review: hold the hosts region through composition, and say so in plan

A declaration-level test composes the shipped networking module with a
resolver and asserts /etc/hosts arrives as mesh-wireguard.fact-node-names
with into: block and region-only content, that the resolver's restart-on
still names it, and that a resource's at passes through untouched — a
composition step dropping into would otherwise go unnoticed. plan --show
marks files written into, so a region is not read as the whole file.
The rollout order is spelled out: every node's host, the controller's
own included, must be block-aware before this controller ships (hq 128).

internal/catalogue/hosts_region_test.go

@@ -0,0 +1,106 @@
+package catalogue_test
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+	"github.com/novox/mesh-controller/internal/overlay"
+)
+
+// novox/hq issue 128, held where the host will see it: the shipped networking module's names,
+// through the whole composition, and not only through FactsInto.
+//
+// Composition prefixes a fact's id with the module that asked for it and passes everything else
+// through; a step that dropped `into` on the way would send the region as a whole file, and the
+// host would write the machine's hosts file over again with every unit test above still green.
+
+// onTheNetwork stands in for the overlay's generator: the node is part of the private network,
+// and what the generator writes is not what is under test here.
+type onTheNetwork struct{}
+
+func (onTheNetwork) Resources(string) ([]map[string]any, bool, error) {
+	return []map[string]any{{"id": "overlay-config", "type": "file",
+		"path": "/etc/wireguard/mesh0.conf", "mode": "0600", "content": "[Interface]\n"}}, true, nil
+}
+
+func TestTheHostsRegionArrivesAsTheHostWillReadIt(t *testing.T) {
+	shelf := provided(t)
+	// A resolver restarting on the names another module put on the machine, and one resource it
+	// only runs at start — neither of which composition has any business changing.
+	resolver, err := catalogue.ParseManifest([]byte(`{
+		"module": "resolver", "version": "1", "requires": ["mesh-addressing"],
+		"resources": [
+			{"id": "seed", "type": "file", "path": "/etc/resolver/seed", "mode": "0644",
+			 "content": "seed\n", "at": "start"},
+			{"id": "daemon", "type": "service", "unit": "resolver.service", "state": "running",
+			 "restart-on": ["seed", "mesh-wireguard.fact-node-names"]}
+		]}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	shelf[resolver.Module] = resolver
+
+	got, err := catalogue.Resolve(shelf, []string{overlay.Domain, "resolver"},
+		catalogue.Node{Name: "homer", At: "homer.internal"}, catalogue.World{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	names := map[string]string{"homer.internal": "10.42.0.1", "marge.internal": "10.42.0.2"}
+	out, err := got.Declaration(catalogue.Rendering{
+		Names: names, Machines: names, Suffix: "internal",
+		Generators: map[string]catalogue.Generator{overlay.Name: onTheNetwork{}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	ids := map[string]map[string]any{}
+	for _, r := range out {
+		ids[r["id"].(string)] = r
+	}
+
+	hosts := ids[overlay.Name+".fact-node-names"]
+	if hosts == nil {
+		t.Fatalf("no names reached the machine; the declaration has %v", keys(ids))
+	}
+	if hosts["path"] != "/etc/hosts" || hosts["into"] != "block" {
+		t.Fatalf("the hosts file is not written into as a region: %v", hosts)
+	}
+	content := hosts["content"].(string)
+	if !strings.Contains(content, "10.42.0.1\thomer.internal\thomer\t# this machine\n") {
+		t.Errorf("the region does not name the machine:\n%s", content)
+	}
+	for _, floor := range []string{"Generated by the mesh", "localhost", "127.0.1.1"} {
+		if strings.Contains(content, floor) {
+			t.Errorf("the region carries %q, which is the machine's:\n%s", floor, content)
+		}
+	}
+
+	// The resolver's reference to it still names a resource the host will be sent.
+	daemon := ids["resolver.daemon"]
+	if daemon == nil {
+		t.Fatalf("the resolver's service was not composed: %v", keys(ids))
+	}
+	for _, named := range daemon["restart-on"].([]any) {
+		if ids[named.(string)] == nil {
+			t.Errorf("the resolver restarts on %v, which is nothing the host is sent", named)
+		}
+	}
+	if !reflect.DeepEqual(daemon["restart-on"], []any{"resolver.seed", overlay.Name + ".fact-node-names"}) {
+		t.Errorf("restart-on is %v", daemon["restart-on"])
+	}
+
+	// And a resource's own `at` passes through as the manifest wrote it.
+	if seed := ids["resolver.seed"]; seed == nil || seed["at"] != "start" {
+		t.Errorf("a resource's at did not survive composition: %v", seed)
+	}
+}
+
+func keys(m map[string]map[string]any) []string {
+	out := make([]string, 0, len(m))
+	for k := range m {
+		out = append(out, k)
+	}
+	return out
+}