Accept a modules value with the new fingerprint alone, and say mesh/check is the required status (issue 1851, S7 and S16, ADR 0453 §6, ADR 0503 §3 §5)

FP and None stay readable for build-agent's path until R4g; the gate's status
comment and the checks verb now name mesh/check as what a main requires.

go/catalogue/delivery_seat.go

@@ -42,7 +42,8 @@ func deliveryVerbs() []Verb {
 				"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, " +
+			"of the commit's mesh statuses (mesh/check, the one a main requires from R4e, hq's 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.",

go/delivery/gate.go

@@ -29,7 +29,9 @@ const (
 )
 
 // The contexts of the statuses the mesh owes the forge: the gate's, the delivery's view of a head and of a group
-// (design 47), and the checker's, which is not required until R4f (ADR 0484 clause 2, ADR 0453 clause 4).
+// (design 47), and the checker's. From R4e the checker's mesh/check is the one status a repository's main requires,
+// in place of mesh/merge-gate and mesh/repo-check (ADR 0503 clauses 3 and 5); a main that requires only
+// mesh/repo-check (hq) keeps it, and the controller writes it for a repository the checker writes no status for.
 const (
 	StatusContextMergeGate     = "mesh/merge-gate"
 	StatusContextDelivery      = "mesh/delivery"

go/delivery/module_newform_test.go

@@ -0,0 +1,32 @@
+package delivery
+
+import (
+	"strings"
+	"testing"
+)
+
+// From R4e a modules value needs no old form (novox/hq ADR 0453 clause 6; issue 1851): a value with only the new
+// fingerprint, or only the reason it has none, is written and read; an old value with fp or none still reads; a value
+// with no fingerprint of either form and no reason is still refused.
+func TestAModulesValueNeedsNoOldForm(t *testing.T) {
+	fp2 := SourcePrefix2 + strings.Repeat("a", 64)
+	for _, m := range []Module{
+		{FP2: fp2, Repository: "a/b", Commit: "c"},
+		{None2: "no lock", Repository: "a/b", Commit: "c"},
+	} {
+		data, err := EncodeModule(m)
+		if err != nil {
+			t.Fatalf("%+v was refused: %v", m, err)
+		}
+		back, err := DecodeModule(data)
+		if err != nil || back.FP2 != m.FP2 || back.None2 != m.None2 || back.FP != "" || back.None != "" {
+			t.Fatalf("decoded %+v, %v", back, err)
+		}
+	}
+	if _, err := DecodeModule([]byte(`{"v":1,"fp":"src1:x","repository":"a/b","commit":"c"}`)); err != nil {
+		t.Fatalf("an old value no longer reads: %v", err)
+	}
+	if _, err := EncodeModule(Module{Repository: "a/b", Commit: "c"}); err == nil {
+		t.Fatal("a value with no fingerprint and no reason was accepted")
+	}
+}

go/delivery/source.go

@@ -370,12 +370,13 @@ func (c Candidate) check() error {
 
 // Module is the value of a modules key: the module's source fingerprint, or none and why (a build that resolves
 // packages when built gets none, said here), and where it was computed: Repository and Commit are the trunk merge,
-// Src the source revision then (0 before the gate writes source.main). Exactly one of FP and None is set.
+// Src the source revision then (0 before the gate writes source.main). At most one of FP and None is set.
 //
 // From R4c the indexer writes the new form beside it (novox/hq ADR 0453 clause 6): FP2, the fingerprint of the new
 // form (`src2:` and 64 hex digits), or None2, why it has none; one of them whenever either is, never both; and
 // Manifest, the digest of the module's manifest, the key of the manifests bucket. A value written before has none of
-// the three, and a reader of today's shape ignores them.
+// the three, and a reader of today's shape ignores them. From R4e a value may carry the new form alone (ADR 0453 clause
+// 6; issue 1851): one of FP, None, FP2 and None2 is set, and the old form stays readable.
 type Module struct {
 	V        int    `json:"v"`
 	FP       string `json:"fp,omitempty"`
@@ -443,8 +444,8 @@ func (m Module) check() error {
 	switch {
 	case m.FP != "" && m.None != "":
 		return fmt.Errorf("delivery: a module has both a fingerprint and a reason for none")
-	case m.FP == "" && m.None == "":
-		return fmt.Errorf("delivery: a module has neither a fingerprint nor a reason for none")
+	case m.FP == "" && m.None == "" && m.FP2 == "" && m.None2 == "":
+		return fmt.Errorf("delivery: a module has neither a fingerprint nor a reason for none, of either form")
 	case m.FP != "" && !strings.HasPrefix(m.FP, SourcePrefix):
 		return fmt.Errorf("delivery: the module's fingerprint %q does not start with %s", m.FP, SourcePrefix)
 	case m.FP2 != "" && m.None2 != "":