Sync writes __source__ falsely into frozen layers

CHANGELOG

@@ -1,3 +1,7 @@
+23.1.3+dev.2
+  * Fixed a bug where zodbsync was writing `source`-files into frozen
+    layers
+
 23.1.2+dev.1
   * Fix layer-update for cases where the last element in both checksum files is
     not the same.

perfact/zodbsync/tests/test_sync.py

@@ -1654,7 +1654,7 @@ def test_playback_hook_failed(self):
             assert 'NewFolder2' not in self.app.objectIds()
 
     @contextmanager
-    def addlayer(self, seqnum='00'):
+    def addlayer(self, seqnum='00', frozen=True):
         """
         Create a temp directory and add a config that uses this as additional
         code layer.
@@ -1666,6 +1666,7 @@ def addlayer(self, seqnum='00'):
         with tempfile.TemporaryDirectory() as layer:
             with open(path, 'w') as f:
                 f.write('base_dir = "{}"\n'.format(layer))
+                f.write('frozen = {}\n'.format(frozen))
             os.mkdir(os.path.join(layer, '__root__'))
             # Force re-reading config
             if hasattr(self, 'runner'):
@@ -2152,3 +2153,36 @@ def test_layer_update_warn(self, caplog):
             assert expect + '/Test' in caplog.text
             assert 'AttributeError' not in caplog.text
             assert expect + '/ToDelete/Sub' in caplog.text
+
+    def test_layer_frozen(self):
+        """
+        Verify that changed files are properly written into the custom
+        layer in case the layer below is frozen.
+        """
+        with self.runner.sync.tm:
+            self.app.manage_addProduct['OFSP'].manage_addFile(id='blob')
+
+        with self.addlayer(frozen=True) as layer:
+            self.run('record', '/blob')
+            shutil.move(
+                '{}/__root__/blob'.format(self.repo.path),
+                '{}/__root__/blob'.format(layer),
+            )
+            with self.runner.sync.tm:
+                self.app.blob.manage_edit(
+                    filedata='text_content',
+                    content_type='text/plain',
+                    title='BLOB'
+                )
+            self.run('record', '/')
+            root = os.path.join(self.repo.path, '__root__')
+            # both meta and source file are in custom layer
+            assert os.path.exists(os.path.join(root, 'blob/__meta__'))
+            assert os.path.exists(os.path.join(root, 'blob/__source__.txt'))
+            source_fmt = '{}/__root__/blob/__source__.txt'
+            with open(source_fmt.format(layer)) as f:
+                # source in layer should still be empty
+                assert f.read() == ''
+            with open(source_fmt.format(self.repo.path)) as f:
+                # ... content is in custom layer!
+                assert f.read() == 'text_content'

perfact/zodbsync/zodbsync.py

@@ -470,7 +470,6 @@ def fs_write(self, path, data):
                 obj_id=path.rstrip('/').rsplit('/', 1)[-1],
             )
             src_fname = '{}.{}'.format(base, ext)
-            src_path = os.path.join(base_dir, src_fname)
             new_data['src_fnames'] = [src_fname]
             new_data['source'] = source
 
@@ -494,7 +493,7 @@ def fs_write(self, path, data):
                 self.logger.debug(
                     "Will write %d bytes of source" % len(source)
                 )
-                with open(src_path, 'wb') as f:
+                with open(os.path.join(write_base, src_fname), 'wb') as f:
                     f.write(source)
 
             # We wrote the object to the topmost layer, so the index where the