Send the bus's machine before the grants, and only when its user list moved (hq issue 249)

Grants first could hold back the very declaration that lets the controller
issue them. The holder now goes first, then buckets and memberships, then
the rest; a membership that fails holds back only its own machine, and an
announcement whose send stopped at its grants is asked again. Whether the
holder must go first is read from a digest of the user list it was last
sent, not its whole declaration. Migration renumbered to 0058.

internal/inventory/nodes.go

@@ -921,6 +921,26 @@ func (i *Inventory) RecordSent(ctx context.Context, node, digest string) error {
 	return err
 }
 
+// RecordSentBusUsers keeps a digest of the bus's user list a machine was just sent, by its name
+// (novox/hq issue 249): whether the machine holding the bus must go first is whether this differs
+// from the list composed now.
+func (i *Inventory) RecordSentBusUsers(ctx context.Context, name, digest string) error {
+	_, err := i.store.Pool().Exec(ctx,
+		`update node set sent_bus_users = $2 where name = $1`, name, digest)
+	return err
+}
+
+// SentBusUsers is the digest of the bus's user list a machine was last sent, empty for none.
+func (i *Inventory) SentBusUsers(ctx context.Context, name string) (string, error) {
+	var sent string
+	err := i.store.Pool().QueryRow(ctx,
+		`select sent_bus_users from node where name = $1`, name).Scan(&sent)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return "", nil
+	}
+	return sent, err
+}
+
 // Outstanding is the digest of the declaration a machine was last sent, by its name, and empty
 // for one that has never been sent anything.
 //