accounts: implement kvdb DebitAccount

This commit introduces the `DebitAccount` function in the kvdb store,
to support decreasing an existing off-chain account’s balance by a
specified amount in the database.

accounts/interface.go

@@ -230,6 +230,11 @@ type Store interface {
 	CreditAccount(ctx context.Context, id AccountID,
 		amount lnwire.MilliSatoshi) error
 
+	// DebitAccount decreases the balance of the account with the
+	// given ID by the given amount.
+	DebitAccount(ctx context.Context, id AccountID,
+		amount lnwire.MilliSatoshi) error
+
 	// UpsertAccountPayment updates or inserts a payment entry for the given
 	// account. Various functional options can be passed to modify the
 	// behavior of the method. The returned boolean is true if the payment

accounts/store_kvdb.go

@@ -244,6 +244,33 @@ func (s *BoltStore) CreditAccount(_ context.Context, id AccountID,
 	return s.updateAccount(id, update)
 }
 
+// DebitAccount decreases the balance of the account with the given ID
+// by the given amount.
+//
+// NOTE: This is part of the Store interface.
+func (s *BoltStore) DebitAccount(_ context.Context, id AccountID,
+	amount lnwire.MilliSatoshi) error {
+
+	if amount > math.MaxInt64 {
+		return fmt.Errorf("amount %v exceeds the maximum of %v",
+			amount, int64(math.MaxInt64))
+	}
+
+	update := func(account *OffChainBalanceAccount) error {
+		if account.CurrentBalance-int64(amount) < 0 {
+			return fmt.Errorf("cannot debit %v from the account "+
+				"balance, as the resulting balance would be "+
+				"below 0", int64(amount/1000))
+		}
+
+		account.CurrentBalance -= int64(amount)
+
+		return nil
+	}
+
+	return s.updateAccount(id, update)
+}
+
 // UpsertAccountPayment updates or inserts a payment entry for the given
 // account. Various functional options can be passed to modify the behavior of
 // the method. The returned boolean is true if the payment was already known

accounts/store_test.go

@@ -2,6 +2,7 @@ package accounts
 
 import (
 	"context"
+	"github.com/lightningnetwork/lnd/lnwire"
 	"testing"
 	"time"
 
@@ -71,6 +72,14 @@ func TestAccountStore(t *testing.T) {
 	)
 	require.NoError(t, err)
 
+	// Adjust the account balance by first crediting 10000, and then
+	// debiting 5000.
+	err = store.CreditAccount(ctx, acct1.ID, lnwire.MilliSatoshi(10000))
+	require.NoError(t, err)
+
+	err = store.DebitAccount(ctx, acct1.ID, lnwire.MilliSatoshi(5000))
+	require.NoError(t, err)
+
 	// Update the in-memory account so that we can compare it with the
 	// account we get from the store.
 	acct1.CurrentBalance = -500
@@ -85,11 +94,30 @@ func TestAccountStore(t *testing.T) {
 	}
 	acct1.Invoices[lntypes.Hash{12, 34, 56, 78}] = struct{}{}
 	acct1.Invoices[lntypes.Hash{34, 56, 78, 90}] = struct{}{}
+	acct1.CurrentBalance += 10000
+	acct1.CurrentBalance -= 5000
 
 	dbAccount, err = store.Account(ctx, acct1.ID)
 	require.NoError(t, err)
 	assertEqualAccounts(t, acct1, dbAccount)
 
+	// Test that adjusting the balance to exactly 0 should work, while
+	// adjusting the balance to below 0 should fail.
+	err = store.DebitAccount(
+		ctx, acct1.ID, lnwire.MilliSatoshi(acct1.CurrentBalance),
+	)
+	require.NoError(t, err)
+
+	acct1.CurrentBalance = 0
+
+	dbAccount, err = store.Account(ctx, acct1.ID)
+	require.NoError(t, err)
+	assertEqualAccounts(t, acct1, dbAccount)
+
+	// Adjusting the value to below 0 should fail.
+	err = store.DebitAccount(ctx, acct1.ID, lnwire.MilliSatoshi(1))
+	require.ErrorContains(t, err, "balance would be below 0")
+
 	// Sleep just a tiny bit to make sure we are never too quick to measure
 	// the expiry, even though the time is nanosecond scale and writing to
 	// the store and reading again should take at least a couple of