The mesh builds: a machine takes the work, and the catalogue shows it

A build is work, not state. Everything else the control plane sends a
node is a declaration — this is what you should be — reconciled forever.
A build happens once and is finished. Putting it in a declaration would
mean rebuilding on every reconcile, or a declaration carrying "and I
already did this", which is state about an event rather than about a
machine.

So it travels on its own queue and the answer comes back correlated. One
queue, so several build machines share the work and each request is done
exactly once — which a per-machine routing key would not give.

mesh-builder is the program a build machine runs. Not the control plane,
which must not run commands on a machine; not the host, which would then
need a container runtime and git everywhere to do something almost no
machine will ever do. It holds its own broker credential and nothing
else.

Three properties that are decisions:

- a request is acknowledged only once the answer is away, so a builder
  that dies mid-build leaves the work for another machine rather than
  losing it with nobody ever hearing why
- one build at a time. Five at once against one runtime finishes all five
  slower than it would have finished the first, and the queue is what
  shares work between machines
- a failure is a RESULT. A build that fails silently is
  indistinguishable from a builder that is not running, and those want
  different responses

And `module list` is a catalogue: what exists, at which version, built
from which commit or handed over by hand or shipped with the control
plane, whether it is behind its source, and which machines run it. All of
that was recorded from the first build and none of it was shown, so "is
this current?" could only be answered by reading the database.

Proven against a real broker, registry and store: the mesh asked, a
builder consumed, built, published, answered; the manifest was recorded
with its commit; the source moved and the catalogue said "behind";
rebuilding caught it up with a new digest because the content changed.

internal/inventory/catalogue.go

@@ -515,3 +515,63 @@ func (i *Inventory) pinRows(ctx context.Context, nodeName string) (int, error) {
 		`select count(*) from provision_pin where node = $1`, node.ID).Scan(&n)
 	return n, err
 }
+
+// Entry is one module as a catalogue shows it: what it is, where it came from, and who runs it.
+type Entry struct {
+	Manifest catalogue.Manifest
+	Source   Source
+	// On is every node this module is assigned to, sorted.
+	On []string
+	// Provided is true when the module came with the control plane rather than from a repository.
+	Provided bool
+}
+
+// Catalogued is every module the mesh knows about, with everything a person asks about one.
+//
+// **One query rather than a call per module.** A catalogue that costs a round trip per row is a
+// catalogue nobody lists, and the questions here — what is this, where did it come from, who is
+// running it — are asked together every time.
+func (i *Inventory) Catalogued(ctx context.Context) ([]Entry, error) {
+	rows, err := i.store.Pool().Query(ctx,
+		`select m.name, m.manifest,
+		        coalesce(m.source, ''), coalesce(m.ref, ''),
+		        coalesce(m.built_from, ''), coalesce(m.source_head, ''),
+		        coalesce(array_agg(n.name order by n.name) filter (where n.name is not null), '{}')
+		 from module m
+		 left join assignment a on a.module = m.name
+		 left join node n on n.id = a.node
+		 group by m.name, m.manifest, m.source, m.ref, m.built_from, m.source_head
+		 order by m.name`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var out []Entry
+	for rows.Next() {
+		var raw []byte
+		var name string
+		var source Source
+		var on []string
+		if err := rows.Scan(&name, &raw, &source.Repository, &source.Ref,
+			&source.BuiltFrom, &source.Head, &on); err != nil {
+			return nil, err
+		}
+		var m catalogue.Manifest
+		if err := json.Unmarshal(raw, &m); err != nil {
+			return nil, err
+		}
+		entry := Entry{Manifest: m, Source: source, On: on}
+		if source.Repository == providedBy {
+			// It came with the control plane. Not a repository, and showing it as one would have
+			// somebody go looking for it.
+			entry.Provided = true
+			entry.Source = Source{}
+		}
+		out = append(out, entry)
+	}
+	return out, rows.Err()
+}
+
+// providedBy is what the source column says for a module the control plane ships.
+const providedBy = "the control plane"