Promise the delivery seat's checks verb, optional until its holder serves it (hq ADR 0239)

What the mesh's checks said of a pull request had no verb: the verdict was read from this
controller's journal. mesh-delivery answers it as checks. A verb added to a mesh seat whose holder
lives in another repository deadlocked: this controller would refuse the holder that does not serve
it, the one before it the holder that does, and every catalogue check between the two would fail on
mesh-delivery. So a verb can be marked optional — served or not, the holder holds — until every
holder serves it.

internal/catalogue/delivery_seat.go

@@ -9,7 +9,7 @@ package catalogue
 // DeliverySeat is the seat mesh-delivery holds.
 const DeliverySeat = "mesh-delivery"
 
-// deliveryVerbs are the delivery seat's tools: five that read, and the acts of a person and of healer H2.
+// deliveryVerbs are the delivery seat's tools: six that read, and the acts of a person and of healer H2.
 func deliveryVerbs() []Verb {
 	return []Verb{
 		{Name: "deliveries", Description: "Every delivery not final, and those that ended in the last day, one line " +
@@ -32,6 +32,16 @@ func deliveryVerbs() []Verb {
 			Input: schema(map[string]string{"repository": "owner/repository",
 				"paths": "the files it changes, comma-separated, from the repository's root",
 				"base":  "the branch it merges into (default main)"}, []string{"repository", "paths"})},
+		{Name: "checks", Description: "What the mesh's checks said of a pull request's head or of one commit: each " +
+			"of the commit's mesh statuses (mesh/merge-gate, mesh/repo-check, mesh/delivery, …) with its state, " +
+			"description and when it was set; the merge check's full verdict as the controller said it — each " +
+			"layer's summary, the machine that ran it, when, its build id and its report; and whether the branch's " +
+			"protection would let it merge, every required status being success.",
+			Input: schema(map[string]string{"repository": "owner/repository",
+				"number": "a pull request's number: its head is read",
+				"commit": "a commit's sha, or the start of one, instead of a pull request"}, []string{"repository"}),
+			// Added after the seat's first holder shipped: optional until mesh-delivery serves it everywhere.
+			Optional: true},
 		{Name: "table", Description: "The state table every delivery runs by: each transition with its guard, " +
 			"each state's bound and what healer H2 may do once it has passed; and the machine steps' table.",
 			Input: schema(map[string]string{}, nil)},

internal/catalogue/delivery_seat_test.go

@@ -0,0 +1,60 @@
+package catalogue
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+)
+
+// The delivery seat answers "what did the mesh's checks say of this pull request?" as a verb of its own,
+// `checks` (novox/hq ADR 0239): read by repository and a pull request's number or a commit.
+func TestTheDeliverySeatPromisesChecks(t *testing.T) {
+	seat, ok := SeatNamed(DeliverySeat)
+	if !ok {
+		t.Fatal("the delivery seat is not in the set")
+	}
+	var checks *Verb
+	for i := range seat.Serves {
+		if seat.Serves[i].Name == "checks" {
+			checks = &seat.Serves[i]
+		}
+	}
+	if checks == nil {
+		t.Fatalf("the delivery seat promises %v and not checks", VerbNames(seat.Serves))
+	}
+	props, _ := checks.Input["properties"].(map[string]any)
+	for _, arg := range []string{"repository", "number", "commit"} {
+		if _, has := props[arg]; !has {
+			t.Errorf("checks takes no %q", arg)
+		}
+	}
+	if req, _ := checks.Input["required"].([]string); !reflect.DeepEqual(req, []string{"repository"}) {
+		t.Errorf("checks requires %v, wanted only the repository", req)
+	}
+
+	// Added after the seat's holder shipped, it is optional: the holder serving the ten verbs before it still
+	// holds the seat, and one serving all eleven does too — so the controller and the catalogue can move in
+	// either order, and no check of the catalogue fails on a module nobody touched between the two.
+	if !checks.Optional {
+		t.Fatal("checks is a condition of holding before its holder serves it")
+	}
+	var before []string
+	for _, v := range VerbNames(seat.Serves) {
+		if v != "checks" {
+			before = append(before, v)
+		}
+	}
+	m := Manifest{Module: "mesh-delivery", Claims: []Claim{{Name: DeliverySeat, Scope: ScopeMesh, Serves: before}}}
+	if err := CanHold(m, seat); err != nil {
+		t.Fatalf("a holder without checks yet: %v", err)
+	}
+	m.Claims[0].Serves = VerbNames(seat.Serves)
+	if err := CanHold(m, seat); err != nil {
+		t.Fatalf("a holder serving every verb: %v", err)
+	}
+	// Every other verb is still a condition of holding.
+	m.Claims[0].Serves = append([]string{"checks"}, before[1:]...)
+	if err := CanHold(m, seat); err == nil || !strings.Contains(err.Error(), before[0]) {
+		t.Fatalf("a holder without %s: %v", before[0], err)
+	}
+}

internal/catalogue/verbs.go

@@ -22,6 +22,14 @@ type Verb struct {
 	Description string         `json:"description,omitempty"`
 	Input       map[string]any `json:"input,omitempty"`
 	Output      map[string]any `json:"output,omitempty"`
+	// Optional marks a verb added to a mesh seat whose holder lives in another repository (design 33 §7,
+	// additive within a version): a holder that serves it is accepted, and one that does not yet still
+	// holds the seat. Without it the addition would be a deadlock — this controller refusing the holder that
+	// does not serve the verb, the controller before it refusing the holder that does — and every check of
+	// the catalogue between the two would fail on a module nobody touched. Once every holder serves it,
+	// the mark is removed and the verb is a condition of holding like the rest. Never stored or said: the
+	// seat set's protocol is the compiled one.
+	Optional bool `json:"-"`
 }
 
 func (v *Verb) UnmarshalJSON(raw []byte) error {
@@ -442,7 +450,7 @@ func unservedVerbs(tools []string, promised []Verb) []string {
 	}
 	var missing []string
 	for _, v := range promised {
-		if !has[v.Name] {
+		if !has[v.Name] && !v.Optional {
 			missing = append(missing, v.Name)
 		}
 	}