feat(audio): add external-PCM mode (playPcmStream) for Spotify sidecar

Adds AudioPlayer.playPcmStream(readable, {onExternalEnd}) that feeds a
long-lived external 48kHz/s16le/stereo Readable into the existing pcmBuffer +
20ms frame loop + Opus encoder without spawning a per-URL ffmpeg. Reuses the
same high/low-water backpressure (pausing/resuming the Readable), suppresses the
underrun trackEnd drain/stall branches while external (emitting a silence frame
to keep the 20ms timeline), tears down externalMode in stop() by DETACHING the
shared readable (remove our data/end/error listeners + pause, never destroy the
sidecar stream) and clearing onExternalEnd, and makes seek() a local no-op in
external mode. The url play() path and all exported pure functions are unchanged.

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

src/audio/player.test.ts

@@ -2,7 +2,9 @@ import { describe, it, expect } from "vitest";
 import { mkdtempSync, writeFileSync, existsSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { buildFfmpegArgs, shouldUsePowerShellDownload, cleanupTempDir, shouldEndOnStall, volumeToFactor } from "./player.js";
+import { Readable } from "node:stream";
+import { buildFfmpegArgs, shouldUsePowerShellDownload, cleanupTempDir, shouldEndOnStall, volumeToFactor, AudioPlayer } from "./player.js";
+import type { Logger } from "../logger.js";
 
 function getHeadersArg(args: string[]): string {
   const idx = args.indexOf("-headers");
@@ -200,3 +202,192 @@ describe("shouldEndOnStall (#89 mid-track stall watchdog)", () => {
     expect(shouldEndOnStall(10, false, MAX_EMPTY, MAX_STALL)).toBe(false);
   });
 });
+
+// Minimal stub: AudioPlayer only calls debug/info/warn/error; child() returns self.
+const silentLogger = {
+  debug() {},
+  info() {},
+  warn() {},
+  error() {},
+  fatal() {},
+  trace() {},
+  child() {
+    return silentLogger;
+  },
+} as unknown as Logger;
+
+// A readable we fully control: no underlying source; we push PCM manually and
+// keep it open (never push(null)) to model the long-lived go-librespot sidecar.
+function openPcmReadable(): Readable {
+  return new Readable({ read() {} });
+}
+
+const wait = (ms: number): Promise<void> => new Promise<void>((r) => setTimeout(r, ms));
+const FRAME_BYTES = 3840; // PCM_FRAME_BYTES: 960 samples * 2ch * 2 bytes @48k s16le
+
+describe("AudioPlayer external-PCM mode (playPcmStream)", () => {
+  it("emits Opus 'frame' events from the external PCM stream without spawning ffmpeg", async () => {
+    const player = new AudioPlayer(silentLogger);
+    const frames: Buffer[] = [];
+    player.on("frame", (f) => frames.push(f));
+
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, {});
+    stream.push(Buffer.alloc(FRAME_BYTES * 10)); // ~10 frames of PCM
+
+    await wait(150); // ~7 frame ticks at 20ms
+
+    expect(player.getState()).toBe("playing");
+    expect(frames.length).toBeGreaterThan(0);
+    expect(Buffer.isBuffer(frames[0])).toBe(true);
+    player.stop();
+  });
+
+  it("does NOT emit 'trackEnd' on underrun while external (stream stays open)", async () => {
+    const player = new AudioPlayer(silentLogger);
+    let ended = 0;
+    const frames: Buffer[] = [];
+    player.on("trackEnd", () => ended++);
+    player.on("frame", (f) => frames.push(f));
+
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, {});
+    stream.push(Buffer.alloc(FRAME_BYTES * 2)); // only 2 frames, then underrun
+
+    await wait(200); // long after those 2 frames have drained
+
+    // In the url path, ffmpeg===null + empty buffer would fire trackEnd; here it must not.
+    expect(ended).toBe(0);
+    // Silence frames keep the 20ms timeline alive -> more than the 2 fed frames emitted.
+    expect(frames.length).toBeGreaterThan(2);
+    expect(player.getState()).toBe("playing");
+    player.stop();
+  });
+
+  // CORRECTION C2 (c): stop() DETACHES the shared readable — it must NOT be destroyed
+  // (destroying the sidecar's long-lived ffmpeg stdout would kill it for every future
+  // track). The sessionId bump + listener removal fence stale PCM out of pcmBuffer.
+  it("stop() detaches external mode without destroying the readable, and fences via sessionId", async () => {
+    const player = new AudioPlayer(silentLogger);
+    const frames: Buffer[] = [];
+    player.on("frame", (f) => frames.push(f));
+
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, {});
+    expect(stream.listenerCount("data")).toBe(1);
+    stream.push(Buffer.alloc(FRAME_BYTES * 5));
+    await wait(80);
+
+    player.stop();
+    expect(player.getState()).toBe("idle");
+    // C2: the shared sidecar stream must NOT be destroyed by teardown.
+    expect(stream.destroyed).toBe(false);
+    // Player's listeners are removed on detach (data/end/error).
+    expect(stream.listenerCount("data")).toBe(0);
+    expect(stream.listenerCount("end")).toBe(0);
+    expect(stream.listenerCount("error")).toBe(0);
+
+    const countAtStop = frames.length;
+    // sessionId fence + detached listeners: PCM pushed after stop must not
+    // resurrect the timeline or re-feed pcmBuffer.
+    stream.push(Buffer.alloc(FRAME_BYTES * 5));
+    await wait(80);
+    expect(frames.length).toBe(countAtStop);
+  });
+
+  // CORRECTION C2 (a): a gapless track change is driven by the sidecar pushing LATER
+  // PCM over the SAME already-attached stream. The player must NOT detach/re-attach
+  // (no second playPcmStream) — one persistent data listener serves every track.
+  it("(C2-a) feeds a later chunk over the SAME single attachment — gapless track change, no re-attach", async () => {
+    const player = new AudioPlayer(silentLogger);
+    const frames: Buffer[] = [];
+    player.on("frame", (f) => frames.push(f));
+
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, {});
+    expect(stream.listenerCount("data")).toBe(1); // attached exactly once
+
+    stream.push(Buffer.alloc(FRAME_BYTES * 4)); // "track 1" PCM
+    await wait(120);
+    const afterFirst = frames.length;
+    expect(afterFirst).toBeGreaterThan(0);
+
+    stream.push(Buffer.alloc(FRAME_BYTES * 4)); // sidecar seamlessly rolls into "track 2"
+    await wait(120);
+    expect(frames.length).toBeGreaterThan(afterFirst);
+
+    // Still exactly ONE listener — no detach/re-attach across the handoff.
+    expect(stream.listenerCount("data")).toBe(1);
+    expect(player.getState()).toBe("playing");
+    player.stop();
+  });
+
+  // CORRECTION C2 (b): a second playPcmStream detaches the first (NOT destroyed, and it
+  // stops feeding pcmBuffer) and attaches the second.
+  it("(C2-b) a second playPcmStream detaches the first (not destroyed, stops feeding) and attaches the second", async () => {
+    const player = new AudioPlayer(silentLogger);
+    const frames: Buffer[] = [];
+    player.on("frame", (f) => frames.push(f));
+
+    const first = openPcmReadable();
+    player.playPcmStream(first, {});
+    first.push(Buffer.alloc(FRAME_BYTES * 4));
+    await wait(120);
+    expect(frames.length).toBeGreaterThan(0);
+    expect(first.listenerCount("data")).toBe(1);
+
+    const second = openPcmReadable();
+    player.playPcmStream(second, {}); // fences + detaches `first`, attaches `second`
+
+    // C2: `first` is DETACHED, not destroyed.
+    expect(first.destroyed).toBe(false);
+    // `first` no longer feeds pcmBuffer — its data listener was removed.
+    expect(first.listenerCount("data")).toBe(0);
+    // `second` is now the attached source.
+    expect(second.listenerCount("data")).toBe(1);
+    expect(player.getState()).toBe("playing");
+    player.stop();
+  });
+
+  it("fires onExternalEnd when the readable ends (drives controller-based advance)", async () => {
+    const player = new AudioPlayer(silentLogger);
+    let endedCb = 0;
+
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, { onExternalEnd: () => endedCb++ });
+    stream.push(Buffer.alloc(FRAME_BYTES));
+    await wait(40);
+    stream.push(null); // end-of-stream
+    await wait(40);
+
+    expect(endedCb).toBe(1);
+    player.stop();
+  });
+
+  it("seek() is a local no-op in external mode (never respawns ffmpeg on a spotify sentinel)", async () => {
+    const player = new AudioPlayer(silentLogger);
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, {});
+    stream.push(Buffer.alloc(FRAME_BYTES * 3));
+    await wait(40);
+
+    expect(() => player.seek(30)).not.toThrow();
+    // Still external, still playing — no url-ffmpeg respawn, state unchanged.
+    expect(player.getState()).toBe("playing");
+    player.stop();
+  });
+
+  it("pause()/resume() still gate local emission in external mode (unchanged semantics)", async () => {
+    const player = new AudioPlayer(silentLogger);
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, {});
+    stream.push(Buffer.alloc(FRAME_BYTES * 3));
+    await wait(40);
+
+    player.pause();
+    expect(player.getState()).toBe("paused");
+    player.resume();
+    expect(player.getState()).toBe("playing");
+    player.stop();
+  });
+});

src/audio/player.ts

@@ -5,6 +5,7 @@ import { accessSync, chmodSync, constants, mkdtempSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { createOpusEncoder, PCM_FRAME_BYTES, type Encoder } from "./encoder.js";
+import type { Readable } from "node:stream";
 import type { Logger } from "../logger.js";
 
 const require = createRequire(import.meta.url);
@@ -187,6 +188,22 @@ export class AudioPlayer extends EventEmitter {
   private static readonly MAX_STALL_ATTEMPTS = 3000;
   private currentSongDuration = 0; // 当前歌曲总时长（秒）
 
+  // --- External PCM mode (Stage 2: go-librespot Spotify sidecar) ---
+  // When true, PCM arrives from a long-lived external Readable instead of a
+  // per-URL ffmpeg: this.ffmpeg stays null, and the underrun-driven trackEnd
+  // branches are suppressed (advance is driven by the controller, not EOF).
+  //
+  // CORRECTION C2: externalStream is the backend's LONG-LIVED, SHARED ffmpeg
+  // stdout (one stream reused across every track). Teardown must DETACH (remove
+  // the listeners we added + pause), never destroy it. We keep references to the
+  // exact handler functions so detach can removeListener precisely.
+  private externalMode = false;
+  private externalStream: Readable | null = null;
+  private onExternalEnd: (() => void) | null = null;
+  private externalDataHandler: ((chunk: Buffer) => void) | null = null;
+  private externalEndHandler: (() => void) | null = null;
+  private externalErrorHandler: ((err: Error) => void) | null = null;
+
   constructor(logger: Logger) {
     super();
     this.encoder = createOpusEncoder();
@@ -390,6 +407,108 @@ export class AudioPlayer extends EventEmitter {
     this.startFrameLoop();
   }
 
+  /**
+   * External-PCM mode (Stage 2 go-librespot Spotify sidecar).
+   *
+   * Feeds an already-normalized 48kHz/s16le/stereo PCM Readable (the
+   * go-librespot FIFO -> ffmpeg output) straight into the existing pcmBuffer +
+   * 20ms frame loop + Opus encoder + "frame" emission, WITHOUT spawning a
+   * per-URL ffmpeg. The url play() path is left completely untouched.
+   *
+   * Track advance is NOT driven by buffer underrun here (the sidecar stream is
+   * continuous and never EOFs per song); the caller drives advance via the
+   * SpotifyController "trackEnded" WebSocket event. onExternalEnd fires only if
+   * the underlying readable itself ends or errors.
+   *
+   * CORRECTION C2: the readable is the backend's long-lived, SHARED ffmpeg
+   * stdout reused across every track — a gapless track change is just LATER PCM
+   * on this SAME already-attached stream (no re-attach). Teardown DETACHES
+   * (removes our listeners + pauses); it never destroys the shared stream.
+   */
+  playPcmStream(readable: Readable, opts: { onExternalEnd?: () => void } = {}): void {
+    // 1. Fence current playback: stop() bumps sessionId, clears pcmBuffer, kills
+    //    any ffmpeg, and DETACHES (never destroys) any prior external stream.
+    this.stop();
+
+    const currentSessionId = this.sessionId;
+    this.externalMode = true;
+    this.externalStream = readable;
+    this.onExternalEnd = opts.onExternalEnd ?? null;
+    // Leave this.ffmpeg = null; clear currentUrl so seek() cannot respawn ffmpeg.
+    this.currentUrl = "";
+    this.seekOffset = 0;
+    this.framesPlayed = 0;
+    this.healthyFrames = 0;
+    this.ffmpegPaused = false;
+    this.spawnFailed = false;
+    this.emptyFrameAttempts = 0;
+    this.currentSongDuration = 0;
+
+    // Same ingestion + high-water backpressure as the ffmpeg.stdout handler,
+    // but pausing the Readable instead of ffmpeg.stdout. sessionId-guarded so
+    // stale sidecar PCM can't leak into a new track after stop()/skip. Handler
+    // refs are stored so detach can remove exactly these listeners (C2).
+    const onData = (chunk: Buffer): void => {
+      if (this.sessionId !== currentSessionId) return;
+      this.pcmBuffer = Buffer.concat([this.pcmBuffer, chunk]);
+      if (
+        this.pcmBuffer.length > AudioPlayer.BUFFER_HIGH_WATER &&
+        !this.ffmpegPaused &&
+        this.externalStream === readable
+      ) {
+        readable.pause();
+        this.ffmpegPaused = true;
+      }
+    };
+    const onEnd = (): void => {
+      if (this.sessionId !== currentSessionId) return;
+      this.onExternalEnd?.();
+    };
+    const onError = (err: Error): void => {
+      if (this.sessionId !== currentSessionId) return;
+      this.logger.warn({ err }, "External PCM stream error");
+      this.onExternalEnd?.();
+    };
+
+    this.externalDataHandler = onData;
+    this.externalEndHandler = onEnd;
+    this.externalErrorHandler = onError;
+
+    readable.on("data", onData);
+    readable.on("end", onEnd);
+    readable.on("error", onError);
+
+    this.state = "playing";
+    this.startFrameLoop();
+  }
+
+  /**
+   * CORRECTION C2: DETACH, never destroy. The external readable is the backend's
+   * long-lived, SHARED ffmpeg stdout reused across every track; destroying it
+   * would kill the sidecar pipe for all future tracks. Remove only the listeners
+   * WE added and pause the flow so stale PCM stops landing in pcmBuffer, then
+   * clear the external-mode state.
+   */
+  private detachExternalStream(): void {
+    const stream = this.externalStream;
+    if (stream) {
+      if (this.externalDataHandler) stream.off("data", this.externalDataHandler);
+      if (this.externalEndHandler) stream.off("end", this.externalEndHandler);
+      if (this.externalErrorHandler) stream.off("error", this.externalErrorHandler);
+      try {
+        stream.pause();
+      } catch {
+        /* best-effort: never destroy the shared sidecar stream */
+      }
+    }
+    this.externalDataHandler = null;
+    this.externalEndHandler = null;
+    this.externalErrorHandler = null;
+    this.externalStream = null;
+    this.externalMode = false;
+    this.onExternalEnd = null;
+  }
+
   stop(): void {
     // 3. 递增 ID 是最有效的逻辑“隔离墙”
     this.sessionId++; 
@@ -419,6 +538,11 @@ export class AudioPlayer extends EventEmitter {
       this.currentTempDir = null;
     }
 
+    // CORRECTION C2: tear down external mode by DETACHING (remove our listeners +
+    // pause) — never destroy the shared, long-lived sidecar stream. The
+    // sessionId++ above already fences the external data/end/error handlers.
+    this.detachExternalStream();
+
     this.ffmpegPaused = false;
     this.spawnFailed = false;
     this.state = "idle";
@@ -480,7 +604,10 @@ export class AudioPlayer extends EventEmitter {
         ? (this.currentSongDuration - elapsed) <= 5 // 距离结尾不足5秒
         : true; // 未知时长时保守处理
       
-      if (this.ffmpeg !== null && this.pcmBuffer.length < PCM_FRAME_BYTES) {
+      // External mode: the sidecar PCM stream is continuous and never EOFs per
+      // song; a transient underrun must NOT end the track (advance is driven by
+      // the controller). Skip BOTH drain/stall branches while externalMode.
+      if (!this.externalMode && this.ffmpeg !== null && this.pcmBuffer.length < PCM_FRAME_BYTES) {
         this.emptyFrameAttempts++;
         
         // End the track when FFmpeg has gone silent: quickly if we're near the
@@ -526,7 +653,7 @@ export class AudioPlayer extends EventEmitter {
         this.emptyFrameAttempts = 0;
       }
 
-      if (!this.ffmpeg && this.pcmBuffer.length < PCM_FRAME_BYTES) {
+      if (!this.externalMode && !this.ffmpeg && this.pcmBuffer.length < PCM_FRAME_BYTES) {
         this.frameLoopRunning = false;
         if (this.state !== "idle") {
           this.state = "idle";
@@ -542,13 +669,24 @@ export class AudioPlayer extends EventEmitter {
   }
 
   private sendNextFrame(): void {
-    if (this.pcmBuffer.length < PCM_FRAME_BYTES) return;
+    if (this.pcmBuffer.length < PCM_FRAME_BYTES) {
+      // External mode: the sidecar PCM stream is long-lived and must NOT end on
+      // a transient underrun. Emit an encoded silence frame so the 20ms voice
+      // timeline stays continuous instead of returning (which would desync TS).
+      if (this.externalMode) this.emitSilenceFrame();
+      return;
+    }
     const pcmFrame = this.pcmBuffer.subarray(0, PCM_FRAME_BYTES);
     this.pcmBuffer = this.pcmBuffer.subarray(PCM_FRAME_BYTES);
 
-    if (this.ffmpegPaused && this.pcmBuffer.length < AudioPlayer.BUFFER_LOW_WATER && this.ffmpeg?.stdout) {
-      this.ffmpeg.stdout.resume();
-      this.ffmpegPaused = false;
+    if (this.ffmpegPaused && this.pcmBuffer.length < AudioPlayer.BUFFER_LOW_WATER) {
+      if (this.externalMode && this.externalStream) {
+        this.externalStream.resume();
+        this.ffmpegPaused = false;
+      } else if (this.ffmpeg?.stdout) {
+        this.ffmpeg.stdout.resume();
+        this.ffmpegPaused = false;
+      }
     }
 
     try {
@@ -566,6 +704,16 @@ export class AudioPlayer extends EventEmitter {
     }
   }
 
+  private emitSilenceFrame(): void {
+    try {
+      const opusFrame = this.encoder.encode(Buffer.alloc(PCM_FRAME_BYTES));
+      this.emit("frame", opusFrame);
+      this.framesPlayed++;
+    } catch (err) {
+      this.emit("error", err as Error);
+    }
+  }
+
   private applyVolume(pcm: Buffer): Buffer {
     const factor = volumeToFactor(this.volume);
     // factor === 1 only at volume 100; skip the per-sample loop at full loudness.
@@ -578,8 +726,16 @@ export class AudioPlayer extends EventEmitter {
     return out;
   }
 
+  // NOTE: in external (Spotify sidecar) mode getElapsed() is frame-count based
+  // (framesPlayed includes silence frames emitted on underrun) and therefore
+  // only APPROXIMATE — the authoritative position is the controller's live
+  // status.track.position. This approximation is acceptable for Spotify.
   getElapsed(): number { return this.seekOffset + (this.framesPlayed * FRAME_DURATION_MS) / 1000; }
-  seek(seconds: number): void { 
+  seek(seconds: number): void {
+    // External (Spotify sidecar) mode: local seek is a no-op. Respawning ffmpeg
+    // on the spotify: sentinel would collide with the continuous PCM source;
+    // transport is delegated to the SpotifyController by the caller (Task 7).
+    if (this.externalMode) return;
     if (this.currentUrl && Number.isFinite(seconds) && seconds >= 0) {
       this.play(this.currentUrl, seconds, this.currentSongDuration);
     }