Review before merge: refuse a silent disagreement, and bound the sweep

Three things found reading this back, each of which would have been quiet.

A consumer that keeps several holders of one provision (ADR 0094) gets a
login per holder, and a provider derives from the login — so it would make a
resource per holder while the consumer is told one value for the requirement.
That is issue 124's own failure one case to the side: authenticate, then be
refused on every object. Refused now, naming both ends.

The sweep runs inside somebody's build and was unbounded. At most two hundred
artifacts and sixty seconds, stopping at the first refusal because a store
that refuses one refuses all; the rest is offered again next build.

The citation and migration renumbers are in the commit before this one.

internal/catalogue/consumer_into_serves.go

@@ -177,7 +177,7 @@ func sortedAnyKeys(values map[string]any) []string {
 // choice servedOnThisMachine makes for the consumer's half. Nothing serving it on this machine is
 // not an error: a contribution can reach a machine whose provider is a record or an adapter, and
 // then there is nothing derived to tell.
-func (r Resolution) derivedFor(provision, as string, settings SettingsBy) (map[string]any, error) {
+func (r Resolution) derivedFor(provision, as, consumer, local string, settings SettingsBy) (map[string]any, error) {
 	for _, m := range r.Modules {
 		serves, said := m.Serves[provision]
 		if !said {
@@ -195,6 +195,27 @@ func (r Resolution) derivedFor(provision, as string, settings SettingsBy) (map[s
 		if names == nil {
 			return nil, nil
 		}
+		// **A consumer that keeps several holders of this provision is refused** — this is issue
+		// 124's own failure one case to the side, and it would be just as quiet.
+		//
+		// Each holder gets its own login, `…_<local>` (ADR 0094), and a provider derives from the
+		// login, so it would make one resource per holder. The consumer's side has no such
+		// dimension: one binding file per provision, one `${bound:<provision>:<key>}`, both
+		// derived from the un-suffixed identity. So the provider would create the holder's
+		// resource and the consumer would be configured against a name nothing made — it would
+		// authenticate successfully and be refused on every object, which reads like a credential
+		// fault and is not one.
+		//
+		// Lifting this means giving the consumer's side a local dimension. That is a decision,
+		// not an omission, and until it is taken the mesh says so rather than guessing.
+		if local != "" {
+			return nil, fmt.Errorf(
+				"%s keeps several holders of %s (this one is %q), and %s derives %s for each "+
+					"consumer from the login the mesh minted. Each holder has its own login, and a "+
+					"consumer is told one value per requirement — so the two ends would name "+
+					"different things and nothing would compare them (novox/hq ADR 0201)",
+				consumer, local, provision, m.Module, orNothing(sortedAnyKeys(names)))
+		}
 		settled, err := Settle(names, settings[m.Module])
 		if err != nil {
 			return nil, fmt.Errorf("%s serving %s: %w", m.Module, provision, err)

internal/catalogue/declaration.go

@@ -1307,7 +1307,7 @@ func (r Resolution) contributions(settings SettingsBy, grants []Grant,
 			continue
 		}
 		as := holderAs(ConsumerIdentity(g.Consumer, IdentitySource(g.Slug, g.From)), g.Local)
-		derived, err := r.derivedFor(g.Provision, as, settings)
+		derived, err := r.derivedFor(g.Provision, as, g.From, g.Local, settings)
 		if err != nil {
 			return nil, err
 		}

internal/catalogue/derived_for_consumer_test.go

@@ -295,3 +295,49 @@ func storeGrants(t *testing.T, out []map[string]any) []Contribution {
 	t.Fatalf("the provider was given no contributions file: %v", out)
 	return nil
 }
+
+// A consumer that keeps SEVERAL holders of one provision is refused, rather than told one thing
+// while its provider is told another.
+//
+// **This is issue 124's own failure, one case to the side.** The mesh gives each holder its own
+// login — `mesh_node_mod_<local>` (ADR 0094) — and the provider derives from the login, so it
+// would make one resource per holder. The consumer's side has no such dimension: there is one
+// binding file per provision and one `${bound:<provision>:<key>}`, both derived from the
+// un-suffixed identity. So the provider would create `…-mod-cold` and the consumer would be
+// configured against `…-mod`: it would authenticate successfully and be refused on every object,
+// which is exactly the fault this whole record exists to end.
+//
+// Refused, loudly, at the one place that can see both halves. Lifting it means giving the
+// consumer's side a local dimension, which is a decision and not an omission.
+func TestAConsumerWithSeveralHoldersOfADerivingProviderIsRefused(t *testing.T) {
+	m := files()
+	// Two holders of the one provision, the shape ADR 0094 gives a module that keeps several.
+	m.Secrets = nil
+	m.SecretsMany = map[string]map[string]string{"s3-bucket": {
+		"hot":  "/var/lib/files/hot.secret",
+		"cold": "/var/lib/files/cold.secret",
+	}}
+	m.Resources = []map[string]any{{
+		"id": "env", "type": "file", "path": "/var/lib/files/env", "mode": "0600",
+		"content": "BUCKET=${bound:s3-bucket:bucket}\n",
+	}}
+	r, err := Resolve(shelf(store(), m), []string{"store", "files"}, reachable(), World{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = r.Declaration(Rendering{Grants: []Grant{
+		{Provision: "s3-bucket", Consumer: "workstation", From: "files", Slug: "files",
+			Local: "hot", Values: map[string]any{}, Sealed: "c2VhbGVk"},
+		{Provision: "s3-bucket", Consumer: "workstation", From: "files", Slug: "files",
+			Local: "cold", Values: map[string]any{}, Sealed: "c2VhbGVk"},
+	}})
+	if err == nil {
+		t.Fatal("a consumer with several holders of a deriving provider was accepted; " +
+			"its two ends would have disagreed in silence")
+	}
+	for _, want := range []string{"files", "s3-bucket", "bucket"} {
+		if !strings.Contains(err.Error(), want) {
+			t.Errorf("the refusal does not name %q: %v", want, err)
+		}
+	}
+}