Include requesting principal in CMDs (#329)

There are situations where it is useful to know the identity of the
principal who requested a CMD. Include this information in the CMD
packet in the following form:

* The `uuid` field of the CMD is set to the Factory+ UUID.
* The `body` of the CMD is a UTF-8 string starting with a scheme name
and a colon.
* The scheme `uuid:` is followed by a principal UUID in canonical string
form.
* The scheme `kerberos:` is followed by a Kerberos principal name.
* The schema `sparkplug:` is followed by a Sparkplug address in
`Group/Node` form.

(Currently Devices are not permitted to request command escalation, as
they aren't security principals and can't be authorised.)

This is a rebase of #269 onto `main`. The previous merge was lost when
we changed policy to 'main is always releasable'.

acs-cmdesc/lib/cmdescd.js

@@ -168,7 +168,14 @@ export default class CmdEscD {
             }
         }
 
-        this.mqtt.publish(to, "CMD", [cmd]);
+        this.mqtt.publish({
+            address:    to,
+            type:       "CMD",
+            metrics:    [cmd],
+            from:       from instanceof Address 
+                ? `sparkplug:${from}`
+                : `kerberos:${from}`,
+        });
         return log(200);
     }
 }

acs-cmdesc/lib/mqttcli.js

@@ -156,7 +156,11 @@ export default class MqttCli {
         //    Object.values(Changed).map(v =>
         //        ({ name: `Last_Changed/${v}`, type: "UUID", value: "" })));
 
-        this.publish(this.address, "BIRTH", metrics);
+        this.publish({
+            address:    this.address,
+            type:       "BIRTH",
+            metrics,
+        });
     }
 
     decode_request(metric) {
@@ -189,27 +193,33 @@ export default class MqttCli {
         return [address, {name: tag, value}];
     }
 
-    encode_metrics(metrics, kind) {
+    encode_metrics (opts) {
+        const { type, metrics, from } = opts;
+
         const payload = {
             timestamp: Date.now(),
-            metrics: metrics,
+            metrics,
         };
 
         /* CMDs don't increment the seq, because by spec they are only
          * sent by the Primary App. */
-        if (kind != "CMD") {
+        if (type != "CMD") {
             payload.seq = this.seq;
             this.seq = (this.seq < 255) ? (this.seq + 1) : 0;
         }
-        if (kind == "BIRTH")
+        if (type == "BIRTH" || type == "CMD")
             payload.uuid = UUIDs.FactoryPlus;
+        if (type == "CMD") {
+            const sender = from ?? `uuid:${this.device_uuid}`;
+            payload.body = Buffer.from(sender.toString());
+        }
 
         return SpB.encodePayload(payload);
     }
 
-    async publish(addr, kind, metrics) {
-        const topic = addr.topic(kind);
-        const payload = this.encode_metrics(metrics, kind);
+    async publish (opts) {
+        const topic = opts.address.topic(opts.type);
+        const payload = this.encode_metrics(opts);
 
         this.log("mqtt", `Publishing to ${topic}`);
         this.mqtt.publish(topic, payload);
@@ -224,7 +234,11 @@ export default class MqttCli {
 
         const resp = MetricBuilder.cmd.command_escalation_response(
             to, cmd.name, stat);
-        this.publish(from, "CMD", resp);
+        this.publish({
+            address:    from,
+            type:       "CMD",
+            metrics:    resp,
+        });
     }
 }