fix(bilibili): keep long videos playing when the CDN drops the stream (#161)

Long B站 videos (2-3 h) still stopped ~15-20 min in, the same symptom as
#89. Reconnecting to the same URL is not enough once the CDN session is
gone, so:

- Prefer an upos/cos mirror from baseUrl + backupUrl over PCDN hosts
  (*.mcdn.bilivideo.cn, *.szbdyd.com), which are the ones that cut off.
- When a B站 track ends more than 30 s before its known duration, fetch
  a fresh URL and resume at the current position instead of advancing.
  Up to 3 attempts without real progress, then advance as before; a
  track the user started meanwhile is never clobbered.
- Seek B站 URLs input-side (-ss before -i). Their CDN serves Range, so a
  resume jumps to the byte offset instead of re-downloading everything
  before it (measured locally: 0.2 s vs a full-file download at 1.5 h
  into a 2 h fMP4), which would otherwise trip the 60 s stall watchdog.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/music/bilibili.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect, vi } from "vitest";
-import { BiliBiliProvider } from "./bilibili.js";
+import { BiliBiliProvider, pickStableAudioUrl } from "./bilibili.js";
 
 describe("BiliBiliProvider.search pagination", () => {
   function mockProvider() {
@@ -162,3 +162,44 @@ describe("BiliBiliProvider multi-P support", () => {
     expect(playurlCall![1].params.bvid).toBe("BV1multiP"); // 纯净 bvid
   });
 });
+
+describe("pickStableAudioUrl (#161 long streams dying mid-play)", () => {
+  const pcdn = "https://xy1x2x3x4xy.mcdn.bilivideo.cn:4483/upgcxcode/1/2/3/3-1-30280.m4s?e=x&deadline=1";
+  const szbdyd = "https://cn-hk-eq-01-01.szbdyd.com/upgcxcode/1/2/3/3-1-30280.m4s?deadline=1";
+  const upos = "https://upos-sz-mirrorcos.bilivideo.com/upgcxcode/1/2/3/3-1-30280.m4s?deadline=1";
+  const upos2 = "https://upos-sz-mirror08c.bilivideo.com/upgcxcode/1/2/3/3-1-30280.m4s?deadline=1";
+
+  it("prefers an upos/cos mirror over a PCDN baseUrl", () => {
+    expect(pickStableAudioUrl({ baseUrl: pcdn, backupUrl: [szbdyd, upos] })).toBe(upos);
+  });
+
+  it("keeps the baseUrl when it is already a stable host", () => {
+    expect(pickStableAudioUrl({ baseUrl: upos, backupUrl: [upos2] })).toBe(upos);
+  });
+
+  it("accepts the snake_case field names", () => {
+    expect(pickStableAudioUrl({ base_url: pcdn, backup_url: [upos2] })).toBe(upos2);
+  });
+
+  it("falls back to the baseUrl when every candidate is PCDN", () => {
+    expect(pickStableAudioUrl({ baseUrl: pcdn, backupUrl: [szbdyd] })).toBe(pcdn);
+  });
+
+  it("returns undefined when there is no url at all", () => {
+    expect(pickStableAudioUrl({})).toBeUndefined();
+  });
+
+  it("getSongUrl returns the stable mirror of the best stream", async () => {
+    const p = new BiliBiliProvider();
+    (p as any).cidCache.set("BV1abc", 42);
+    (p as any).api = {
+      get: vi.fn().mockResolvedValue({
+        data: { data: { dash: { audio: [
+          { bandwidth: 64000, baseUrl: "https://upos-sz-mirrorcos.bilivideo.com/low.m4s" },
+          { bandwidth: 320000, baseUrl: pcdn, backupUrl: [upos] },
+        ] } } },
+      }),
+    };
+    expect((await p.getSongUrl("BV1abc"))?.url).toBe(upos);
+  });
+});

src/music/bilibili.ts

@@ -42,6 +42,37 @@ export interface BiliVideoPartsResult {
   parts: BiliVideoPart[];
 }
 
+/**
+ * PCDN / P2P edge hosts (xy*.mcdn.bilivideo.cn:<port>, *.szbdyd.com). Their
+ * sessions get cut mid-file, which kills long streams partway (#89, #161),
+ * and a reconnect to the same host rarely recovers.
+ */
+const BILI_PCDN_HOST = /\.mcdn\.bilivideo\.cn$|\.szbdyd\.com$/i;
+
+/**
+ * Pick the audio URL least likely to die mid-stream: the first upos/cos
+ * mirror among baseUrl + backupUrl, else the baseUrl as before.
+ */
+export function pickStableAudioUrl(stream: {
+  baseUrl?: string;
+  base_url?: string;
+  backupUrl?: string[];
+  backup_url?: string[];
+}): string | undefined {
+  const primary = stream.baseUrl ?? stream.base_url;
+  const candidates = [primary, ...(stream.backupUrl ?? stream.backup_url ?? [])].filter(
+    (u): u is string => typeof u === "string" && u.length > 0,
+  );
+  const stable = candidates.find((u) => {
+    try {
+      return !BILI_PCDN_HOST.test(new URL(u).hostname);
+    } catch {
+      return false;
+    }
+  });
+  return stable ?? primary;
+}
+
 /**
  * 解析带有分P信息的 B站 ID 或 URL。
  * 支持形如 "BVxxxx", "BVxxxx?p=2", "BVxxxx:p2" 以及完整 URL 等格式，默认 page 为 1。
@@ -353,7 +384,7 @@ export class BiliBiliProvider implements MusicProvider {
         (b.bandwidth ?? 0) > (a.bandwidth ?? 0) ? b : a
       );
 
-      const biliUrl = best.baseUrl ?? best.base_url;
+      const biliUrl = pickStableAudioUrl(best);
       return biliUrl ? { url: biliUrl } : null;
     } catch {
       return null;