A group member's own check runs its repository's check alone, a cycle fails the composed check, and mark-ready leaves the delivery seat (issue 977, part D, ADR 0403)

cmd/mesh-controller/delivery.go

@@ -1014,6 +1014,43 @@ func theGraph(ctx context.Context, inv *inventory.Inventory) ([]inventory.Entry,
 	return entries, read, edges, nil
 }
 
+// cycleVerdict is the failing verdict of a group whose modules are ordered in a cycle, and whether there is one: the
+// group's composed check, said without a build. The manifests' change is read from the forge; an answer it cannot give
+// is an error, never read as no cycle.
+func cycleVerdict(ctx context.Context, group string, members []orderMember, reach map[string]orderReach,
+	edges []inventory.Edge) (link.Checked, bool, error) {
+	changes, err := changesOf(ctx, changeSourcesOf(members))
+	if err != nil {
+		return link.Checked{}, false, fmt.Errorf("what the group's members say of their manifests cannot be read: %w", err)
+	}
+	var moved []string
+	for _, m := range members {
+		moved = appendNew(moved, reach[m.ID].Moved...)
+	}
+	said := orderEdges(moved, changes, edges).CycleSaid()
+	if said == "" {
+		return link.Checked{}, false, nil
+	}
+	first := members[0].pull()
+	c := link.Checked{ID: link.NewBuildID(time.Now()), Owner: first.Owner, Repo: first.Repo, Number: first.Number,
+		Commit: first.Commit, Verdict: "fail", Summary: said, Report: said, Group: group,
+		Gate: &link.CheckLayer{Verdict: "fail", Summary: said}}
+	for _, m := range members {
+		p := m.pull()
+		c.Members = append(c.Members, link.CheckedMember{Owner: p.Owner, Repo: p.Repo, Number: p.Number, Commit: p.Commit,
+			Trunk: twelve(p.BaseSHA)})
+	}
+	return c, true, nil
+}
+
+// twelve is a commit as twelve characters.
+func twelve(c string) string {
+	if len(c) > 12 {
+		return c[:12]
+	}
+	return c
+}
+
 // groupPrimary is the head a group's composed check is run from: the one moving the controller, which then
 // judges the group by itself; else the one moving the node-engine, whose validator judges; else the first.
 func groupPrimary(members []orderMember, reach map[string]orderReach) int {
@@ -1036,6 +1073,14 @@ func askGroupCheck(ctx context.Context, open *stores, group string, members []or
 		return "", err
 	}
 	reach := reachOfMembers(members, entries, read, edges)
+	// **A cycle of edges fails the group's composed check, naming every module and edge** (novox/hq ADR 0419 clause 6),
+	// before the build seat is asked: no composition of the heads has an order to be sent in.
+	if failing, cyclic, err := cycleVerdict(ctx, group, members, reach, edges); err != nil {
+		return "", err
+	} else if cyclic {
+		sayChecked(ctx, failing)
+		return failing.ID, nil
+	}
 	primary := groupPrimary(members, reach)
 	p := members[primary].pull()
 	scope := pullScope(p, entries, read, edges)