Say a machine waiting for its push as waiting, not uncomposable (hq issue 275)

Between assign and push a module's own secrets are not made yet; D1 composed
without making them and raised an urgent 'nothing can be sent' that the next
push resolved silently. D1 now composes as the push would (Foreseeing): a
secret the push makes gets a stand-in and is named, one the push is refused on
is refused with the push's words. Waiting is said only past 30 minutes, as a
warning. D3 and D13 expect a holder only once its machine was sent it and
reported or had ten minutes to.

internal/inventory/nodes.go

@@ -976,6 +976,25 @@ func (i *Inventory) RecordSentUnder(ctx context.Context, node, digest string, bu
 	return err
 }
 
+// AwaitingSince is since when a machine has had something waiting for its next send (novox/hq issue
+// 275): the oldest assignment on it made after it was last sent, or — when nothing was assigned since —
+// when it was last sent, or when it joined if it never was. The moment a bound on "not pushed yet" is
+// read from: every change a push carries happened after the last push, and an assignment is the one
+// that says when.
+func (i *Inventory) AwaitingSince(ctx context.Context, name string) (time.Time, error) {
+	var since time.Time
+	err := i.store.Pool().QueryRow(ctx,
+		`select coalesce(
+		    (select min(a.assigned) from assignment a
+		      where a.node = n.id and (n.sent_at is null or a.assigned > n.sent_at)),
+		    n.sent_at, n.created)
+		   from node n where n.name = $1`, name).Scan(&since)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return time.Time{}, fmt.Errorf("%w: %s", ErrNoSuchNode, name)
+	}
+	return since, err
+}
+
 // SentBuilds is the build of each module a machine was last sent, by its name: module to the commit
 // its build was made from (novox/hq issue 259). Known is false when that was not kept — a machine
 // last sent before it was, sent a declaration by hand, or one the mesh does not know.