Move the complicated MagicFolder constructor

All that complexity can be part of MagicFolder itself.

src/allmydata/client.py

@@ -593,23 +593,7 @@ class _Client(node.Node, pollmixin.PollMixin):
 
             for (name, mf_config) in magic_folders.items():
                 self.log("Starting magic_folder '{}'".format(name))
-                db_filename = os.path.join(self.basedir, "private", "magicfolder_{}.sqlite".format(name))
-                local_dir_config = mf_config['directory']
-                try:
-                    poll_interval = int(mf_config["poll_interval"])
-                except ValueError:
-                    raise ValueError("'poll_interval' option must be an int")
-
-                s = magic_folder.MagicFolder(
-                    client=self,
-                    upload_dircap=mf_config["upload_dircap"],
-                    collective_dircap=mf_config["collective_dircap"],
-                    local_path_u=abspath_expanduser_unicode(local_dir_config, base=self.basedir),
-                    dbfile=abspath_expanduser_unicode(db_filename),
-                    umask=mf_config["umask"],
-                    name=name,
-                    downloader_delay=poll_interval,
-                )
+                s = magic_folder.MagicFolder.from_config(self.basedir, mf_config)
                 self._magic_folders[name] = s
                 s.setServiceParent(self)
                 s.startService()

src/allmydata/frontends/magic_folder.py

@@ -244,6 +244,43 @@ def save_magic_folders(node_directory, folders):
 
 class MagicFolder(service.MultiService):
 
+    @classmethod
+    def from_config(cls, client_node, config):
+        """
+        Create a ``MagicFolder`` from a client node and magic-folder
+        configuration.
+
+        :param _Client client_node: The client node the magic-folder is
+            attached to.
+
+        :param dict config: Magic-folder configuration like that in the list
+            returned by ``load_magic_folders``.
+        """
+        db_filename = os.path.join(
+            client_node.basedir,
+            "private",
+            "magicfolder_{}.sqlite".format(name),
+        )
+        local_dir_config = config['directory']
+        try:
+            poll_interval = int(config["poll_interval"])
+        except ValueError:
+            raise ValueError("'poll_interval' option must be an int")
+
+        return cls(
+            client=client_node,
+            upload_dircap=config["upload_dircap"],
+            collective_dircap=config["collective_dircap"],
+            local_path_u=abspath_expanduser_unicode(
+                local_dir_config,
+                base=client_node.basedir,
+            ),
+            dbfile=abspath_expanduser_unicode(db_filename),
+            umask=config["umask"],
+            name=name,
+            downloader_delay=poll_interval,
+        )
+
     def __init__(self, client, upload_dircap, collective_dircap, local_path_u, dbfile, umask,
                  name, uploader_delay=1.0, clock=None, downloader_delay=60):
         precondition_abspath(local_path_u)