nodered: its MQTT broker comes from the mesh

Node-RED's one broker node pointed at zurag.be:1884, where nothing listens. nodered now requires
mqtt-topic (asking for every topic: flows follow the devices' own) and a run-once `mqtt` step —
declared last, restarted when the binding, credential or settings change — points the mesh's broker
nodes at the bound broker through Node-RED's admin API with the module's api-token: the node the
step makes itself when none is named, or the ones an assignment names in `mqtt.brokers`. Only host,
port, TLS and the login change; the broker is asked first whether it takes the login; the deploy is
against the revision read ("nodes", so only that node restarts) and a digest makes a rerun a no-op.
A broker node nobody named is never touched. settings.js keeps `mqtt` and `topics` out of Node-RED.

modules/nodered/mqtt/probe.ts

@@ -0,0 +1,117 @@
+// Ask the broker, before Node-RED is told anything, whether it takes the login and password the
+// mesh delivered — and whether that login may subscribe to every topic, as flows expect.
+//
+// One MQTT 3.1.1 session: CONNECT (clean, a throwaway client id, so no flow's session is taken
+// over), read the CONNACK, optionally SUBSCRIBE once and read the SUBACK, DISCONNECT. No dependency:
+// the handful of bytes MQTT needs for this are written here.
+
+import { randomBytes } from "node:crypto";
+import { connect } from "node:net";
+
+export interface ProbeResult {
+  /** 0 accepted; 4 bad username or password; 5 not authorised. */
+  connack: number;
+  /** The SUBACK return code for the filter asked about: 0–2 granted, 0x80 refused. */
+  suback?: number;
+}
+
+export type Probe = (host: string, port: number, username: string, password: string, subscribe?: string) => Promise<ProbeResult>;
+
+function str(v: string): Buffer {
+  const b = Buffer.from(v, "utf8");
+  const len = Buffer.alloc(2);
+  len.writeUInt16BE(b.length);
+  return Buffer.concat([len, b]);
+}
+
+function packet(type: number, body: Buffer): Buffer {
+  let remaining = body.length;
+  const lenBytes: number[] = [];
+  do {
+    let byte = remaining % 128;
+    remaining = Math.floor(remaining / 128);
+    if (remaining > 0) byte |= 0x80;
+    lenBytes.push(byte);
+  } while (remaining > 0);
+  return Buffer.concat([Buffer.from([type, ...lenBytes]), body]);
+}
+
+/** The first complete packet in `buf`: its type byte, its body, and how many bytes it took. */
+export function firstPacket(buf: Buffer): { type: number; body: Buffer; used: number } | undefined {
+  if (buf.length < 2) return undefined;
+  let length = 0;
+  let multiplier = 1;
+  let i = 1;
+  for (;;) {
+    if (i >= buf.length) return undefined;
+    const byte = buf[i++];
+    length += (byte & 0x7f) * multiplier;
+    if ((byte & 0x80) === 0) break;
+    multiplier *= 128;
+    if (i > 4) throw new Error("malformed MQTT remaining length");
+  }
+  if (buf.length < i + length) return undefined;
+  return { type: buf[0], body: buf.subarray(i, i + length), used: i + length };
+}
+
+export const probeBroker: Probe = (host, port, username, password, subscribe) => {
+  const connectBody = Buffer.concat([
+    str("MQTT"),
+    Buffer.from([4, 0xc2, 0, 10]), // level 4 (3.1.1); username + password + clean session; keepalive 10s
+    str(`mesh-probe-${randomBytes(6).toString("hex")}`),
+    str(username),
+    str(password),
+  ]);
+  return new Promise((resolve, reject) => {
+    const socket = connect({ host, port });
+    let buf = Buffer.alloc(0);
+    const result: ProbeResult = { connack: -1 };
+    const timer = setTimeout(() => {
+      socket.destroy();
+      reject(new Error(`no answer from the broker at ${host}:${port} within 10s`));
+    }, 10_000);
+    const finish = (): void => {
+      clearTimeout(timer);
+      if (result.connack === 0) socket.end(Buffer.from([0xe0, 0]));
+      else socket.destroy();
+      resolve(result);
+    };
+    socket.on("connect", () => socket.write(packet(0x10, connectBody)));
+    socket.on("data", (chunk) => {
+      buf = Buffer.concat([buf, chunk]);
+      for (;;) {
+        let p;
+        try {
+          p = firstPacket(buf);
+        } catch (err) {
+          clearTimeout(timer);
+          socket.destroy();
+          reject(err);
+          return;
+        }
+        if (!p) return;
+        buf = buf.subarray(p.used);
+        const kind = p.type >> 4;
+        if (kind === 2) {
+          result.connack = p.body[1] ?? -1;
+          if (result.connack !== 0 || !subscribe) return finish();
+          // SUBSCRIBE, packet id 1, one filter at QoS 0.
+          socket.write(packet(0x82, Buffer.concat([Buffer.from([0, 1]), str(subscribe), Buffer.from([0])])));
+        } else if (kind === 9) {
+          result.suback = p.body[2];
+          return finish();
+        }
+      }
+    });
+    socket.on("error", (err) => {
+      clearTimeout(timer);
+      reject(err);
+    });
+    socket.on("close", () => {
+      if (result.connack === -1) {
+        clearTimeout(timer);
+        reject(new Error(`the broker at ${host}:${port} closed the connection without answering`));
+      }
+    });
+  });
+};