Info.Value
 	if sentAmt > totalAmt {
-		return fmt.Errorf("%w: sent=%v, total=%v", ErrSentExceedsTotal,
-			sentAmt, totalAmt)
+		return fmt.Errorf("%w: sent=%v, total=%v",
+			paymentsdb.ErrSentExceedsTotal, sentAmt, totalAmt)
 	}
 
 	// Get any terminal info for this payment.
@@ -451,7 +452,7 @@ func (m *MPPayment) NeedWaitAttempts() (bool, error) {
 		case StatusSucceeded:
 			return false, fmt.Errorf("%w: parts of the payment "+
 				"already succeeded but still have remaining "+
-				"amount %v", ErrPaymentInternal,
+				"amount %v", paymentsdb.ErrPaymentInternal,
 				m.State.RemainingAmt)
 
 		// The payment is failed and we have no inflight HTLCs, no need
@@ -462,7 +463,7 @@ func (m *MPPayment) NeedWaitAttempts() (bool, error) {
 		// Unknown payment status.
 		default:
 			return false, fmt.Errorf("%w: %s",
-				ErrUnknownPaymentStatus, m.Status)
+				paymentsdb.ErrUnknownPaymentStatus, m.Status)
 		}
 	}
 
@@ -472,7 +473,8 @@ func (m *MPPayment) NeedWaitAttempts() (bool, error) {
 	// When the payment is newly created, yet the payment has no remaining
 	// amount, return an error.
 	case StatusInitiated:
-		return false, fmt.Errorf("%w: %v", ErrPaymentInternal, m.Status)
+		return false, fmt.Errorf("%w: %v",
+			paymentsdb.ErrPaymentInternal, m.Status)
 
 	// If the payment is inflight, we must wait.
 	//
@@ -493,12 +495,13 @@ func (m *MPPayment) NeedWaitAttempts() (bool, error) {
 	// marked as failed with a reason, which means the remainingAmt must
 	// not be zero because our sentAmt is zero.
 	case StatusFailed:
-		return false, fmt.Errorf("%w: %v", ErrPaymentInternal, m.Status)
+		return false, fmt.Errorf("%w: %v",
+			paymentsdb.ErrPaymentInternal, m.Status)
 
 	// Unknown payment status.
 	default:
-		return false, fmt.Errorf("%w: %s", ErrUnknownPaymentStatus,
-			m.Status)
+		return false, fmt.Errorf("%w: %s",
+			paymentsdb.ErrUnknownPaymentStatus, m.Status)
 	}
 }
 
@@ -528,7 +531,8 @@ func (m *MPPayment) AllowMoreAttempts() (bool, error) {
 		// remainingAmt, return an error.
 		if m.Status == StatusInitiated {
 			return false, fmt.Errorf("%w: initiated payment has "+
-				"zero remainingAmt", ErrPaymentInternal)
+				"zero remainingAmt",
+				paymentsdb.ErrPaymentInternal)
 		}
 
 		// Otherwise, exit early since all other statuses with zero
@@ -544,8 +548,8 @@ func (m *MPPayment) AllowMoreAttempts() (bool, error) {
 	// as the preimage is received. In this case, return an error state.
 	if m.Status == StatusSucceeded {
 		return false, fmt.Errorf("%w: payment already succeeded but "+
-			"still have remaining amount %v", ErrPaymentInternal,
-			m.State.RemainingAmt)
+			"still have remaining amount %v",
+			paymentsdb.ErrPaymentInternal, m.State.RemainingAmt)
 	}
 
 	// Now check if we can register a new HTLC.
diff --git a/channeldb/mp_payment_test.go b/channeldb/mp_payment_test.go
index 13e3987..455a04d 100644
--- a/channeldb/mp_payment_test.go
+++ b/channeldb/mp_payment_test.go
@@ -8,6 +8,7 @@ import (
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
+	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
 	"github.com/lightningnetwork/lnd/routing/route"
 	"github.com/stretchr/testify/require"
 )
@@ -65,35 +66,35 @@ func TestRegistrable(t *testing.T) {
 			// Test inflight status with settled HTLC but no failed
 			// payment.
 			status:         StatusInFlight,
-			registryErr:    ErrPaymentPendingSettled,
+			registryErr:    paymentsdb.ErrPaymentPendingSettled,
 			hasSettledHTLC: true,
 		},
 		{
 			// Test inflight status with no settled HTLC but failed
 			// payment.
 			status:        StatusInFlight,
-			registryErr:   ErrPaymentPendingFailed,
+			registryErr:   paymentsdb.ErrPaymentPendingFailed,
 			paymentFailed: true,
 		},
 		{
 			// Test error state with settled HTLC and failed
 			// payment.
 			status:         0,
-			registryErr:    ErrUnknownPaymentStatus,
+			registryErr:    paymentsdb.ErrUnknownPaymentStatus,
 			hasSettledHTLC: true,
 			paymentFailed:  true,
 		},
 		{
 			status:      StatusSucceeded,
-			registryErr: ErrPaymentAlreadySucceeded,
+			registryErr: paymentsdb.ErrPaymentAlreadySucceeded,
 		},
 		{
 			status:      StatusFailed,
-			registryErr: ErrPaymentAlreadyFailed,
+			registryErr: paymentsdb.ErrPaymentAlreadyFailed,
 		},
 		{
 			status:      0,
-			registryErr: ErrUnknownPaymentStatus,
+			registryErr: paymentsdb.ErrUnknownPaymentStatus,
 		},
 	}
 
@@ -149,7 +150,7 @@ func TestPaymentSetState(t *testing.T) {
 				},
 			},
 			totalAmt:    1,
-			errExpected: ErrSentExceedsTotal,
+			errExpected: paymentsdb.ErrSentExceedsTotal,
 		},
 		{
 			// Test that when the htlc is failed, the fee is not
@@ -294,7 +295,7 @@ func TestNeedWaitAttempts(t *testing.T) {
 			status:       StatusSucceeded,
 			remainingAmt: 1000,
 			needWait:     false,
-			expectedErr:  ErrPaymentInternal,
+			expectedErr:  paymentsdb.ErrPaymentInternal,
 		},
 		{
 			// Payment is in terminal state, no need to wait.
@@ -309,7 +310,7 @@ func TestNeedWaitAttempts(t *testing.T) {
 			status:       StatusInitiated,
 			remainingAmt: 0,
 			needWait:     false,
-			expectedErr:  ErrPaymentInternal,
+			expectedErr:  paymentsdb.ErrPaymentInternal,
 		},
 		{
 			// With zero remainingAmt we must wait for the results.
@@ -330,21 +331,21 @@ func TestNeedWaitAttempts(t *testing.T) {
 			status:       StatusFailed,
 			remainingAmt: 0,
 			needWait:     false,
-			expectedErr:  ErrPaymentInternal,
+			expectedErr:  paymentsdb.ErrPaymentInternal,
 		},
 		{
 			// Payment is in an unknown status, return an error.
 			status:       0,
 			remainingAmt: 0,
 			needWait:     false,
-			expectedErr:  ErrUnknownPaymentStatus,
+			expectedErr:  paymentsdb.ErrUnknownPaymentStatus,
 		},
 		{
 			// Payment is in an unknown status, return an error.
 			status:       0,
 			remainingAmt: 1000,
 			needWait:     false,
-			expectedErr:  ErrUnknownPaymentStatus,
+			expectedErr:  paymentsdb.ErrUnknownPaymentStatus,
 		},
 	}
 
@@ -397,7 +398,7 @@ func TestAllowMoreAttempts(t *testing.T) {
 			status:       StatusInitiated,
 			remainingAmt: 0,
 			allowMore:    false,
-			expectedErr:  ErrPaymentInternal,
+			expectedErr:  paymentsdb.ErrPaymentInternal,
 		},
 		{
 			// With zero remainingAmt we don't allow more HTLC
@@ -505,7 +506,7 @@ func TestAllowMoreAttempts(t *testing.T) {
 			remainingAmt:   1000,
 			hasSettledHTLC: true,
 			allowMore:      false,
-			expectedErr:    ErrPaymentInternal,
+			expectedErr:    paymentsdb.ErrPaymentInternal,
 		},
 		{
 			// With the payment failed with no inflight HTLCs, we
diff --git a/channeldb/payment_status.go b/channeldb/payment_status.go
index 4501844..179e22f 100644
--- a/channeldb/payment_status.go
+++ b/channeldb/payment_status.go
@@ -1,6 +1,10 @@
 package channeldb
 
-import "fmt"
+import (
+	"fmt"
+
+	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
+)
 
 // PaymentStatus represent current status of payment.
 type PaymentStatus byte
@@ -58,24 +62,25 @@ func (ps PaymentStatus) initializable() error {
 	// again in case other goroutines have already been creating HTLCs for
 	// it.
 	case StatusInitiated:
-		return ErrPaymentExists
+		return paymentsdb.ErrPaymentExists
 
 	// We already have an InFlight payment on the network. We will disallow
 	// any new payments.
 	case StatusInFlight:
-		return ErrPaymentInFlight
+		return paymentsdb.ErrPaymentInFlight
 
 	// The payment has been attempted and is succeeded so we won't allow
 	// creating it again.
 	case StatusSucceeded:
-		return ErrAlreadyPaid
+		return paymentsdb.ErrAlreadyPaid
 
 	// We allow retrying failed payments.
 	case StatusFailed:
 		return nil
 
 	default:
-		return fmt.Errorf("%w: %v", ErrUnknownPaymentStatus, ps)
+		return fmt.Errorf("%w: %v", paymentsdb.ErrUnknownPaymentStatus,
+			ps)
 	}
 }
 
@@ -91,7 +96,7 @@ func (ps PaymentStatus) removable() error {
 	// There are still inflight HTLCs and the payment needs to wait for the
 	// final outcomes.
 	case StatusInFlight:
-		return ErrPaymentInFlight
+		return paymentsdb.ErrPaymentInFlight
 
 	// The payment has been attempted and is succeeded and is allowed to be
 	// removed.
@@ -103,7 +108,8 @@ func (ps PaymentStatus) removable() error {
 		return nil
 
 	default:
-		return fmt.Errorf("%w: %v", ErrUnknownPaymentStatus, ps)
+		return fmt.Errorf("%w: %v", paymentsdb.ErrUnknownPaymentStatus,
+			ps)
 	}
 }
 
@@ -121,13 +127,14 @@ func (ps PaymentStatus) updatable() error {
 
 	// If the payment has a terminal condition, we won't allow any updates.
 	case StatusSucceeded:
-		return ErrPaymentAlreadySucceeded
+		return paymentsdb.ErrPaymentAlreadySucceeded
 
 	case StatusFailed:
-		return ErrPaymentAlreadyFailed
+		return paymentsdb.ErrPaymentAlreadyFailed
 
 	default:
-		return fmt.Errorf("%w: %v", ErrUnknownPaymentStatus, ps)
+		return fmt.Errorf("%w: %v", paymentsdb.ErrUnknownPaymentStatus,
+			ps)
 	}
 }
 
diff --git a/channeldb/payment_status_test.go b/channeldb/payment_status_test.go
index f821623..bd61818 100644
--- a/channeldb/payment_status_test.go
+++ b/channeldb/payment_status_test.go
@@ -5,6 +5,7 @@ import (
 	"testing"
 
 	"github.com/lightningnetwork/lnd/lntypes"
+	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
 	"github.com/stretchr/testify/require"
 )
 
@@ -197,33 +198,33 @@ func TestPaymentStatusActions(t *testing.T) {
 	}{
 		{
 			status:    StatusInitiated,
-			initErr:   ErrPaymentExists,
+			initErr:   paymentsdb.ErrPaymentExists,
 			updateErr: nil,
 			removeErr: nil,
 		},
 		{
 			status:    StatusInFlight,
-			initErr:   ErrPaymentInFlight,
+			initErr:   paymentsdb.ErrPaymentInFlight,
 			updateErr: nil,
-			removeErr: ErrPaymentInFlight,
+			removeErr: paymentsdb.ErrPaymentInFlight,
 		},
 		{
 			status:    StatusSucceeded,
-			initErr:   ErrAlreadyPaid,
-			updateErr: ErrPaymentAlreadySucceeded,
+			initErr:   paymentsdb.ErrAlreadyPaid,
+			updateErr: paymentsdb.ErrPaymentAlreadySucceeded,
 			removeErr: nil,
 		},
 		{
 			status:    StatusFailed,
 			initErr:   nil,
-			updateErr: ErrPaymentAlreadyFailed,
+			updateErr: paymentsdb.ErrPaymentAlreadyFailed,
 			removeErr: nil,
 		},
 		{
 			status:    0,
-			initErr:   ErrUnknownPaymentStatus,
-			updateErr: ErrUnknownPaymentStatus,
-			removeErr: ErrUnknownPaymentStatus,
+			initErr:   paymentsdb.ErrUnknownPaymentStatus,
+			updateErr: paymentsdb.ErrUnknownPaymentStatus,
+			removeErr: paymentsdb.ErrUnknownPaymentStatus,
 		},
 	}
 
diff --git a/channeldb/payments_kv_store.go b/channeldb/payments_kv_store.go
index ab98d42..4c1880a 100644
--- a/channeldb/payments_kv_store.go
+++ b/channeldb/payments_kv_store.go
@@ -16,6 +16,7 @@ import (
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
+	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
 	"github.com/lightningnetwork/lnd/record"
 	"github.com/lightningnetwork/lnd/routing/route"
 	"github.com/lightningnetwork/lnd/tlv"
@@ -31,113 +32,6 @@ const (
 	paymentProgressLogInterval = 30 * time.Second
 )
 
-var (
-	// ErrAlreadyPaid signals we have already paid this payment hash.
-	ErrAlreadyPaid = errors.New("invoice is already paid")
-
-	// ErrPaymentInFlight signals that payment for this payment hash is
-	// already "in flight" on the network.
-	ErrPaymentInFlight = errors.New("payment is in transition")
-
-	// ErrPaymentExists is returned when we try to initialize an already
-	// existing payment that is not failed.
-	ErrPaymentExists = errors.New("payment already exists")
-
-	// ErrPaymentInternal is returned when performing the payment has a
-	// conflicting state, such as,
-	// - payment has StatusSucceeded but remaining amount is not zero.
-	// - payment has StatusInitiated but remaining amount is zero.
-	// - payment has StatusFailed but remaining amount is zero.
-	ErrPaymentInternal = errors.New("internal error")
-
-	// ErrPaymentNotInitiated is returned if the payment wasn't initiated.
-	ErrPaymentNotInitiated = errors.New("payment isn't initiated")
-
-	// ErrPaymentAlreadySucceeded is returned in the event we attempt to
-	// change the status of a payment already succeeded.
-	ErrPaymentAlreadySucceeded = errors.New("payment is already succeeded")
-
-	// ErrPaymentAlreadyFailed is returned in the event we attempt to alter
-	// a failed payment.
-	ErrPaymentAlreadyFailed = errors.New("payment has already failed")
-
-	// ErrUnknownPaymentStatus is returned when we do not recognize the
-	// existing state of a payment.
-	ErrUnknownPaymentStatus = errors.New("unknown payment status")
-
-	// ErrPaymentTerminal is returned if we attempt to alter a payment that
-	// already has reached a terminal condition.
-	ErrPaymentTerminal = errors.New("payment has reached terminal " +
-		"condition")
-
-	// ErrAttemptAlreadySettled is returned if we try to alter an already
-	// settled HTLC attempt.
-	ErrAttemptAlreadySettled = errors.New("attempt already settled")
-
-	// ErrAttemptAlreadyFailed is returned if we try to alter an already
-	// failed HTLC attempt.
-	ErrAttemptAlreadyFailed = errors.New("attempt already failed")
-
-	// ErrValueMismatch is returned if we try to register a non-MPP attempt
-	// with an amount that doesn't match the payment amount.
-	ErrValueMismatch = errors.New("attempted value doesn't match payment " +
-		"amount")
-
-	// ErrValueExceedsAmt is returned if we try to register an attempt that
-	// would take the total sent amount above the payment amount.
-	ErrValueExceedsAmt = errors.New("attempted value exceeds payment " +
-		"amount")
-
-	// ErrNonMPPayment is returned if we try to register an MPP attempt for
-	// a payment that already has a non-MPP attempt registered.
-	ErrNonMPPayment = errors.New("payment has non-MPP attempts")
-
-	// ErrMPPayment is returned if we try to register a non-MPP attempt for
-	// a payment that already has an MPP attempt registered.
-	ErrMPPayment = errors.New("payment has MPP attempts")
-
-	// ErrMPPRecordInBlindedPayment is returned if we try to register an
-	// attempt with an MPP record for a payment to a blinded path.
-	ErrMPPRecordInBlindedPayment = errors.New("blinded payment cannot " +
-		"contain MPP records")
-
-	// ErrBlindedPaymentTotalAmountMismatch is returned if we try to
-	// register an HTLC shard to a blinded route where the total amount
-	// doesn't match existing shards.
-	ErrBlindedPaymentTotalAmountMismatch = errors.New("blinded path " +
-		"total amount mismatch")
-
-	// ErrMPPPaymentAddrMismatch is returned if we try to register an MPP
-	// shard where the payment address doesn't match existing shards.
-	ErrMPPPaymentAddrMismatch = errors.New("payment address mismatch")
-
-	// ErrMPPTotalAmountMismatch is returned if we try to register an MPP
-	// shard where the total amount doesn't match existing shards.
-	ErrMPPTotalAmountMismatch = errors.New("mp payment total amount " +
-		"mismatch")
-
-	// ErrPaymentPendingSettled is returned when we try to add a new
-	// attempt to a payment that has at least one of its HTLCs settled.
-	ErrPaymentPendingSettled = errors.New("payment has settled htlcs")
-
-	// ErrPaymentPendingFailed is returned when we try to add a new attempt
-	// to a payment that already has a failure reason.
-	ErrPaymentPendingFailed = errors.New("payment has failure reason")
-
-	// ErrSentExceedsTotal is returned if the payment's current total sent
-	// amount exceed the total amount.
-	ErrSentExceedsTotal = errors.New("total sent exceeds total amount")
-
-	// errNoAttemptInfo is returned when no attempt info is stored yet.
-	errNoAttemptInfo = errors.New("unable to find attempt info for " +
-		"inflight payment")
-
-	// errNoSequenceNrIndex is returned when an attempt to lookup a payment
-	// index is made for a sequence number that is not indexed.
-	errNoSequenceNrIndex = errors.New("payment sequence number index " +
-		"does not exist")
-)
-
 //nolint:ll
 var (
 	// paymentsRootBucket is the name of the top-level bucket within the
@@ -223,26 +117,6 @@ var (
 	paymentsIndexBucket = []byte("payments-index-bucket")
 )
 
-var (
-	// ErrNoSequenceNumber is returned if we look up a payment which does
-	// not have a sequence number.
-	ErrNoSequenceNumber = errors.New("sequence number not found")
-
-	// ErrDuplicateNotFound is returned when we lookup a payment by its
-	// index and cannot find a payment with a matching sequence number.
-	ErrDuplicateNotFound = errors.New("duplicate payment not found")
-
-	// ErrNoDuplicateBucket is returned when we expect to find duplicates
-	// when looking up a payment from its index, but the payment does not
-	// have any.
-	ErrNoDuplicateBucket = errors.New("expected duplicate bucket")
-
-	// ErrNoDuplicateNestedBucket is returned if we do not find duplicate
-	// payments in their own sub-bucket.
-	ErrNoDuplicateNestedBucket = errors.New("nested duplicate bucket not " +
-		"found")
-)
-
 // KVPaymentsDB implements persistence for payments and payment attempts.
 type KVPaymentsDB struct {
 	paymentSeqMx     sync.Mutex
@@ -306,7 +180,7 @@ func (p *KVPaymentsDB) InitPayment(paymentHash lntypes.Hash,
 
 		// Otherwise, if the error is not `ErrPaymentNotInitiated`,
 		// we'll return the error.
-		case !errors.Is(err, ErrPaymentNotInitiated):
+		case !errors.Is(err, paymentsdb.ErrPaymentNotInitiated):
 			return err
 		}
 
@@ -480,7 +354,7 @@ func (p *KVPaymentsDB) RegisterAttempt(paymentHash lntypes.Hash,
 
 		// MPP records should not be set for attempts to blinded paths.
 		if isBlinded && mpp != nil {
-			return ErrMPPRecordInBlindedPayment
+			return paymentsdb.ErrMPPRecordInBlindedPayment
 		}
 
 		for _, h := range payment.InFlightHTLCs() {
@@ -489,7 +363,7 @@ func (p *KVPaymentsDB) RegisterAttempt(paymentHash lntypes.Hash,
 			// If this is a blinded payment, then no existing HTLCs
 			// should have MPP records.
 			if isBlinded && hMpp != nil {
-				return ErrMPPRecordInBlindedPayment
+				return paymentsdb.ErrMPPRecordInBlindedPayment
 			}
 
 			// If this is a blinded payment, then we just need to
@@ -501,7 +375,7 @@ func (p *KVPaymentsDB) RegisterAttempt(paymentHash lntypes.Hash,
 					h.Route.FinalHop().TotalAmtMsat {
 
 					//nolint:ll
-					return ErrBlindedPaymentTotalAmountMismatch
+					return paymentsdb.ErrBlindedPaymentTotalAmountMismatch
 				}
 
 				continue
@@ -511,12 +385,12 @@ func (p *KVPaymentsDB) RegisterAttempt(paymentHash lntypes.Hash,
 			// We tried to register a non-MPP attempt for a MPP
 			// payment.
 			case mpp == nil && hMpp != nil:
-				return ErrMPPayment
+				return paymentsdb.ErrMPPayment
 
 			// We tried to register a MPP shard for a non-MPP
 			// payment.
 			case mpp != nil && hMpp == nil:
-				return ErrNonMPPayment
+				return paymentsdb.ErrNonMPPayment
 
 			// Non-MPP payment, nothing more to validate.
 			case mpp == nil:
@@ -525,11 +399,11 @@ func (p *KVPaymentsDB) RegisterAttempt(paymentHash lntypes.Hash,
 
 			// Check that MPP options match.
 			if mpp.PaymentAddr() != hMpp.PaymentAddr() {
-				return ErrMPPPaymentAddrMismatch
+				return paymentsdb.ErrMPPPaymentAddrMismatch
 			}
 
 			if mpp.TotalMsat() != hMpp.TotalMsat() {
-				return ErrMPPTotalAmountMismatch
+				return paymentsdb.ErrMPPTotalAmountMismatch
 			}
 		}
 
@@ -538,15 +412,15 @@ func (p *KVPaymentsDB) RegisterAttempt(paymentHash lntypes.Hash,
 		// attempt.
 		amt := attempt.Route.ReceiverAmt()
 		if !isBlinded && mpp == nil && amt != payment.Info.Value {
-			return ErrValueMismatch
+			return paymentsdb.ErrValueMismatch
 		}
 
 		// Ensure we aren't sending more than the total payment amount.
 		sentAmt, _ := payment.SentAmt()
 		if sentAmt+amt > payment.Info.Value {
 			return fmt.Errorf("%w: attempted=%v, payment amount="+
-				"%v", ErrValueExceedsAmt, sentAmt+amt,
-				payment.Info.Value)
+				"%v", paymentsdb.ErrValueExceedsAmt,
+				sentAmt+amt, payment.Info.Value)
 		}
 
 		htlcsBucket, err := bucket.CreateBucketIfNotExists(
@@ -651,12 +525,12 @@ func (p *KVPaymentsDB) updateHtlcKey(paymentHash lntypes.Hash,
 		// Make sure the shard is not already failed or settled.
 		failKey := htlcBucketKey(htlcFailInfoKey, aid)
 		if htlcsBucket.Get(failKey) != nil {
-			return ErrAttemptAlreadyFailed
+			return paymentsdb.ErrAttemptAlreadyFailed
 		}
 
 		settleKey := htlcBucketKey(htlcSettleInfoKey, aid)
 		if htlcsBucket.Get(settleKey) != nil {
-			return ErrAttemptAlreadySettled
+			return paymentsdb.ErrAttemptAlreadySettled
 		}
 
 		// Add or update the key for this htlc.
@@ -696,8 +570,8 @@ func (p *KVPaymentsDB) Fail(paymentHash lntypes.Hash,
 
 		prefetchPayment(tx, paymentHash)
 		bucket, err := fetchPaymentBucketUpdate(tx, paymentHash)
-		if errors.Is(err, ErrPaymentNotInitiated) {
-			updateErr = ErrPaymentNotInitiated
+		if errors.Is(err, paymentsdb.ErrPaymentNotInitiated) {
+			updateErr = paymentsdb.ErrPaymentNotInitiated
 			return nil
 		} else if err != nil {
 			return err
@@ -708,8 +582,8 @@ func (p *KVPaymentsDB) Fail(paymentHash lntypes.Hash,
 		// failure to the KVPaymentsDB without synchronizing with
 		// other attempts.
 		_, err = fetchPaymentStatus(bucket)
-		if errors.Is(err, ErrPaymentNotInitiated) {
-			updateErr = ErrPaymentNotInitiated
+		if errors.Is(err, paymentsdb.ErrPaymentNotInitiated) {
+			updateErr = paymentsdb.ErrPaymentNotInitiated
 			return nil
 		} else if err != nil {
 			return err
@@ -802,12 +676,12 @@ func fetchPaymentBucket(tx kvdb.RTx, paymentHash lntypes.Hash) (
 
 	payments := tx.ReadBucket(paymentsRootBucket)
 	if payments == nil {
-		return nil, ErrPaymentNotInitiated
+		return nil, paymentsdb.ErrPaymentNotInitiated
 	}
 
 	bucket := payments.NestedReadBucket(paymentHash[:])
 	if bucket == nil {
-		return nil, ErrPaymentNotInitiated
+		return nil, paymentsdb.ErrPaymentNotInitiated
 	}
 
 	return bucket, nil
@@ -820,12 +694,12 @@ func fetchPaymentBucketUpdate(tx kvdb.RwTx, paymentHash lntypes.Hash) (
 
 	payments := tx.ReadWriteBucket(paymentsRootBucket)
 	if payments == nil {
-		return nil, ErrPaymentNotInitiated
+		return nil, paymentsdb.ErrPaymentNotInitiated
 	}
 
 	bucket := payments.NestedReadWriteBucket(paymentHash[:])
 	if bucket == nil {
-		return nil, ErrPaymentNotInitiated
+		return nil, paymentsdb.ErrPaymentNotInitiated
 	}
 
 	return bucket, nil
@@ -882,7 +756,7 @@ func fetchPaymentStatus(bucket kvdb.RBucket) (PaymentStatus, error) {
 	// Creation info should be set for all payments, regardless of state.
 	// If not, it is unknown.
 	if bucket.Get(paymentCreationInfoKey) == nil {
-		return 0, ErrPaymentNotInitiated
+		return 0, paymentsdb.ErrPaymentNotInitiated
 	}
 
 	payment, err := fetchPayment(bucket)
@@ -1128,7 +1002,7 @@ func fetchHtlcAttempts(bucket kvdb.RBucket) ([]HTLCAttempt, error) {
 
 	// Sanity check that all htlcs have an attempt info.
 	if attemptInfoCount != len(htlcsMap) {
-		return nil, errNoAttemptInfo
+		return nil, paymentsdb.ErrNoAttemptInfo
 	}
 
 	keys := make([]uint64, len(htlcsMap))
@@ -1367,7 +1241,7 @@ func fetchPaymentWithSequenceNumber(tx kvdb.RTx, paymentHash lntypes.Hash,
 	// the payment we are actually looking for.
 	seqBytes := bucket.Get(paymentSequenceKey)
 	if seqBytes == nil {
-		return nil, ErrNoSequenceNumber
+		return nil, paymentsdb.ErrNoSequenceNumber
 	}
 
 	// If this top level payment has the sequence number we are looking for,
@@ -1382,7 +1256,7 @@ func fetchPaymentWithSequenceNumber(tx kvdb.RTx, paymentHash lntypes.Hash,
 	// find a duplicate payments bucket here, something is wrong.
 	dup := bucket.NestedReadBucket(duplicatePaymentsBucket)
 	if dup == nil {
-		return nil, ErrNoDuplicateBucket
+		return nil, paymentsdb.ErrNoDuplicateBucket
 	}
 
 	var duplicatePayment *MPPayment
@@ -1390,7 +1264,7 @@ func fetchPaymentWithSequenceNumber(tx kvdb.RTx, paymentHash lntypes.Hash,
 		subBucket := dup.NestedReadBucket(k)
 		if subBucket == nil {
 			// We one bucket for each duplicate to be found.
-			return ErrNoDuplicateNestedBucket
+			return paymentsdb.ErrNoDuplicateNestedBucket
 		}
 
 		seqBytes := subBucket.Get(duplicatePaymentSequenceKey)
@@ -1419,7 +1293,7 @@ func fetchPaymentWithSequenceNumber(tx kvdb.RTx, paymentHash lntypes.Hash,
 	// failed to find the payment with this sequence number; something is
 	// wrong.
 	if duplicatePayment == nil {
-		return nil, ErrDuplicateNotFound
+		return nil, paymentsdb.ErrDuplicateNotFound
 	}
 
 	return duplicatePayment, nil
diff --git a/channeldb/payments_kv_store_test.go b/channeldb/payments_kv_store_test.go
index 3e82559..4083fa3 100644
--- a/channeldb/payments_kv_store_test.go
+++ b/channeldb/payments_kv_store_test.go
@@ -16,6 +16,7 @@ import (
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
+	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
 	"github.com/lightningnetwork/lnd/record"
 	"github.com/lightningnetwork/lnd/routing/route"
 	"github.com/lightningnetwork/lnd/tlv"
@@ -193,7 +194,7 @@ func TestKVPaymentsDBSwitchFail(t *testing.T) {
 	// Attempt a final payment, which should now fail since the prior
 	// payment succeed.
 	err = paymentDB.InitPayment(info.PaymentIdentifier, info)
-	if !errors.Is(err, ErrAlreadyPaid) {
+	if !errors.Is(err, paymentsdb.ErrAlreadyPaid) {
 		t.Fatalf("unable to send htlc message: %v", err)
 	}
 }
@@ -228,7 +229,7 @@ func TestKVPaymentsDBSwitchDoubleSend(t *testing.T) {
 	// payment hash, should result in error indicating that payment has
 	// already been sent.
 	err = paymentDB.InitPayment(info.PaymentIdentifier, info)
-	require.ErrorIs(t, err, ErrPaymentExists)
+	require.ErrorIs(t, err, paymentsdb.ErrPaymentExists)
 
 	// Record an attempt.
 	_, err = paymentDB.RegisterAttempt(info.PaymentIdentifier, attempt)
@@ -246,7 +247,7 @@ func TestKVPaymentsDBSwitchDoubleSend(t *testing.T) {
 
 	// Sends base htlc message which initiate StatusInFlight.
 	err = paymentDB.InitPayment(info.PaymentIdentifier, info)
-	if !errors.Is(err, ErrPaymentInFlight) {
+	if !errors.Is(err, paymentsdb.ErrPaymentInFlight) {
 		t.Fatalf("payment control wrong behaviour: " +
 			"double sending must trigger ErrPaymentInFlight error")
 	}
@@ -269,7 +270,7 @@ func TestKVPaymentsDBSwitchDoubleSend(t *testing.T) {
 	)
 
 	err = paymentDB.InitPayment(info.PaymentIdentifier, info)
-	if !errors.Is(err, ErrAlreadyPaid) {
+	if !errors.Is(err, paymentsdb.ErrAlreadyPaid) {
 		t.Fatalf("unable to send htlc message: %v", err)
 	}
 }
@@ -294,9 +295,7 @@ func TestKVPaymentsDBSuccessesWithoutInFlight(t *testing.T) {
 			Preimage: preimg,
 		},
 	)
-	if err != ErrPaymentNotInitiated {
-		t.Fatalf("expected ErrPaymentNotInitiated, got %v", err)
-	}
+	require.ErrorIs(t, err, paymentsdb.ErrPaymentNotInitiated)
 }
 
 // TestKVPaymentsDBFailsWithoutInFlight checks that a strict payment
@@ -314,9 +313,7 @@ func TestKVPaymentsDBFailsWithoutInFlight(t *testing.T) {
 
 	// Calling Fail should return an error.
 	_, err = paymentDB.Fail(info.PaymentIdentifier, FailureReasonNoRoute)
-	if err != ErrPaymentNotInitiated {
-		t.Fatalf("expected ErrPaymentNotInitiated, got %v", err)
-	}
+	require.ErrorIs(t, err, paymentsdb.ErrPaymentNotInitiated)
 }
 
 // TestKVPaymentsDBDeleteNonInFlight checks that calling DeletePayments only
@@ -780,7 +777,7 @@ func TestKVPaymentsDBMultiShard(t *testing.T) {
 		b := *attempt
 		b.AttemptID = 3
 		_, err = paymentDB.RegisterAttempt(info.PaymentIdentifier, &b)
-		require.ErrorIs(t, err, ErrValueExceedsAmt)
+		require.ErrorIs(t, err, paymentsdb.ErrValueExceedsAmt)
 
 		// Fail the second attempt.
 		a := attempts[1]
@@ -880,9 +877,13 @@ func TestKVPaymentsDBMultiShard(t *testing.T) {
 		b.AttemptID = 3
 		_, err = paymentDB.RegisterAttempt(info.PaymentIdentifier, &b)
 		if test.settleFirst {
-			require.ErrorIs(t, err, ErrPaymentPendingSettled)
+			require.ErrorIs(
+				t, err, paymentsdb.ErrPaymentPendingSettled,
+			)
 		} else {
-			require.ErrorIs(t, err, ErrPaymentPendingFailed)
+			require.ErrorIs(
+				t, err, paymentsdb.ErrPaymentPendingFailed,
+			)
 		}
 
 		assertPaymentStatus(
@@ -951,21 +952,21 @@ func TestKVPaymentsDBMultiShard(t *testing.T) {
 		// settled.
 		case test.settleFirst && !test.settleLast:
 			finalStatus = StatusSucceeded
-			registerErr = ErrPaymentAlreadySucceeded
+			registerErr = paymentsdb.ErrPaymentAlreadySucceeded
 
 		case !test.settleFirst && test.settleLast:
 			finalStatus = StatusSucceeded
-			registerErr = ErrPaymentAlreadySucceeded
+			registerErr = paymentsdb.ErrPaymentAlreadySucceeded
 
 		// If both failed, we end up in a failed status.
 		case !test.settleFirst && !test.settleLast:
 			finalStatus = StatusFailed
-			registerErr = ErrPaymentAlreadyFailed
+			registerErr = paymentsdb.ErrPaymentAlreadyFailed
 
 		// Otherwise, the payment has a succeed status.
 		case test.settleFirst && test.settleLast:
 			finalStatus = StatusSucceeded
-			registerErr = ErrPaymentAlreadySucceeded
+			registerErr = paymentsdb.ErrPaymentAlreadySucceeded
 		}
 
 		assertPaymentStatus(
@@ -1021,27 +1022,21 @@ func TestKVPaymentsDBMPPRecordValidation(t *testing.T) {
 	b.AttemptID = 1
 	b.Route.FinalHop().MPP = nil
 	_, err = paymentDB.RegisterAttempt(info.PaymentIdentifier, &b)
-	if err != ErrMPPayment {
-		t.Fatalf("expected ErrMPPayment, got: %v", err)
-	}
+	require.ErrorIs(t, err, paymentsdb.ErrMPPayment)
 
 	// Try to register attempt one with a different payment address.
 	b.Route.FinalHop().MPP = record.NewMPP(
 		info.Value, [32]byte{2},
 	)
 	_, err = paymentDB.RegisterAttempt(info.PaymentIdentifier, &b)
-	if err != ErrMPPPaymentAddrMismatch {
-		t.Fatalf("expected ErrMPPPaymentAddrMismatch, got: %v", err)
-	}
+	require.ErrorIs(t, err, paymentsdb.ErrMPPPaymentAddrMismatch)
 
 	// Try registering one with a different total amount.
 	b.Route.FinalHop().MPP = record.NewMPP(
 		info.Value/2, [32]byte{1},
 	)
 	_, err = paymentDB.RegisterAttempt(info.PaymentIdentifier, &b)
-	if err != ErrMPPTotalAmountMismatch {
-		t.Fatalf("expected ErrMPPTotalAmountMismatch, got: %v", err)
-	}
+	require.ErrorIs(t, err, paymentsdb.ErrMPPTotalAmountMismatch)
 
 	// Create and init a new payment. This time we'll check that we cannot
 	// register an MPP attempt if we already registered a non-MPP one.
@@ -1063,9 +1058,7 @@ func TestKVPaymentsDBMPPRecordValidation(t *testing.T) {
 	)
 
 	_, err = paymentDB.RegisterAttempt(info.PaymentIdentifier, &b)
-	if err != ErrNonMPPayment {
-		t.Fatalf("expected ErrNonMPPayment, got: %v", err)
-	}
+	require.ErrorIs(t, err, paymentsdb.ErrNonMPPayment)
 }
 
 // TestDeleteFailedAttempts checks that DeleteFailedAttempts properly removes
@@ -1171,7 +1164,7 @@ func assertPaymentStatus(t *testing.T, p *KVPaymentsDB,
 	t.Helper()
 
 	payment, err := p.FetchPayment(hash)
-	if errors.Is(err, ErrPaymentNotInitiated) {
+	if errors.Is(err, paymentsdb.ErrPaymentNotInitiated) {
 		return
 	}
 	if err != nil {
@@ -1271,7 +1264,7 @@ func fetchPaymentIndexEntry(_ *testing.T, p *KVPaymentsDB,
 
 		indexValue := indexBucket.Get(key)
 		if indexValue == nil {
-			return errNoSequenceNrIndex
+			return paymentsdb.ErrNoSequenceNrIndex
 		}
 
 		r := bytes.NewReader(indexValue)
@@ -1307,7 +1300,7 @@ func assertPaymentIndex(t *testing.T, p *KVPaymentsDB,
 // exist.
 func assertNoIndex(t *testing.T, p *KVPaymentsDB, seqNr uint64) {
 	_, err := fetchPaymentIndexEntry(t, p, seqNr)
-	require.Equal(t, errNoSequenceNrIndex, err)
+	require.Equal(t, paymentsdb.ErrNoSequenceNrIndex, err)
 }
 
 // payment is a helper structure that holds basic information on a test payment,
@@ -1669,14 +1662,14 @@ func TestFetchPaymentWithSequenceNumber(t *testing.T) {
 			name:           "lookup non-existent duplicate",
 			paymentHash:    hasDuplicates.PaymentIdentifier,
 			sequenceNumber: 999999,
-			expectedErr:    ErrDuplicateNotFound,
+			expectedErr:    paymentsdb.ErrDuplicateNotFound,
 		},
 		{
 			name: "lookup duplicate, no duplicates " +
 				"bucket",
 			paymentHash:    noDuplicates.PaymentIdentifier,
 			sequenceNumber: duplicateTwoSeqNr,
-			expectedErr:    ErrNoDuplicateBucket,
+			expectedErr:    paymentsdb.ErrNoDuplicateBucket,
 		},
 	}
 
diff --git a/lnrpc/routerrpc/router_server.go b/lnrpc/routerrpc/router_server.go
index 2e65aed..843222e 100644
--- a/lnrpc/routerrpc/router_server.go
+++ b/lnrpc/routerrpc/router_server.go
@@ -22,6 +22,7 @@ import (
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/macaroons"
+	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
 	"github.com/lightningnetwork/lnd/routing"
 	"github.com/lightningnetwork/lnd/routing/route"
 	"github.com/lightningnetwork/lnd/zpay32"
@@ -373,9 +374,9 @@ func (s *Server) SendPaymentV2(req *SendPaymentRequest,
 			payment.Identifier(), err)
 
 		// Transform user errors to grpc code.
-		if errors.Is(err, channeldb.ErrPaymentExists) ||
-			errors.Is(err, channeldb.ErrPaymentInFlight) ||
-			errors.Is(err, channeldb.ErrAlreadyPaid) {
+		if errors.Is(err, paymentsdb.ErrPaymentExists) ||
+			errors.Is(err, paymentsdb.ErrPaymentInFlight) ||
+			errors.Is(err, paymentsdb.ErrAlreadyPaid) {
 
 			return status.Error(
 				codes.AlreadyExists, err.Error(),
@@ -955,13 +956,13 @@ func (s *Server) SendToRouteV2(ctx context.Context,
 
 	// Transform user errors to grpc code.
 	switch {
-	case errors.Is(err, channeldb.ErrPaymentExists):
+	case errors.Is(err, paymentsdb.ErrPaymentExists):
 		fallthrough
 
-	case errors.Is(err, channeldb.ErrPaymentInFlight):
+	case errors.Is(err, paymentsdb.ErrPaymentInFlight):
 		fallthrough
 
-	case errors.Is(err, channeldb.ErrAlreadyPaid):
+	case errors.Is(err, paymentsdb.ErrAlreadyPaid):
 		return nil, status.Error(
 			codes.AlreadyExists, err.Error(),
 		)
@@ -1368,7 +1369,7 @@ func (s *Server) subscribePayment(identifier lntypes.Hash) (
 	sub, err := router.Tower.SubscribePayment(identifier)
 
 	switch {
-	case errors.Is(err, channeldb.ErrPaymentNotInitiated):
+	case errors.Is(err, paymentsdb.ErrPaymentNotInitiated):
 		return nil, status.Error(codes.NotFound, err.Error())
 
 	case err != nil:
diff --git a/payments/db/errors.go b/payments/db/errors.go
new file mode 100644
index 0000000..40e37d9
--- /dev/null
+++ b/payments/db/errors.go
@@ -0,0 +1,133 @@
+package paymentsdb
+
+import "errors"
+
+var (
+	// ErrAlreadyPaid signals we have already paid this payment hash.
+	ErrAlreadyPaid = errors.New("invoice is already paid")
+
+	// ErrPaymentInFlight signals that payment for this payment hash is
+	// already "in flight" on the network.
+	ErrPaymentInFlight = errors.New("payment is in transition")
+
+	// ErrPaymentExists is returned when we try to initialize an already
+	// existing payment that is not failed.
+	ErrPaymentExists = errors.New("payment already exists")
+
+	// ErrPaymentInternal is returned when performing the payment has a
+	// conflicting state, such as,
+	// - payment has StatusSucceeded but remaining amount is not zero.
+	// - payment has StatusInitiated but remaining amount is zero.
+	// - payment has StatusFailed but remaining amount is zero.
+	ErrPaymentInternal = errors.New("internal error")
+
+	// ErrPaymentNotInitiated is returned if the payment wasn't initiated.
+	ErrPaymentNotInitiated = errors.New("payment isn't initiated")
+
+	// ErrPaymentAlreadySucceeded is returned in the event we attempt to
+	// change the status of a payment already succeeded.
+	ErrPaymentAlreadySucceeded = errors.New("payment is already succeeded")
+
+	// ErrPaymentAlreadyFailed is returned in the event we attempt to alter
+	// a failed payment.
+	ErrPaymentAlreadyFailed = errors.New("payment has already failed")
+
+	// ErrUnknownPaymentStatus is returned when we do not recognize the
+	// existing state of a payment.
+	ErrUnknownPaymentStatus = errors.New("unknown payment status")
+
+	// ErrPaymentTerminal is returned if we attempt to alter a payment that
+	// already has reached a terminal condition.
+	ErrPaymentTerminal = errors.New("payment has reached terminal " +
+		"condition")
+
+	// ErrAttemptAlreadySettled is returned if we try to alter an already
+	// settled HTLC attempt.
+	ErrAttemptAlreadySettled = errors.New("attempt already settled")
+
+	// ErrAttemptAlreadyFailed is returned if we try to alter an already
+	// failed HTLC attempt.
+	ErrAttemptAlreadyFailed = errors.New("attempt already failed")
+
+	// ErrValueMismatch is returned if we try to register a non-MPP attempt
+	// with an amount that doesn't match the payment amount.
+	ErrValueMismatch = errors.New("attempted value doesn't match payment " +
+		"amount")
+
+	// ErrValueExceedsAmt is returned if we try to register an attempt that
+	// would take the total sent amount above the payment amount.
+	ErrValueExceedsAmt = errors.New("attempted value exceeds payment " +
+		"amount")
+
+	// ErrNonMPPayment is returned if we try to register an MPP attempt for
+	// a payment that already has a non-MPP attempt registered.
+	ErrNonMPPayment = errors.New("payment has non-MPP attempts")
+
+	// ErrMPPayment is returned if we try to register a non-MPP attempt for
+	// a payment that already has an MPP attempt registered.
+	ErrMPPayment = errors.New("payment has MPP attempts")
+
+	// ErrMPPRecordInBlindedPayment is returned if we try to register an
+	// attempt with an MPP record for a payment to a blinded path.
+	ErrMPPRecordInBlindedPayment = errors.New("blinded payment cannot " +
+		"contain MPP records")
+
+	// ErrBlindedPaymentTotalAmountMismatch is returned if we try to
+	// register an HTLC shard to a blinded route where the total amount
+	// doesn't match existing shards.
+	ErrBlindedPaymentTotalAmountMismatch = errors.New("blinded path " +
+		"total amount mismatch")
+
+	// ErrMPPPaymentAddrMismatch is returned if we try to register an MPP
+	// shard where the payment address doesn't match existing shards.
+	ErrMPPPaymentAddrMismatch = errors.New("payment address mismatch")
+
+	// ErrMPPTotalAmountMismatch is returned if we try to register an MPP
+	// shard where the total amount doesn't match existing shards.
+	ErrMPPTotalAmountMismatch = errors.New("mp payment total amount " +
+		"mismatch")
+
+	// ErrPaymentPendingSettled is returned when we try to add a new
+	// attempt to a payment that has at least one of its HTLCs settled.
+	ErrPaymentPendingSettled = errors.New("payment has settled htlcs")
+
+	// ErrPaymentPendingFailed is returned when we try to add a new attempt
+	// to a payment that already has a failure reason.
+	ErrPaymentPendingFailed = errors.New("payment has failure reason")
+
+	// ErrSentExceedsTotal is returned if the payment's current total sent
+	// amount exceed the total amount.
+	ErrSentExceedsTotal = errors.New("total sent exceeds total amount")
+
+	// ErrNoAttemptInfo is returned when no attempt info is stored yet.
+	ErrNoAttemptInfo = errors.New("unable to find attempt info for " +
+		"inflight payment")
+)
+
+// KV backend specific errors.
+var (
+	// ErrNoSequenceNumber is returned if we look up a payment which does
+	// not have a sequence number.
+	ErrNoSequenceNumber = errors.New("sequence number not found")
+
+	// ErrDuplicateNotFound is returned when we lookup a payment by its
+	// index and cannot find a payment with a matching sequence number.
+	ErrDuplicateNotFound = errors.New("duplicate payment not found")
+
+	// ErrNoDuplicateBucket is returned when we expect to find duplicates
+	// when looking up a payment from its index, but the payment does not
+	// have any.
+	ErrNoDuplicateBucket = errors.New("expected duplicate bucket")
+
+	// ErrNoDuplicateNestedBucket is returned if we do not find duplicate
+	// payments in their own sub-bucket.
+	ErrNoDuplicateNestedBucket = errors.New("nested duplicate bucket not " +
+		"found")
+
+	// ErrNoSequenceNrIndex is returned when an attempt to lookup a payment
+	// index is made for a sequence number that is not indexed.
+	//
+	// NOTE: Only used for the kv backend.
+	ErrNoSequenceNrIndex = errors.New("payment sequence number index " +
+		"does not exist")
+)
diff --git a/routing/control_tower_test.go b/routing/control_tower_test.go
index 697fa74..ceb5cb6 100644
--- a/routing/control_tower_test.go
+++ b/routing/control_tower_test.go
@@ -13,6 +13,7 @@ import (
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lntypes"
+	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
 	"github.com/lightningnetwork/lnd/routing/route"
 	"github.com/stretchr/testify/require"
 )
@@ -53,7 +54,7 @@ func TestControlTowerSubscribeUnknown(t *testing.T) {
 
 	// Subscription should fail when the payment is not known.
 	_, err := pControl.SubscribePayment(lntypes.Hash{1})
-	require.ErrorIs(t, err, channeldb.ErrPaymentNotInitiated)
+	require.ErrorIs(t, err, paymentsdb.ErrPaymentNotInitiated)
 }
 
 // TestControlTowerSubscribeSuccess tests that payment updates for a
diff --git a/routing/mock_test.go b/routing/mock_test.go
index d360163..d952825 100644
--- a/routing/mock_test.go
+++ b/routing/mock_test.go
@@ -13,6 +13,7 @@ import (
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
+	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
 	"github.com/lightningnetwork/lnd/record"
 	"github.com/lightningnetwork/lnd/routing/route"
 	"github.com/lightningnetwork/lnd/routing/shards"
@@ -308,7 +309,7 @@ func (m *mockControlTowerOld) InitPayment(phash lntypes.Hash,
 
 	// Don't allow re-init a successful payment.
 	if _, ok := m.successful[phash]; ok {
-		return channeldb.ErrAlreadyPaid
+		return paymentsdb.ErrAlreadyPaid
 	}
 
 	_, failed := m.failed[phash]
@@ -316,7 +317,7 @@ func (m *mockControlTowerOld) InitPayment(phash lntypes.Hash,
 
 	// If the payment is known, only allow re-init if failed.
 	if ok && !failed {
-		return channeldb.ErrPaymentInFlight
+		return paymentsdb.ErrPaymentInFlight
 	}
 
 	delete(m.failed, phash)
@@ -330,7 +331,7 @@ func (m *mockControlTowerOld) InitPayment(phash lntypes.Hash,
 func (m *mockControlTowerOld) DeleteFailedAttempts(phash lntypes.Hash) error {
 	p, ok := m.payments[phash]
 	if !ok {
-		return channeldb.ErrPaymentNotInitiated
+		return paymentsdb.ErrPaymentNotInitiated
 	}
 
 	var inFlight bool
@@ -347,7 +348,7 @@ func (m *mockControlTowerOld) DeleteFailedAttempts(phash lntypes.Hash) error {
 	}
 
 	if inFlight {
-		return channeldb.ErrPaymentInFlight
+		return paymentsdb.ErrPaymentInFlight
 	}
 
 	return nil
@@ -366,7 +367,7 @@ func (m *mockControlTowerOld) RegisterAttempt(phash lntypes.Hash,
 	// Lookup payment.
 	p, ok := m.payments[phash]
 	if !ok {
-		return channeldb.ErrPaymentNotInitiated
+		return paymentsdb.ErrPaymentNotInitiated
 	}
 
 	var inFlight bool
@@ -387,15 +388,15 @@ func (m *mockControlTowerOld) RegisterAttempt(phash lntypes.Hash,
 	_, failed := m.failed[phash]
 
 	if settled || failed {
-		return channeldb.ErrPaymentTerminal
+		return paymentsdb.ErrPaymentTerminal
 	}
 
 	if settled && !inFlight {
-		return channeldb.ErrPaymentAlreadySucceeded
+		return paymentsdb.ErrPaymentAlreadySucceeded
 	}
 
 	if failed && !inFlight {
-		return channeldb.ErrPaymentAlreadyFailed
+		return paymentsdb.ErrPaymentAlreadyFailed
 	}
 
 	// Add attempt to payment.
@@ -421,7 +422,7 @@ func (m *mockControlTowerOld) SettleAttempt(phash lntypes.Hash,
 	// Only allow setting attempts if the payment is known.
 	p, ok := m.payments[phash]
 	if !ok {
-		return nil, channeldb.ErrPaymentNotInitiated
+		return nil, paymentsdb.ErrPaymentNotInitiated
 	}
 
 	// Find the attempt with this pid, and set the settle info.
@@ -431,10 +432,10 @@ func (m *mockControlTowerOld) SettleAttempt(phash lntypes.Hash,
 		}
 
 		if a.Settle != nil {
-			return nil, channeldb.ErrAttemptAlreadySettled
+			return nil, paymentsdb.ErrAttemptAlreadySettled
 		}
 		if a.Failure != nil {
-			return nil, channeldb.ErrAttemptAlreadyFailed
+			return nil, paymentsdb.ErrAttemptAlreadyFailed
 		}
 
 		p.attempts[i].Settle = settleInfo
@@ -462,7 +463,7 @@ func (m *mockControlTowerOld) FailAttempt(phash lntypes.Hash, pid uint64,
 	// Only allow failing attempts if the payment is known.
 	p, ok := m.payments[phash]
 	if !ok {
-		return nil, channeldb.ErrPaymentNotInitiated
+		return nil, paymentsdb.ErrPaymentNotInitiated
 	}
 
 	// Find the attempt with this pid, and set the failure info.
@@ -472,10 +473,10 @@ func (m *mockControlTowerOld) FailAttempt(phash lntypes.Hash, pid uint64,
 		}
 
 		if a.Settle != nil {
-			return nil, channeldb.ErrAttemptAlreadySettled
+			return nil, paymentsdb.ErrAttemptAlreadySettled
 		}
 		if a.Failure != nil {
-			return nil, channeldb.ErrAttemptAlreadyFailed
+			return nil, paymentsdb.ErrAttemptAlreadyFailed
 		}
 
 		p.attempts[i].Failure = failInfo
@@ -499,7 +500,7 @@ func (m *mockControlTowerOld) FailPayment(phash lntypes.Hash,
 
 	// Payment must be known.
 	if _, ok := m.payments[phash]; !ok {
-		return channeldb.ErrPaymentNotInitiated
+		return paymentsdb.ErrPaymentNotInitiated
 	}
 
 	m.failed[phash] = reason
@@ -521,7 +522,7 @@ func (m *mockControlTowerOld) fetchPayment(phash lntypes.Hash) (
 
 	p, ok := m.payments[phash]
 	if !ok {
-		return nil, channeldb.ErrPaymentNotInitiated
+		return nil, paymentsdb.ErrPaymentNotInitiated
 	}
 
 	mp := &channeldb.MPPayment{
diff --git a/routing/router.go b/routing/router.go
index dcece25..f7347c8 100644
--- a/routing/router.go
+++ b/routing/router.go
@@ -24,6 +24,7 @@ import (
 	"github.com/lightningnetwork/lnd/lnutils"
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwire"
+	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
 	"github.com/lightningnetwork/lnd/record"
 	"github.com/lightningnetwork/lnd/routing/route"
 	"github.com/lightningnetwork/lnd/routing/shards"
@@ -1133,8 +1134,8 @@ func (r *ChannelRouter) sendToRoute(htlcHash lntypes.Hash, rt *route.Route,
 	switch {
 	// If this is an MPP attempt and the hash is already registered with
 	// the database, we can go on to launch the shard.
-	case mpp != nil && errors.Is(err, channeldb.ErrPaymentInFlight):
-	case mpp != nil && errors.Is(err, channeldb.ErrPaymentExists):
+	case mpp != nil && errors.Is(err, paymentsdb.ErrPaymentInFlight):
+	case mpp != nil && errors.Is(err, paymentsdb.ErrPaymentExists):
 
 	// Any other error is not tolerated.
 	case err != nil: