feat(spotify): GoLibrespotBackend sidecar (FIFO + ffmpeg PCM + REST/WS)

Implements SpotifyAudioBackend over a go-librespot sidecar: start() mkfifos
the pipe, spawns the FIFO->48k s16le ffmpeg reader BEFORE go-librespot, writes
config.yml, polls the REST /  until ready, then connects the WS event stream.
Maps not_playing/stopped -> trackEnded and metadata -> SpotifyNowPlaying;
play/pause/resume/seek delegate to the REST client. All child_process/fs/REST/WS
seams are injectable so the lifecycle is fully unit-tested without a real binary.

Correction C1: ffmpeg is resolved via getFfmpegCommand() (now exported from
src/audio/player.ts) so the bundled ffmpeg-static fallback is honored in Docker;
the ffmpeg command is overridable via deps.ffmpegCommand for tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/music/spotify/go-librespot.test.ts

@@ -0,0 +1,221 @@
+import { describe, it, expect, vi } from "vitest";
+import { EventEmitter } from "node:events";
+import { PassThrough } from "node:stream";
+import pino from "pino";
+import { GoLibrespotBackend } from "./go-librespot.js";
+
+const log = pino({ level: "silent" });
+
+/** A minimal stand-in for a spawned ChildProcess with real Readable stdout/stderr. */
+function makeFakeChild() {
+  const child: any = new EventEmitter();
+  child.stdout = new PassThrough();
+  child.stderr = new PassThrough();
+  child.kill = vi.fn();
+  return child;
+}
+
+function makeHarness() {
+  const calls: string[] = [];
+  const ffmpegChild = makeFakeChild();
+  const gliChild = makeFakeChild();
+
+  const spawn = vi.fn((cmd: string, ..._rest: any[]) => {
+    const isGli = cmd.includes("go-librespot");
+    calls.push(`spawn:${isGli ? "go-librespot" : cmd}`);
+    return isGli ? gliChild : ffmpegChild;
+  });
+  const execFileSync = vi.fn((cmd: string) => {
+    calls.push(`exec:${cmd}`);
+    return Buffer.from("");
+  });
+  const writeFileSync = vi.fn(() => calls.push("write:config"));
+  const mkdirSync = vi.fn();
+  const unlinkSync = vi.fn(() => calls.push("unlink:fifo"));
+  const existsSync = vi.fn(() => false);
+
+  const rest = {
+    ping: vi.fn(async () => true),
+    playTrack: vi.fn(async () => {}),
+    pause: vi.fn(async () => {}),
+    resume: vi.fn(async () => {}),
+    stop: vi.fn(async () => {}),
+    seek: vi.fn(async () => {}),
+    getStatus: vi.fn(async () => null),
+  };
+  const events: any = new EventEmitter();
+  events.start = vi.fn(() => calls.push("ws:start"));
+  events.stop = vi.fn();
+
+  const backend = new GoLibrespotBackend({
+    deviceName: "Test Bot",
+    bitrate: 320,
+    workDir: "/tmp/work",
+    configDir: "/tmp/cfg",
+    apiPort: 3678,
+    logger: log,
+    deps: {
+      spawn,
+      execFileSync,
+      writeFileSync,
+      mkdirSync,
+      unlinkSync,
+      existsSync,
+      // C1: pin the ffmpeg command so the arg-array/order assertions below stay
+      // stable while production resolves ffmpeg via getFfmpegCommand() (which
+      // falls back to bundled ffmpeg-static when `ffmpeg` isn't on PATH).
+      ffmpegCommand: "ffmpeg",
+      findBinary: () => "/bin/go-librespot",
+      makeRest: () => rest,
+      makeEvents: () => events,
+      sleep: async () => {},
+      pollIntervalMs: 1,
+      pollTimeoutMs: 100,
+    } as any,
+  });
+
+  return { backend, calls, spawn, execFileSync, writeFileSync, existsSync, unlinkSync, rest, events, ffmpegChild, gliChild };
+}
+
+describe("GoLibrespotBackend.start", () => {
+  it("creates the FIFO with mkfifo before spawning ffmpeg, and spawns ffmpeg BEFORE go-librespot", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+
+    expect(h.execFileSync).toHaveBeenCalledWith("mkfifo", ["/tmp/work/go-librespot.fifo"]);
+    expect(h.writeFileSync).toHaveBeenCalled();
+
+    const mkfifoIdx = h.calls.indexOf("exec:mkfifo");
+    const ffmpegIdx = h.calls.indexOf("spawn:ffmpeg");
+    const gliIdx = h.calls.indexOf("spawn:go-librespot");
+    expect(mkfifoIdx).toBeGreaterThanOrEqual(0);
+    expect(ffmpegIdx).toBeGreaterThan(mkfifoIdx); // ffmpeg attaches to the FIFO first
+    expect(gliIdx).toBeGreaterThan(ffmpegIdx);     // then the writer (go-librespot)
+    expect(h.calls.indexOf("ws:start")).toBeGreaterThan(gliIdx); // WS connects last
+  });
+
+  it("passes --config_dir to go-librespot using the resolved binary path", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    expect(h.spawn).toHaveBeenCalledWith(
+      "/bin/go-librespot",
+      ["--config_dir", "/tmp/cfg"],
+      expect.anything(),
+    );
+  });
+
+  it("uses the 44100->48000 s16le ffmpeg command reading the FIFO", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const ffmpegArgs = h.spawn.mock.calls.find((c) => c[0] === "ffmpeg")![1] as string[];
+    expect(ffmpegArgs).toEqual([
+      "-hide_banner", "-loglevel", "error",
+      "-f", "s16le", "-ar", "44100", "-ac", "2", "-i", "/tmp/work/go-librespot.fifo",
+      "-f", "s16le", "-ar", "48000", "-ac", "2", "-acodec", "pcm_s16le", "pipe:1",
+    ]);
+  });
+
+  it("emits 'ready' and reports isReady() true once the REST ping succeeds", async () => {
+    const h = makeHarness();
+    const ready = vi.fn();
+    h.backend.on("ready", ready);
+    await h.backend.start();
+    expect(h.rest.ping).toHaveBeenCalled();
+    expect(ready).toHaveBeenCalledTimes(1);
+    expect(h.backend.isReady()).toBe(true);
+  });
+
+  it("keeps polling ping() until it returns true", async () => {
+    const h = makeHarness();
+    h.rest.ping.mockResolvedValueOnce(false).mockResolvedValueOnce(false).mockResolvedValue(true);
+    await h.backend.start();
+    expect(h.rest.ping).toHaveBeenCalledTimes(3);
+    expect(h.backend.isReady()).toBe(true);
+  });
+});
+
+describe("GoLibrespotBackend WebSocket event mapping", () => {
+  it("maps a not_playing event to trackEnded{reason:'ended'}", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.events.emit("not_playing", { uri: "spotify:track:abc" });
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:abc", reason: "ended" });
+  });
+
+  it("maps a stopped event to trackEnded{reason:'stopped'}", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.events.emit("stopped", { uri: "spotify:track:xyz" });
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:xyz", reason: "stopped" });
+  });
+
+  it("maps a metadata event to a SpotifyNowPlaying and updates getPositionMs()", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const meta = vi.fn();
+    h.backend.on("metadata", meta);
+    h.events.emit("metadata", {
+      uri: "spotify:track:abc",
+      name: "Song",
+      artist_names: ["A", "B"],
+      album_name: "Alb",
+      album_cover_url: "http://x/y.jpg",
+      position: 1234,
+      duration: 200000,
+    });
+    expect(meta).toHaveBeenCalledWith({
+      uri: "spotify:track:abc",
+      name: "Song",
+      artist: "A, B",
+      album: "Alb",
+      coverUrl: "http://x/y.jpg",
+      durationMs: 200000,
+    });
+    expect(h.backend.getPositionMs()).toBe(1234);
+  });
+});
+
+describe("GoLibrespotBackend transport delegation + PCM", () => {
+  it("playTrack delegates to the REST client", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    await h.backend.playTrack("spotify:track:go");
+    expect(h.rest.playTrack).toHaveBeenCalledWith("spotify:track:go");
+  });
+
+  it("pause/resume/seek delegate to the REST client and seek updates position", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    await h.backend.pause();
+    await h.backend.resume();
+    await h.backend.seek(5000);
+    expect(h.rest.pause).toHaveBeenCalled();
+    expect(h.rest.resume).toHaveBeenCalled();
+    expect(h.rest.seek).toHaveBeenCalledWith(5000);
+    expect(h.backend.getPositionMs()).toBe(5000);
+  });
+
+  it("getPcmStream() returns the ffmpeg stdout Readable", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    expect(h.backend.getPcmStream()).toBe(h.ffmpegChild.stdout);
+  });
+});
+
+describe("GoLibrespotBackend.stop", () => {
+  it("kills ffmpeg + go-librespot, stops the WS, removes the FIFO, and clears ready", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    h.existsSync.mockReturnValue(true); // FIFO now present, so stop() unlinks it
+    h.backend.stop();
+    expect(h.ffmpegChild.kill).toHaveBeenCalled();
+    expect(h.gliChild.kill).toHaveBeenCalled();
+    expect(h.events.stop).toHaveBeenCalled();
+    expect(h.unlinkSync).toHaveBeenCalledWith("/tmp/work/go-librespot.fifo");
+    expect(h.backend.isReady()).toBe(false);
+  });
+});