J2: the tool runner tells a subscribed bundle how its module's consumer stands, as mesh/heartbeat (stdio.OnHeartbeat), at each idle beat and once Missed (ADR 0615, ADR 0621, issue 1966)

Claude-Session: https://claude.ai/code/session_01JG73KJNrH5EgFXMKKYSrig

node-tools/internal/launch/launch.go

@@ -80,7 +80,9 @@ type Registration struct {
 // Bus is what a launched bundle reaches the mesh through (novox/hq ADR 0193, ADR 0198): the runtime
 // publishes, asks and subscribes on the module's behalf. Subscribe is asked each time the module's
 // code subscribes; the runtime binds the module's consumer once and calls deliver for every event,
-// acknowledging it on the bus when deliver returns nil.
+// acknowledging it on the bus when deliver returns nil. beat is the runtime's word on that consumer between
+// events (a bus.Beat as JSON), which the launch hands the child that subscribed as `mesh/heartbeat`, so a bundle
+// can tell a silent source from a quiet one (stdio.OnHeartbeat).
 //
 // State and Watch reach the module's state (novox/hq ADR 0201): State answers `get`, `put`, `delete`
 // and `keys`; Watch hands deliver the current values and then every change, and returns once the
@@ -88,7 +90,7 @@ type Registration struct {
 type Bus interface {
 	Publish(params json.RawMessage) error
 	Ask(params json.RawMessage) (json.RawMessage, error)
-	Subscribe(deliver func(envelope json.RawMessage) error) error
+	Subscribe(deliver func(envelope json.RawMessage) error, beat func(heartbeat json.RawMessage)) error
 	State(verb string, params json.RawMessage) (json.RawMessage, error)
 	Watch(params json.RawMessage, deliver func(change json.RawMessage) error) (stop func(), err error)
 	// Log answers a bundle's `append`, `read`, `last` and `info` on its module's log (novox/hq ADR 0297).
@@ -261,6 +263,17 @@ func Start(module, entry string, env []string, mesh Bus, logf func(string, ...an
 		_, err := c.ask(module, "mesh/event", map[string]any{"envelope": envelope}, EventTimeout)
 		return err
 	}
+	// The heartbeat of the module's consumer goes to the child that last subscribed, as a notification: the
+	// bundle answers nothing, and a child not running misses it, as it misses the events it did not take.
+	beat := func(heartbeat json.RawMessage) {
+		mu.Lock()
+		c := subscriber
+		mu.Unlock()
+		if c == nil {
+			return
+		}
+		_ = c.write(map[string]any{"jsonrpc": "2.0", "method": "mesh/heartbeat", "params": heartbeat})
+	}
 	var restart func(after time.Duration)
 	var startedAt time.Time
 	// The restart pauses are read once, here: the exit handling runs on its own goroutine and must not
@@ -344,7 +357,7 @@ func Start(module, entry string, env []string, mesh Bus, logf func(string, ...an
 							mu.Lock()
 							subscriber = c
 							mu.Unlock()
-							err = mesh.Subscribe(deliver)
+							err = mesh.Subscribe(deliver, beat)
 						case "mesh/seat.publish", "mesh/seat.prove", "mesh/seat.record", "mesh/seat.kinds", "mesh/seat.pull", "mesh/seat.settle":
 							var answered json.RawMessage
 							if answered, err = mesh.Seat(strings.TrimPrefix(m.Method, "mesh/seat."), m.Params, nil); err == nil {