delivery: the grants part's grant keys and the source the assigner reads (a1; issue 774)

Why: the grants part writes grants.<module>.<fp> in shadow beside the controller's; the assigner switches source only when the lease is handed away.

go/buscompose/buscompose_test.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"git.novox.be/novox/mesh-sdk/go/announce"
+	"git.novox.be/novox/mesh-sdk/go/delivery"
 	"github.com/nats-io/nats.go/jetstream"
 	"os"
 	"path/filepath"
@@ -415,6 +416,11 @@ func TestTheCopiedComposerComposesAsTheControllers(t *testing.T) {
 			if on, why := LeaseHandedAway([]byte(row.raw), row.found, row.err); on != row.on || why == "" {
 				t.Errorf("%s: on %v (%s), want %v", row.name, on, why, row.on)
 			}
+			// The assigner's grants source follows it: the grants part's bucket only for a handed-away lease (issue 774).
+			on, _ := LeaseHandedAway([]byte(row.raw), row.found, row.err)
+			if bucket, _ := delivery.GrantSource(on); (bucket == delivery.PartGrantsBucket) != row.on {
+				t.Errorf("%s: the grants source is %s", row.name, bucket)
+			}
 		}
 		announced := map[string]announce.States{ControllerSeat: ControllerAnnouncement("novox")}
 		for reader, want := range map[string]bool{VaultSeat: true, GrantsModule: true, "mesh-delivery-view": false} {