payments/migration1: add external migration test

diff --git a/payments/db/migration1/migration_external_test.go b/payments/db/migration1/migration_external_test.go
new file mode 100644
index 0000000..a5ddc42
--- /dev/null
+++ b/payments/db/migration1/migration_external_test.go
@@ -0,0 +1,367 @@
+//go:build test_db_postgres || test_db_sqlite
+
+package migration1
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/btcsuite/btclog/v2"
+	"github.com/lightningnetwork/lnd/kvdb"
+	"github.com/lightningnetwork/lnd/kvdb/postgres"
+	"github.com/lightningnetwork/lnd/kvdb/sqlbase"
+	"github.com/lightningnetwork/lnd/kvdb/sqlite"
+	"github.com/lightningnetwork/lnd/lntypes"
+	"github.com/lightningnetwork/lnd/payments/db/migration1/sqlc"
+	"github.com/lightningnetwork/lnd/sqldb"
+	"github.com/stretchr/testify/require"
+)
+
+// TestMigrationWithExternalDB tests the migration of the payment store from a
+// bolt backed channel.db or a kvdb channel.sqlite to a SQL database. Note that
+// this test does not attempt to be a complete migration test for all payment
+// store types but rather is added as a tool for developers and users to debug
+// payment migration issues with an actual channel.db/channel.sqlite file.
+//
+// NOTE: To use this test, place either of those files in the
+// payments/db/migration1/testdata directory, uncomment the "Skipf" line, and
+// set the "fileName" variable to the name of the channel database file you
+// want to use for the migration test.
+func TestMigrationWithExternalDB(t *testing.T) {
+	ctx := context.Background()
+
+	// NOTE: comment this line out to run the test.
+	t.Skipf("skipping test meant for local debugging only")
+
+	// NOTE: set this to the name of the channel database file you want
+	// to use for the migration test. This may be either a bbolt ".db" file
+	// or a SQLite ".sqlite" file. If you want to migrate from a
+	// bbolt channel.db file, set this to "channel.db".
+	const fileName = "channel.db"
+
+	// NOTE: if set, this test will prefer migrating from a Postgres-backed
+	// kvdb source instead of a local file. Leave empty to use fileName.
+	const postgresKVDSN = ""
+	const postgresKVPfx = "channeldb"
+	const logSequenceOrder = false
+
+	// Determine if we are using a SQLite file or a Bolt DB file.
+	isSqlite := strings.HasSuffix(fileName, ".sqlite")
+
+	// Set up logging for the test.
+	logger := btclog.NewSLogger(btclog.NewDefaultHandler(os.Stdout))
+	UseLogger(logger)
+
+	// migrate runs the migration from the kvdb store to the SQL store
+	// and then performs a batched deep comparison of every migrated
+	// payment to ensure all data (including HTLC details) was preserved
+	// correctly.
+	migrate := func(t *testing.T, kvBackend kvdb.Backend) {
+		sqlStore := setupTestSQLDB(t)
+
+		// Run migration in a transaction.
+		err := sqlStore.db.ExecTx(
+			ctx, sqldb.WriteTxOpt(), func(tx SQLQueries) error {
+				return MigratePaymentsKVToSQL(
+					ctx, kvBackend, tx, &SQLStoreConfig{
+						QueryCfg: sqlStore.cfg.QueryCfg,
+					},
+				)
+			}, sqldb.NoOpReset,
+		)
+		require.NoError(t, err)
+
+		t.Log("========================================")
+		t.Log("   Deep Validation")
+		t.Log("========================================")
+
+		// Deep compare all migrated payments in batches.
+		deepValidateAllPayments(
+			t, ctx, kvBackend, sqlStore,
+		)
+
+		_ = logSequenceOrder
+	}
+
+	connectPostgres := func(t *testing.T, dsn, prefix string) kvdb.Backend {
+		dsn = strings.TrimSpace(dsn)
+		if dsn == "" {
+			t.Fatalf("missing postgres kvdb dsn")
+		}
+
+		prefix = strings.TrimSpace(prefix)
+		if prefix == "" {
+			prefix = "channeldb"
+		}
+
+		const (
+			timeout  = 10 * time.Second
+			maxConns = 5
+		)
+		sqlbase.Init(maxConns)
+
+		dbCfg := &postgres.Config{
+			Dsn:            dsn,
+			Timeout:        timeout,
+			MaxConnections: maxConns,
+		}
+
+		kvStore, err := kvdb.Open(
+			kvdb.PostgresBackendName, ctx, dbCfg, prefix,
+		)
+		require.NoError(t, err)
+
+		t.Cleanup(func() { _ = kvStore.Close() })
+
+		return kvStore
+	}
+
+	connectPostgresKV := func(t *testing.T) kvdb.Backend {
+		return connectPostgres(t, postgresKVDSN, postgresKVPfx)
+	}
+
+	connectBBolt := func(t *testing.T, dbPath string) kvdb.Backend {
+		cfg := &kvdb.BoltBackendConfig{
+			DBPath:            dbPath,
+			DBFileName:        fileName,
+			NoFreelistSync:    true,
+			AutoCompact:       false,
+			AutoCompactMinAge: kvdb.DefaultBoltAutoCompactMinAge,
+			DBTimeout:         kvdb.DefaultDBTimeout,
+			ReadOnly:          true,
+		}
+
+		kvStore, err := kvdb.GetBoltBackend(cfg)
+		require.NoError(t, err)
+
+		t.Cleanup(func() { _ = kvStore.Close() })
+
+		return kvStore
+	}
+
+	connectSQLite := func(t *testing.T, dbPath string) kvdb.Backend {
+		const (
+			timeout  = 10 * time.Second
+			maxConns = 5
+		)
+		sqlbase.Init(maxConns)
+
+		cfg := &sqlite.Config{
+			Timeout:        timeout,
+			BusyTimeout:    timeout,
+			MaxConnections: maxConns,
+		}
+
+		kvStore, err := kvdb.Open(
+			kvdb.SqliteBackendName, ctx, cfg,
+			dbPath, fileName,
+			// NOTE: we use the raw string here else we get an
+			// import cycle if we try to import lncfg.NSChannelDB.
+			"channeldb",
+		)
+		require.NoError(t, err)
+
+		t.Cleanup(func() { _ = kvStore.Close() })
+
+		return kvStore
+	}
+
+	tests := []struct {
+		name   string
+		dbPath string
+	}{
+		{
+			name:   "testdata",
+			dbPath: "testdata",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if postgresKVDSN != "" {
+				migrate(t, connectPostgresKV(t))
+				return
+			}
+
+			chanDBPath := path.Join(test.dbPath, fileName)
+			t.Logf("Connecting to channel DB at: %s", chanDBPath)
+
+			connectDB := connectBBolt
+			if isSqlite {
+				connectDB = connectSQLite
+			}
+
+			migrate(t, connectDB(t, test.dbPath))
+		})
+	}
+}
+
+// kvPaymentRef holds a KV payment and its hash for batched deep validation.
+type kvPaymentRef struct {
+	hash    lntypes.Hash
+	payment *MPPayment
+}
+
+// deepValidateAllPayments iterates all KV payments in batches and performs a
+// deep comparison against their SQL counterparts. For each batch, it fetches
+// the full SQL payment data (including HTLCs, routes, custom records) via
+// batch queries and compares field-by-field with the KV data.
+func deepValidateAllPayments(t *testing.T, ctx context.Context,
+	kvBackend kvdb.Backend, sqlStore *SQLStore) {
+
+	t.Helper()
+
+	batchSize := int(sqlStore.cfg.QueryCfg.MaxBatchSize)
+
+	// Get total payment count from SQL so we can show progress.
+	var totalPayments int64
+	err := sqlStore.db.ExecTx(
+		ctx, sqldb.ReadTxOpt(), func(db SQLQueries) error {
+			var err error
+			totalPayments, err = db.CountPayments(ctx)
+			return err
+		}, sqldb.NoOpReset,
+	)
+	require.NoError(t, err)
+
+	var totalValidated int
+
+	// Iterate all KV payments, collecting them into batches. For each
+	// full batch, perform a batched deep comparison against SQL.
+	var batch []kvPaymentRef
+	err = kvdb.View(kvBackend, func(kvTx kvdb.RTx) error {
+		paymentsBucket := kvTx.ReadBucket(paymentsRootBucket)
+		if paymentsBucket == nil {
+			return nil
+		}
+
+		return paymentsBucket.ForEach(func(k, v []byte) error {
+			bucket := paymentsBucket.NestedReadBucket(k)
+			if bucket == nil {
+				return nil
+			}
+
+			payment, err := fetchPayment(bucket)
+			if err != nil {
+				return err
+			}
+
+			var hash lntypes.Hash
+			copy(hash[:], k)
+
+			batch = append(batch, kvPaymentRef{
+				hash:    hash,
+				payment: payment,
+			})
+
+			if len(batch) >= batchSize {
+				deepCompareBatch(
+					t, ctx, sqlStore, batch,
+				)
+				totalValidated += len(batch)
+				t.Logf("Deep validated %d/%d payments",
+					totalValidated, totalPayments)
+
+				batch = batch[:0]
+			}
+
+			return nil
+		})
+	}, func() {
+		batch = nil
+		totalValidated = 0
+	})
+	require.NoError(t, err)
+
+	// Validate any remaining payments in the last batch.
+	if len(batch) > 0 {
+		deepCompareBatch(t, ctx, sqlStore, batch)
+		totalValidated += len(batch)
+	}
+
+	t.Logf("Deep validated %d/%d payments (complete)",
+		totalValidated, totalPayments)
+}
+
+// deepCompareBatch performs a batched deep comparison of KV payments against
+// their SQL counterparts. It fetches SQL payment data in a single read
+// transaction using batch queries.
+func deepCompareBatch(t *testing.T, ctx context.Context,
+	sqlStore *SQLStore, batch []kvPaymentRef) {
+
+	t.Helper()
+
+	err := sqlStore.db.ExecTx(
+		ctx, sqldb.ReadTxOpt(), func(db SQLQueries) error {
+			// Look up each payment by hash to get the SQL row
+			// with payment ID.
+			ids := make([]int64, 0, len(batch))
+			rowsByHash := make(
+				map[lntypes.Hash]sqlc.FetchPaymentsByIDsRow,
+				len(batch),
+			)
+			for _, ref := range batch {
+				row, err := fetchPaymentByHash(
+					ctx, db, ref.hash,
+				)
+				if err != nil {
+					return fmt.Errorf("fetch SQL payment "+
+						"%x: %w", ref.hash[:8], err)
+				}
+				ids = append(ids, row.Payment.ID)
+			}
+
+			// Batch-fetch full payment details.
+			rows, err := db.FetchPaymentsByIDs(ctx, ids)
+			if err != nil {
+				return fmt.Errorf("batch fetch payments: %w",
+					err)
+			}
+			for _, row := range rows {
+				var hash lntypes.Hash
+				copy(hash[:], row.PaymentIdentifier)
+				rowsByHash[hash] = row
+			}
+
+			batchData, err := batchLoadPaymentDetailsData(
+				ctx, sqlStore.cfg.QueryCfg, db, ids,
+			)
+			if err != nil {
+				return fmt.Errorf("batch load details: %w",
+					err)
+			}
+
+			// Deep compare each payment.
+			for _, ref := range batch {
+				row, ok := rowsByHash[ref.hash]
+				require.True(t, ok,
+					"SQL payment %x not in batch "+
+						"results", ref.hash[:8])
+
+				sqlPayment, err := buildPaymentFromBatchData(
+					row, batchData,
+				)
+				require.NoError(t, err,
+					"build SQL payment %x",
+					ref.hash[:8])
+
+				normalizePaymentData(ref.payment)
+				normalizePaymentData(sqlPayment)
+
+				require.Equal(
+					t, ref.payment, sqlPayment,
+					"payment mismatch %x",
+					ref.hash[:8],
+				)
+			}
+
+			return nil
+		}, sqldb.NoOpReset,
+	)
+	require.NoError(t, err)
+}
diff --git a/payments/db/migration1/testdata/README.md b/payments/db/migration1/testdata/README.md
new file mode 100644
index 0000000..08c5fb0
--- /dev/null
+++ b/payments/db/migration1/testdata/README.md
@@ -0,0 +1,52 @@
+# Payment Migration External Testdata
+
+This directory holds a real `channel.db` (bbolt) or `channel.sqlite` file for
+testing the payments KV to SQL migration locally. You can also point the test
+at an existing Postgres-backed kvdb instance.
+
+## How to use
+
+1. Copy your `channel.db` or `channel.sqlite` file into this folder.
+2. Edit `migration_external_test.go`:
+
+   ```go
+   // Comment out this line to enable the test
+   t.Skipf("skipping test meant for local debugging only")
+
+   // Set to your database filename
+   const fileName = "channel.db" // or "channel.sqlite"
+   ```
+
+3. Run the test:
+
+   ```bash
+   # For Postgres backend
+   go test -v -tags="test_db_postgres" -run TestMigrationWithExternalDB
+   ```
+
+## SQLite kvdb source
+
+To migrate from a `channel.sqlite` file, run with the `kvdb_sqlite` build
+tag:
+
+```bash
+go test -v -tags="test_db_sqlite kvdb_sqlite" \
+  -run TestMigrationWithExternalDB
+```
+
+## Postgres kvdb source
+
+To migrate from an existing Postgres-backed kvdb instance, edit
+`postgresKVDSN` in `migration_external_test.go` (set it non-empty), then
+run with the `kvdb_postgres` build tag:
+
+```bash
+go test -v -tags="kvdb_postgres test_db_postgres" \
+  -run TestMigrationWithExternalDB
+```
+
+## Notes
+
+- The external database is opened read-only.
+- The test creates a fresh SQL database for each run.
+- Do not commit production data; keep the file local.