fix: don't remove obsolete files when synth raises exception (#715)

synthtool/metadata.py

@@ -237,19 +237,22 @@ def __enter__(self):
         self.observer.start()
 
     def __exit__(self, type, value, traceback):
-        if should_track_obsolete_files():
-            time.sleep(2)  # Finish collecting observations about modified files.
-            self.observer.stop()
-            self.observer.join()
-            for path in git_ignore(self.handler.get_touched_file_paths()):
-                _metadata.generated_files.append(path)
-            _remove_obsolete_files(self.old_metadata)
+        if value:
+            pass  # An exception was raised.  Don't write metadata or clean up.
         else:
-            self.observer.stop()
-        _clear_local_paths(get())
-        _metadata.sources.sort(key=_source_key)
-        if _enable_write_metadata:
-            write(self.metadata_file_path)
+            if should_track_obsolete_files():
+                time.sleep(2)  # Finish collecting observations about modified files.
+                self.observer.stop()
+                self.observer.join()
+                for path in git_ignore(self.handler.get_touched_file_paths()):
+                    _metadata.generated_files.append(path)
+                _remove_obsolete_files(self.old_metadata)
+            else:
+                self.observer.stop()
+            _clear_local_paths(get())
+            _metadata.sources.sort(key=_source_key)
+            if _enable_write_metadata:
+                write(self.metadata_file_path)
 
 
 def _get_git_source_map(metadata) -> Dict[str, object]:

tests/test_metadata.py

@@ -216,6 +216,28 @@ def test_nothing_happens_when_disabled(source_tree, preserve_track_obsolete_file
     assert os.path.exists("code/c")
 
 
+def test_nothing_happens_when_exception(source_tree, preserve_track_obsolete_file_flag):
+    metadata.set_track_obsolete_files(True)
+
+    with metadata.MetadataTrackerAndWriter(source_tree.tmpdir / "synth.metadata"):
+        source_tree.write("code/b")
+        source_tree.write("code/c")
+
+    metadata.reset()
+    try:
+        with metadata.MetadataTrackerAndWriter(source_tree.tmpdir / "synth.metadata"):
+            source_tree.write("code/c")
+            raise "Exception!"
+    except:  # noqa: E722
+        pass
+
+    assert 0 == len(metadata.get().new_files)
+
+    # Confirm no files were deleted.
+    assert os.path.exists("code/b")
+    assert os.path.exists("code/c")
+
+
 def test_old_file_ignored_by_git_not_removed(
     source_tree, preserve_track_obsolete_file_flag
 ):