fix(fetchart): prevent deletion of configured fallback cover art (#6283)

When `import.delete` or `import.move` is enabled, the `assign_art`
method calls `task.prune(candidate.path)` unconditionally. This
incorrectly deletes the configured `fetchart.fallback` file. Add
explicit check to skip pruning when the candidate path matches the
configured fallback.

beetsplug/fetchart.py

@@ -1446,6 +1446,16 @@ class FetchArtPlugin(plugins.BeetsPlugin, RequestMixin):
             "move"
         ].get(bool)
 
+    def _is_candidate_fallback(self, candidate: Candidate) -> bool:
+        try:
+            return (
+                candidate.path is not None
+                and self.fallback is not None
+                and os.path.samefile(candidate.path, self.fallback)
+            )
+        except OSError:
+            return False
+
     # Asynchronous; after music is added to the library.
     def fetch_art(self, session: ImportSession, task: ImportTask) -> None:
         """Find art for the album being imported."""
@@ -1494,7 +1504,7 @@ class FetchArtPlugin(plugins.BeetsPlugin, RequestMixin):
 
             self._set_art(task.album, candidate, not removal_enabled)
 
-            if removal_enabled:
+            if removal_enabled and not self._is_candidate_fallback(candidate):
                 task.prune(candidate.path)
 
     # Manual album art fetching.

docs/changelog.rst

@@ -43,6 +43,7 @@ Bug fixes
   ampersand.
 - :doc:`plugins/zero`: When the ``omit_single_disc`` option is set,
   ``disctotal`` is zeroed alongside ``disc``.
+- :doc:`plugins/fetchart`: Prevent deletion of configured fallback cover art
 
 For plugin developers
 ~~~~~~~~~~~~~~~~~~~~~

test/plugins/test_art.py

@@ -310,6 +310,16 @@ class FSArtTest(UseThePlugin):
         ]
         assert candidates == paths
 
+    @patch("os.path.samefile")
+    def test_is_candidate_fallback_os_error(self, mock_samefile):
+        mock_samefile.side_effect = OSError("os error")
+        fallback = os.path.join(self.temp_dir, b"a.jpg")
+        self.plugin.fallback = fallback
+        candidate = fetchart.Candidate(logger, self.source.ID, fallback)
+        result = self.plugin._is_candidate_fallback(candidate)
+        mock_samefile.assert_called_once()
+        assert not result
+
 
 class CombinedTest(FetchImageTestCase, CAAHelper):
     ASIN = "xxxx"