Don't crash when a tracked file is missing (#88)

entangled/io/filedb.py

@@ -50,8 +50,11 @@ def managed_files(self) -> set[Path]:
         return {Path(p) for p in self.targets}
 
     def changed_files(self, fs: AbstractFileCache) -> Generator[Path]:
+        # A tracked file that no longer exists (e.g. a source that was moved or
+        # deleted) counts as changed. Without this guard `fs[Path(p)]` would
+        # raise `FileNotFoundError` and crash, see issue #88.
         return (Path(p) for p, known_stat in self.files.items()
-                if fs[Path(p)].stat != known_stat)
+                if Path(p) not in fs or fs[Path(p)].stat != known_stat)
 
     def create_target(self, fs: AbstractFileCache, path: Path):
         if path.is_absolute():

test/io/test_filedb.py

@@ -50,3 +50,24 @@ def test_filedb(example_files: Path):
             assert list(db.changed_files(fs)) == [Path("d")]
             db.update(fs, Path("d"))
             assert list(db.changed_files(fs)) == []
+
+
+def test_changed_files_missing(tmp_path: Path):
+    """Regression test for issue #88: a tracked file that no longer exists on
+    disk (e.g. a source markdown file that was moved or renamed) should be
+    reported as changed rather than crashing with `FileNotFoundError`."""
+    with chdir(tmp_path):
+        with open("source", "w") as f:
+            _ = f.write("hello")
+
+        fs = FileCache()
+        with filedb(fs=fs) as db:
+            db.update(fs, Path("source"))
+            assert list(db.changed_files(fs)) == []
+
+        # the file is moved/removed while still tracked in the db
+        Path("source").unlink()
+        fs.reset()
+
+        with filedb(fs=fs) as db:
+            assert list(db.changed_files(fs)) == [Path("source")]