lnd+paymentsdb: introduce harness for the payment sql backend

diff --git a/config_builder.go b/config_builder.go
index 3fe62cd..07196b2 100644
--- a/config_builder.go
+++ b/config_builder.go
@@ -1228,6 +1228,26 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 
 			return nil, nil, err
 		}
+
+		// Create the payments DB.
+		//
+		// NOTE:  In the regular build, this will construct a kvdb
+		// backed payments backend. With the test_native_sql tag, it
+		// will build a SQL payments backend.
+		sqlPaymentsDB, err := d.getPaymentsStore(
+			baseDB, dbs.ChanStateDB.Backend,
+			paymentsdb.WithKeepFailedPaymentAttempts(
+				cfg.KeepFailedPaymentAttempts,
+			),
+		)
+		if err != nil {
+			err = fmt.Errorf("unable to get payments store: %w",
+				err)
+
+			return nil, nil, err
+		}
+
+		dbs.PaymentsDB = sqlPaymentsDB
 	} else {
 		// Check if the invoice bucket tombstone is set. If it is, we
 		// need to return and ask the user switch back to using the
@@ -1256,40 +1276,35 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 		if err != nil {
 			return nil, nil, err
 		}
-	}
 
-	dbs.GraphDB, err = graphdb.NewChannelGraph(graphStore, chanGraphOpts...)
-	if err != nil {
-		cleanUp()
+		// Create the payments DB.
+		kvPaymentsDB, err := paymentsdb.NewKVStore(
+			dbs.ChanStateDB,
+			paymentsdb.WithKeepFailedPaymentAttempts(
+				cfg.KeepFailedPaymentAttempts,
+			),
+		)
+		if err != nil {
+			cleanUp()
 
-		err = fmt.Errorf("unable to open channel graph DB: %w", err)
-		d.logger.Error(err)
+			err = fmt.Errorf("unable to open payments DB: %w", err)
+			d.logger.Error(err)
 
-		return nil, nil, err
-	}
+			return nil, nil, err
+		}
 
-	// Mount the payments DB which is only KV for now.
-	//
-	// TODO(ziggie): Add support for SQL payments DB.
-	// Mount the payments DB for the KV store.
-	paymentsDBOptions := []paymentsdb.OptionModifier{
-		paymentsdb.WithKeepFailedPaymentAttempts(
-			cfg.KeepFailedPaymentAttempts,
-		),
+		dbs.PaymentsDB = kvPaymentsDB
 	}
-	kvPaymentsDB, err := paymentsdb.NewKVStore(
-		dbs.ChanStateDB,
-		paymentsDBOptions...,
-	)
+
+	dbs.GraphDB, err = graphdb.NewChannelGraph(graphStore, chanGraphOpts...)
 	if err != nil {
 		cleanUp()
 
-		err = fmt.Errorf("unable to open payments DB: %w", err)
+		err = fmt.Errorf("unable to open channel graph DB: %w", err)
 		d.logger.Error(err)
 
 		return nil, nil, err
 	}
-	dbs.PaymentsDB = kvPaymentsDB
 
 	// Wrap the watchtower client DB and make sure we clean up.
 	if cfg.WtClient.Active {
diff --git a/config_prod.go b/config_prod.go
index 60dba8b..02b7d2a 100644
--- a/config_prod.go
+++ b/config_prod.go
@@ -6,6 +6,8 @@ import (
 	"context"
 
 	"github.com/lightningnetwork/lnd/kvdb"
+	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
+	"github.com/lightningnetwork/lnd/sqldb"
 	"github.com/lightningnetwork/lnd/sqldb/sqlc"
 )
 
@@ -24,3 +26,12 @@ func (d *DefaultDatabaseBuilder) getSQLMigration(ctx context.Context,
 
 	return nil, false
 }
+
+// getPaymentsStore returns a paymentsdb.DB backed by a paymentsdb.KVStore
+// implementation.
+func (d *DefaultDatabaseBuilder) getPaymentsStore(_ *sqldb.BaseDB,
+	kvBackend kvdb.Backend,
+	opts ...paymentsdb.OptionModifier) (paymentsdb.DB, error) {
+
+	return paymentsdb.NewKVStore(kvBackend, opts...)
+}
diff --git a/config_test_native_sql.go b/config_test_native_sql.go
index 91589fa..efc6ed8 100644
--- a/config_test_native_sql.go
+++ b/config_test_native_sql.go
@@ -4,8 +4,12 @@ package lnd
 
 import (
 	"context"
+	"database/sql"
 
 	"github.com/lightningnetwork/lnd/kvdb"
+	"github.com/lightningnetwork/lnd/lncfg"
+	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
+	"github.com/lightningnetwork/lnd/sqldb"
 	"github.com/lightningnetwork/lnd/sqldb/sqlc"
 )
 
@@ -25,3 +29,28 @@ func (d *DefaultDatabaseBuilder) getSQLMigration(_ context.Context,
 		return nil, false
 	}
 }
+
+// getPaymentsStore returns a paymentsdb.DB backed by a paymentsdb.SQLStore
+// implementation.
+func (d *DefaultDatabaseBuilder) getPaymentsStore(baseDB *sqldb.BaseDB,
+	kvBackend kvdb.Backend,
+	opts ...paymentsdb.OptionModifier) (paymentsdb.DB, error) {
+
+	paymentsExecutor := sqldb.NewTransactionExecutor(
+		baseDB, func(tx *sql.Tx) paymentsdb.SQLQueries {
+			return baseDB.WithTx(tx)
+		},
+	)
+
+	queryConfig := d.cfg.DB.Sqlite.QueryConfig
+	if d.cfg.DB.Backend == lncfg.PostgresBackend {
+		queryConfig = d.cfg.DB.Postgres.QueryConfig
+	}
+
+	return paymentsdb.NewSQLStore(
+		&paymentsdb.SQLStoreConfig{
+			QueryCfg: &queryConfig,
+		},
+		paymentsExecutor, opts...,
+	)
+}
diff --git a/payments/db/sql_store.go b/payments/db/sql_store.go
new file mode 100644
index 0000000..12585ca
--- /dev/null
+++ b/payments/db/sql_store.go
@@ -0,0 +1,67 @@
+package paymentsdb
+
+import (
+	"fmt"
+
+	"github.com/lightningnetwork/lnd/sqldb"
+)
+
+// SQLQueries is a subset of the sqlc.Querier interface that can be used to
+// execute queries against the SQL payments tables.
+type SQLQueries interface {
+}
+
+// BatchedSQLQueries is a version of the SQLQueries that's capable
+// of batched database operations.
+type BatchedSQLQueries interface {
+	SQLQueries
+	sqldb.BatchedTx[SQLQueries]
+}
+
+// SQLStore represents a storage backend.
+type SQLStore struct {
+	// TODO(ziggie): Remove the KVStore once all the interface functions are
+	// implemented.
+	KVStore
+
+	cfg *SQLStoreConfig
+	db  BatchedSQLQueries
+
+	// keepFailedPaymentAttempts is a flag that indicates whether we should
+	// keep failed payment attempts in the database.
+	keepFailedPaymentAttempts bool
+}
+
+// A compile-time constraint to ensure SQLStore implements DB.
+var _ DB = (*SQLStore)(nil)
+
+// SQLStoreConfig holds the configuration for the SQLStore.
+type SQLStoreConfig struct {
+	// QueryConfig holds configuration values for SQL queries.
+	QueryCfg *sqldb.QueryConfig
+}
+
+// NewSQLStore creates a new SQLStore instance given an open
+// BatchedSQLPaymentsQueries storage backend.
+func NewSQLStore(cfg *SQLStoreConfig, db BatchedSQLQueries,
+	options ...OptionModifier) (*SQLStore, error) {
+
+	opts := DefaultOptions()
+	for _, applyOption := range options {
+		applyOption(opts)
+	}
+
+	if opts.NoMigration {
+		return nil, fmt.Errorf("the NoMigration option is not yet " +
+			"supported for SQL stores")
+	}
+
+	return &SQLStore{
+		cfg:                       cfg,
+		db:                        db,
+		keepFailedPaymentAttempts: opts.KeepFailedPaymentAttempts,
+	}, nil
+}
+
+// A compile-time constraint to ensure SQLStore implements DB.
+var _ DB = (*SQLStore)(nil)