Fix deletion of empty MP4 tuple fields

docs/changelog.rst

@@ -6,6 +6,8 @@ Upcoming
 
 - Fixed parsing of compact ``YYYYMMDD`` date strings (no separators) being
   misread as a year value.
+- Fixed a bug where deleting MP4 track and disc metadata left ``trkn`` and
+  ``disk`` atoms containing ``(0, 0)``.
 
 v0.17.0
 -------

mediafile/storage/mp4.py

@@ -48,7 +48,7 @@ def set(self, mutagen_file, value):
         self.store(mutagen_file, items)
 
     def delete(self, mutagen_file):
-        if self.index == 0:
+        if self.index == 0 or self.deserialize(self.fetch(mutagen_file))[0] == 0:
             super().delete(mutagen_file)
         else:
             self.set(mutagen_file, None)

test/test_mediafile.py

@@ -823,6 +823,39 @@ class MP4Test(
         "channels": 2,
     }
 
+    def test_delete_packed_fields_removes_atoms(self):
+        mediafile = self._mediafile_fixture("full")
+
+        delattr(mediafile, "track")
+        delattr(mediafile, "tracktotal")
+        delattr(mediafile, "disc")
+        delattr(mediafile, "disctotal")
+
+        mediafile.save()
+        mediafile = MediaFile(mediafile.filename)
+
+        self.assertNotIn("trkn", mediafile.mgfile.tags)
+        self.assertNotIn("disk", mediafile.mgfile.tags)
+
+    def test_delete_packed_total_without_number_removes_atoms(self):
+        mediafile = self._mediafile_fixture("empty")
+
+        mediafile.tracktotal = 3
+        mediafile.disctotal = 5
+        mediafile.save()
+
+        mediafile = MediaFile(mediafile.filename)
+        self.assertIn("trkn", mediafile.mgfile.tags)
+        self.assertIn("disk", mediafile.mgfile.tags)
+
+        delattr(mediafile, "tracktotal")
+        delattr(mediafile, "disctotal")
+        mediafile.save()
+
+        mediafile = MediaFile(mediafile.filename)
+        self.assertNotIn("trkn", mediafile.mgfile.tags)
+        self.assertNotIn("disk", mediafile.mgfile.tags)
+
     def test_add_tiff_image_fails(self):
         mediafile = self._mediafile_fixture("empty")
         with self.assertRaises(ValueError):