From f33b5a4057a03718a06b3f2a1cc4cb34af140006 Mon Sep 17 00:00:00 2001
From: Andras Banki-Horvath <bhandras@gmail.com>
Date: Fri, 25 Jun 2021 18:21:59 +0200
Subject: [PATCH] payments: allocate payment sequences in blocks

The bucket sequence we use as payment sequence makes the DB update in
InitPayment conflict and therefore queue up when many concurrent
payments happen. This change allocates payment sequences in blocks
of 1000 to avoid these conflicts.
---
 channeldb/payment_control.go | 74 +++++++++++++++++++++++++++---------
 1 file changed, 55 insertions(+), 19 deletions(-)

diff --git a/channeldb/payment_control.go b/channeldb/payment_control.go
index c11bc5c9..454e2831 100644
--- a/channeldb/payment_control.go
+++ b/channeldb/payment_control.go
@@ -6,11 +6,18 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"sync"
 
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/lntypes"
 )
 
+const (
+	// paymentSeqBlockSize is the block size used when we batch allocate
+	// payment sequences for future payments.
+	paymentSeqBlockSize = 1000
+)
+
 var (
 	// ErrAlreadyPaid signals we have already paid this payment hash.
 	ErrAlreadyPaid = errors.New("invoice is already paid")
@@ -84,7 +91,10 @@ var (
 
 // PaymentControl implements persistence for payments and payment attempts.
 type PaymentControl struct {
-	db *DB
+	paymentSeqMx     sync.Mutex
+	currPaymentSeq   uint64
+	storedPaymentSeq uint64
+	db               *DB
 }
 
 // NewPaymentControl creates a new instance of the PaymentControl.
@@ -101,6 +111,14 @@ func NewPaymentControl(db *DB) *PaymentControl {
 func (p *PaymentControl) InitPayment(paymentHash lntypes.Hash,
 	info *PaymentCreationInfo) error {
 
+	// Obtain a new sequence number for this payment. This is used
+	// to sort the payments in order of creation, and also acts as
+	// a unique identifier for each payment.
+	sequenceNum, err := p.nextPaymentSequence()
+	if err != nil {
+		return err
+	}
+
 	var b bytes.Buffer
 	if err := serializePaymentCreationInfo(&b, info); err != nil {
 		return err
@@ -108,7 +126,7 @@ func (p *PaymentControl) InitPayment(paymentHash lntypes.Hash,
 	infoBytes := b.Bytes()
 
 	var updateErr error
-	err := kvdb.Batch(p.db.Backend, func(tx kvdb.RwTx) error {
+	err = kvdb.Batch(p.db.Backend, func(tx kvdb.RwTx) error {
 		// Reset the update error, to avoid carrying over an error
 		// from a previous execution of the batched db transaction.
 		updateErr = nil
@@ -150,14 +168,6 @@ func (p *PaymentControl) InitPayment(paymentHash lntypes.Hash,
 			return nil
 		}
 
-		// Obtain a new sequence number for this payment. This is used
-		// to sort the payments in order of creation, and also acts as
-		// a unique identifier for each payment.
-		sequenceNum, err := nextPaymentSequence(tx)
-		if err != nil {
-			return err
-		}
-
 		// Before we set our new sequence number, we check whether this
 		// payment has a previously set sequence number and remove its
 		// index entry if it exists. This happens in the case where we
@@ -615,19 +625,45 @@ func fetchPaymentBucketUpdate(tx kvdb.RwTx, paymentHash lntypes.Hash) (
 
 // nextPaymentSequence returns the next sequence number to store for a new
 // payment.
-func nextPaymentSequence(tx kvdb.RwTx) ([]byte, error) {
-	payments, err := tx.CreateTopLevelBucket(paymentsRootBucket)
-	if err != nil {
-		return nil, err
-	}
+func (p *PaymentControl) nextPaymentSequence() ([]byte, error) {
+	p.paymentSeqMx.Lock()
+	defer p.paymentSeqMx.Unlock()
+
+	// Set a new upper bound in the DB every 1000 payments to avoid
+	// conflicts on the sequence when using etcd.
+	if p.currPaymentSeq == p.storedPaymentSeq {
+		var currPaymentSeq, newUpperBound uint64
+		if err := kvdb.Update(p.db.Backend, func(tx kvdb.RwTx) error {
+			paymentsBucket, err := tx.CreateTopLevelBucket(
+				paymentsRootBucket,
+			)
+			if err != nil {
+				return err
+			}
 
-	seq, err := payments.NextSequence()
-	if err != nil {
-		return nil, err
+			currPaymentSeq = paymentsBucket.Sequence()
+			newUpperBound = currPaymentSeq + paymentSeqBlockSize
+			return paymentsBucket.SetSequence(newUpperBound)
+		}, func() {}); err != nil {
+			return nil, err
+		}
+
+		// We lazy initialize the cached currPaymentSeq here using the
+		// first nextPaymentSequence() call. This if statement will auto
+		// initialize our stored currPaymentSeq, since by default both
+		// this variable and storedPaymentSeq are zero which in turn
+		// will have us fetch the current values from the DB.
+		if p.currPaymentSeq == 0 {
+			p.currPaymentSeq = currPaymentSeq
+		}
+
+		p.storedPaymentSeq = newUpperBound
 	}
 
+	p.currPaymentSeq++
 	b := make([]byte, 8)
-	binary.BigEndian.PutUint64(b, seq)
+	binary.BigEndian.PutUint64(b, p.currPaymentSeq)
+
 	return b, nil
 }