channeldb+lnd: set invoice bucket tombstone after migration

This commit introduces the functionality to set a tombstone key
in the invoice bucket after the migration to the native SQL
database. The tombstone prevents the user from switching back
to the KV invoice database, ensuring data consistency and
avoiding potential issues like lingering invoices or partial
state in KV tables. The tombstone is checked on startup to
block any manual overrides that attempt to revert the migration.

config_builder.go

@@ -1101,11 +1101,22 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 		// regardless of the flag.
 		if !d.cfg.DB.SkipSQLInvoiceMigration {
 			migrationFn := func(tx *sqlc.Queries) error {
-				return invoices.MigrateInvoicesToSQL(
+				err := invoices.MigrateInvoicesToSQL(
 					ctx, dbs.ChanStateDB.Backend,
 					dbs.ChanStateDB, tx,
 					invoiceMigrationBatchSize,
 				)
+				if err != nil {
+					return fmt.Errorf("failed to migrate "+
+						"invoices to SQL: %w", err)
+				}
+
+				// Set the invoice bucket tombstone to indicate
+				// that the migration has been completed.
+				d.logger.Debugf("Setting invoice bucket " +
+					"tombstone")
+
+				return dbs.ChanStateDB.SetInvoiceBucketTombstone() //nolint:ll
 			}
 
 			// Make sure we attach the custom migration function to
@@ -1147,6 +1158,28 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 
 		dbs.InvoiceDB = sqlInvoiceDB
 	} 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
+		// native SQL store.
+		ripInvoices, err := dbs.ChanStateDB.GetInvoiceBucketTombstone()
+		d.logger.Debugf("Invoice bucket tombstone set to: %v",
+			ripInvoices)
+
+		if err != nil {
+			err = fmt.Errorf("unable to check invoice bucket "+
+				"tombstone: %w", err)
+			d.logger.Error(err)
+
+			return nil, nil, err
+		}
+		if ripInvoices {
+			err = fmt.Errorf("invoices bucket tombstoned, please " +
+				"switch back to native SQL")
+			d.logger.Error(err)
+
+			return nil, nil, err
+		}
+
 		dbs.InvoiceDB = dbs.ChanStateDB
 	}