fix: blank PUBLIC_HOST_AUDIO_URL no longer breaks audio download links

A present-but-empty PUBLIC_HOST_AUDIO_URL (e.g. `PUBLIC_HOST_AUDIO_URL=` in a
compose file) bypassed the default and stayed blank, so the UI built
root-relative audio links like `app.url/song.mp3` that 404'd, while video links
kept working off PUBLIC_HOST_URL.

Backend: a blank PUBLIC_HOST_AUDIO_URL now inherits PUBLIC_HOST_URL, matching the
README ("same as PUBLIC_HOST_URL but for audio downloads"). Both blank still
stays blank, so intentional root serving is preserved.

Frontend: buildDownloadLink and buildChapterDownloadLink fall back to
PUBLIC_HOST_URL when the audio host is blank.

Added backend and UI regression tests, both verified red against the unfixed
code.

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

app/main.py

@@ -117,6 +117,10 @@ class Config:
             if val and not val.endswith('/'):
                 setattr(self, attr, val + '/')
 
+        # A blank PUBLIC_HOST_AUDIO_URL inherits PUBLIC_HOST_URL so audio links aren't left root-relative; both blank stays blank.
+        if not self.PUBLIC_HOST_AUDIO_URL:
+            self.PUBLIC_HOST_AUDIO_URL = self.PUBLIC_HOST_URL
+
         # Convert relative addresses to absolute addresses to prevent the failure of file address comparison
         if self.YTDL_OPTIONS_FILE and self.YTDL_OPTIONS_FILE.startswith('.'):
             self.YTDL_OPTIONS_FILE = str(Path(self.YTDL_OPTIONS_FILE).resolve())

app/tests/test_config.py

@@ -51,6 +51,43 @@ class ConfigTests(unittest.TestCase):
         self.assertEqual(c.PUBLIC_HOST_URL, "")
         self.assertEqual(c.PUBLIC_HOST_AUDIO_URL, "")
 
+    def test_blank_audio_host_inherits_public_host_url(self):
+        # Regression: a blank PUBLIC_HOST_AUDIO_URL must inherit PUBLIC_HOST_URL, not stay root-relative.
+        with patch.dict(
+            os.environ,
+            _base_env(PUBLIC_HOST_URL="download/", PUBLIC_HOST_AUDIO_URL=""),
+            clear=False,
+        ):
+            c = Config()
+        self.assertEqual(c.PUBLIC_HOST_URL, "download/")
+        self.assertEqual(c.PUBLIC_HOST_AUDIO_URL, "download/")
+
+    def test_blank_audio_host_audio_link_is_not_root_relative(self):
+        # Repro of the reported bug. buildDownloadLink uses configuration[PUBLIC_HOST_AUDIO_URL]
+        # as the base for audio, so a blank value made the link "song.mp3" (root-relative, 404)
+        # while video kept working off PUBLIC_HOST_URL. The base must be prefixed instead.
+        with patch.dict(
+            os.environ,
+            _base_env(PUBLIC_HOST_URL="download/", PUBLIC_HOST_AUDIO_URL=""),
+            clear=False,
+        ):
+            c = Config()
+        audio_link = c.PUBLIC_HOST_AUDIO_URL + "song.mp3"
+        video_link = c.PUBLIC_HOST_URL + "video.mp4"
+        self.assertEqual(video_link, "download/video.mp4")
+        self.assertNotEqual(audio_link, "song.mp3")
+        self.assertEqual(audio_link, "download/song.mp3")
+
+    def test_unset_audio_host_keeps_default_route(self):
+        # Truly unset (not blank) keeps the 'audio_download/' default route.
+        env = _base_env()
+        env.pop("PUBLIC_HOST_AUDIO_URL", None)
+        env.pop("PUBLIC_HOST_URL", None)
+        with patch.dict(os.environ, env, clear=True):
+            c = Config()
+        self.assertEqual(c.PUBLIC_HOST_URL, "download/")
+        self.assertEqual(c.PUBLIC_HOST_AUDIO_URL, "audio_download/")
+
     def test_public_host_url_already_slashed_unchanged(self):
         with patch.dict(
             os.environ,

ui/src/app/app.spec.ts

@@ -262,6 +262,49 @@ describe('App', () => {
     expect(payload.clipEnd).toBe('1:20');
   });
 
+  function makeDownload(overrides: Record<string, unknown> = {}) {
+    return {
+      download_type: 'audio',
+      filename: 'song.mp3',
+      folder: '',
+      title: 'Song',
+      ...overrides,
+    } as unknown as Parameters<App['buildDownloadLink']>[0];
+  }
+
+  it('builds audio link from PUBLIC_HOST_AUDIO_URL when set', () => {
+    downloads.configuration['PUBLIC_HOST_URL'] = 'download/';
+    downloads.configuration['PUBLIC_HOST_AUDIO_URL'] = 'audio_download/';
+    const app = TestBed.createComponent(App).componentInstance;
+    expect(app.buildDownloadLink(makeDownload())).toBe('audio_download/song.mp3');
+  });
+
+  it('builds video link from PUBLIC_HOST_URL', () => {
+    downloads.configuration['PUBLIC_HOST_URL'] = 'download/';
+    downloads.configuration['PUBLIC_HOST_AUDIO_URL'] = 'audio_download/';
+    const app = TestBed.createComponent(App).componentInstance;
+    const link = app.buildDownloadLink(makeDownload({ download_type: 'video', filename: 'video.mp4' }));
+    expect(link).toBe('download/video.mp4');
+  });
+
+  it('audio link falls back to PUBLIC_HOST_URL when audio host is blank (regression)', () => {
+    // The reported bug: a blank PUBLIC_HOST_AUDIO_URL produced a root-relative
+    // "song.mp3" that 404'd while video kept working. It must not be root-relative.
+    downloads.configuration['PUBLIC_HOST_URL'] = 'download/';
+    downloads.configuration['PUBLIC_HOST_AUDIO_URL'] = '';
+    const app = TestBed.createComponent(App).componentInstance;
+    const link = app.buildDownloadLink(makeDownload());
+    expect(link).not.toBe('song.mp3');
+    expect(link).toBe('download/song.mp3');
+  });
+
+  it('audio link stays root-relative when both hosts are blank', () => {
+    downloads.configuration['PUBLIC_HOST_URL'] = '';
+    downloads.configuration['PUBLIC_HOST_AUDIO_URL'] = '';
+    const app = TestBed.createComponent(App).componentInstance;
+    expect(app.buildDownloadLink(makeDownload())).toBe('song.mp3');
+  });
+
   it('blocks subscribe with invalid title regex', () => {
     const alertSpy = vi.spyOn(window, 'alert').mockImplementation(() => undefined);
     const fixture = TestBed.createComponent(App);

ui/src/app/app.ts

@@ -1203,7 +1203,9 @@ export class App implements AfterViewInit, OnInit, OnDestroy {
   buildDownloadLink(download: Download) {
     let baseDir = this.downloads.configuration["PUBLIC_HOST_URL"];
     if (download.download_type === 'audio' || download.filename.endsWith('.mp3')) {
-      baseDir = this.downloads.configuration["PUBLIC_HOST_AUDIO_URL"];
+      // Fall back to PUBLIC_HOST_URL when the audio host is blank, else the link goes root-relative and 404s.
+      baseDir = this.downloads.configuration["PUBLIC_HOST_AUDIO_URL"]
+        || this.downloads.configuration["PUBLIC_HOST_URL"];
     }
 
     if (download.folder) {
@@ -1299,7 +1301,9 @@ export class App implements AfterViewInit, OnInit, OnDestroy {
   buildChapterDownloadLink(download: Download, chapterFilename: string) {
     let baseDir = this.downloads.configuration["PUBLIC_HOST_URL"];
     if (download.download_type === 'audio' || chapterFilename.endsWith('.mp3')) {
-      baseDir = this.downloads.configuration["PUBLIC_HOST_AUDIO_URL"];
+      // Fall back to PUBLIC_HOST_URL when the audio host is blank (see buildDownloadLink).
+      baseDir = this.downloads.configuration["PUBLIC_HOST_AUDIO_URL"]
+        || this.downloads.configuration["PUBLIC_HOST_URL"];
     }
 
     if (download.folder) {