dnsmasq: a resolver tool and event (ADR 0044/0046)

The mesh's resolver is more than config after all. Tools: dnsmasq_names (what
this node answers, from the generated wildcard file) and dnsmasq_resolve (resolve
a name through this node's own resolver — the check that wildcard-resolution
actually answers). Event: it watches the wildcard file and emits
module.dnsmasq.name.added/.removed as machines' names become resolvable here —
DNS having propagated to this node, distinct from the mesh's node.* events.
Typechecks; manifest parses.

modules/dnsmasq/client.ts

@@ -0,0 +1,59 @@
+// dnsmasq's own code, living in the module (novox/hq ADR 0044). dnsmasq here is a pure resolver: it
+// answers the mesh's generated wildcard names (<service>.<node>.<suffix>) and forwards nothing. So
+// its code reads what it was told to answer, and can resolve through itself to prove that it does.
+
+import { readFile } from "node:fs/promises";
+import { Resolver } from "node:dns/promises";
+
+export interface AnsweredName {
+  /** A machine's internal name, e.g. "anchor.internal" — it and everything under it resolve here. */
+  name: string;
+  address: string;
+}
+
+export class DnsmasqClient {
+  constructor(
+    private readonly resolverPath: string,
+    private readonly address: string,
+  ) {}
+
+  /**
+   * Build from the environment. Both values are node-local facts with mesh-chosen defaults — the
+   * wildcard file the module is sent, and the loopback address its config listens on — so there is
+   * nothing to be unconfigured about; it never throws.
+   */
+  static fromEnv(env: NodeJS.ProcessEnv = process.env): DnsmasqClient {
+    return new DnsmasqClient(
+      env.MESH_DNSMASQ_RESOLVER_PATH ?? "/etc/mesh-resolver/nodes.conf",
+      env.MESH_DNSMASQ_ADDRESS ?? "127.0.0.55",
+    );
+  }
+
+  /** The names this resolver answers, read from the mesh-generated wildcard file. */
+  async answeredNames(): Promise<AnsweredName[]> {
+    let text: string;
+    try {
+      text = await readFile(this.resolverPath, "utf8");
+    } catch {
+      return []; // not yet on the network, or the file has not been written — no names, not an error
+    }
+    const names: AnsweredName[] = [];
+    for (const line of text.split("\n")) {
+      // dnsmasq wildcard syntax the mesh writes: address=/<node>.<suffix>/<address>
+      const match = line.match(/^address=\/([^/]+)\/(.+)$/);
+      if (match) names.push({ name: match[1], address: match[2] });
+    }
+    return names;
+  }
+
+  /** Resolve a name through this node's own resolver — the check that wildcard-resolution answers. */
+  async resolve(name: string): Promise<string[]> {
+    const resolver = new Resolver();
+    resolver.setServers([this.address]);
+    try {
+      return await resolver.resolve4(name);
+    } catch {
+      return await resolver.resolve6(name);
+    }
+  }
+}