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.

channeldb/invoices.go

@@ -80,6 +80,13 @@ var (
 	//
 	//   settleIndexNo => invoiceKey
 	settleIndexBucket = []byte("invoice-settle-index")
+
+	// invoiceBucketTombstone is a special key that indicates the invoice
+	// bucket has been permanently closed. Its purpose is to prevent the
+	// invoice bucket from being reopened in the future. A key use case for
+	// the tombstone is to ensure users cannot switch back to the KV invoice
+	// database after migrating to the native SQL database.
+	invoiceBucketTombstone = []byte("invoice-tombstone")
 )
 
 const (
@@ -2402,3 +2409,49 @@ func (d *DB) DeleteInvoice(_ context.Context,
 
 	return err
 }
+
+// SetInvoiceBucketTombstone sets the tombstone key in the invoice bucket to
+// mark the bucket as permanently closed. This prevents it from being reopened
+// in the future.
+func (d *DB) SetInvoiceBucketTombstone() error {
+	return kvdb.Update(d, func(tx kvdb.RwTx) error {
+		// Access the top-level invoice bucket.
+		invoices := tx.ReadWriteBucket(invoiceBucket)
+		if invoices == nil {
+			return fmt.Errorf("invoice bucket does not exist")
+		}
+
+		// Add the tombstone key to the invoice bucket.
+		err := invoices.Put(invoiceBucketTombstone, []byte("1"))
+		if err != nil {
+			return fmt.Errorf("failed to set tombstone: %w", err)
+		}
+
+		return nil
+	}, func() {})
+}
+
+// GetInvoiceBucketTombstone checks if the tombstone key exists in the invoice
+// bucket. It returns true if the tombstone is present and false otherwise.
+func (d *DB) GetInvoiceBucketTombstone() (bool, error) {
+	var tombstoneExists bool
+
+	err := kvdb.View(d, func(tx kvdb.RTx) error {
+		// Access the top-level invoice bucket.
+		invoices := tx.ReadBucket(invoiceBucket)
+		if invoices == nil {
+			return fmt.Errorf("invoice bucket does not exist")
+		}
+
+		// Check if the tombstone key exists.
+		tombstone := invoices.Get(invoiceBucketTombstone)
+		tombstoneExists = tombstone != nil
+
+		return nil
+	}, func() {})
+	if err != nil {
+		return false, err
+	}
+
+	return tombstoneExists, nil
+}