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rdiff-backup: Actions Plug-ins

1. Naming convention and actions manager

An action plug-in is:

  1. either in the rdiffbackup.actions namespace, with any module name (this is how built-in action modules are packaged),

  2. or a top-level module with a name rdb_action_[...].py

All those plug-ins are found by the action manager module rdiffbackup.actions_mgr and returned as a hash made of key-value pairs by the function get_actions_dict:

  • key is the name returned by the Action class

  • value is the Action class itself, as returned by the get_plugin_class function of the action plug-in module.

Note
 more information is provided with pydoc rdiffbackup.actions_mgr.

The actions manager also has two functions to get lists of argparse parsers to use with arguments.parse, get_generic_parsers for generic parsers common to all actions, and get_parent_parsers_compat200 to get all parsers of all "traditional" actions, for faking the old CLI.

2. Module interface

Each action plug-in module contains one single Action-class. This action class is returned by the function get_plugin_class of the module. How the action class is named and of which hierarchy it descends is irrelevant (though it makes sense to derive it from the rdiffbackup.actions.BaseAction class).

All further action plug-in interactions are executed through the interface of this single class.

3. Action class interface

The Action class has the following interface, in addition to the generic plugin class requirements:

  • the class method cls.get_security_class() returns the security class of the action plug-in, one of backup, restore, server or validate. A security class of None is only applicable to actions without client/server context (e.g. info).

  • the class method cls.add_action_subparser(sub_handler) returns a subparser as returned by argparse’s sub_handler.add_parser function, so that the sub-options of the action can be parsed by the rdiffbackup.arguments.parse function.

  • the method self.init(args, log, errlog) initializes the class as object, based on the namespace returned by argparse, a Log and an ErrorLog object.

  • the method self.pre_check() just validates that the arguments passed were correct, beyond what argparse could do, and shouldn’t try to connect yet to any remote location. A return value unequal 0 means an error, which can be used as exit code.

  • the method self.connect() returns a context manager object which can be used in a with action.connect() as conn_act: construct. It is expected that the action object itself is the context manager, but it isn’t absolutely necessary (but the default implementation offered by the BaseAction). The returned class must hence have two methods self.enter() and self.exit(self, exc_type, exc_value, traceback) (closing the connections). Again, it is expected that the runtime context object returned by enter is the same action object (aka return self), but it isn’t an obligation.

    This context object has the following interface, usable through the connection(s) started by connect():

    • self.check() to validate the environment before doing any changes. The check method continues to verify the environment and report as many issues as it can, as to help the user to fix them at once. It returns 0 in case of success, else an integer to be used as exit code.

    • self.setup() prepares the environment (define variables, etc), it might create empty directories and logfiles, but it can’t lead under any circumstances to a broken repository requiring a regression. The setup method returns as soon as it finds an issue and doesn’t try to continue. It returns 0 in case of success, else an integer to be used as exit code.

    • self.run() finally does whatever the action is supposed to do. It might return 0 in case of success, else an integer to be used as exit code. It is the only function that might exit directly instead, even though it isn’t the recommended approach (but a matter of fact for most existing actions).

Note
 the context object doesn’t need a "cleanup" action as clean-up is to be done as part of the exit method provided by the context manager.

Of course, action classes inheriting from the BaseAction class don’t need to define all aspects themselves, making the life of plug-in developers easier.

Note
 more information is provided with pydoc rdiffbackup.actions.

4. Pseudo-code

Taking all together, the code to use an action plugin would look as follows, without any error handling:

discovered_actions = actions_mgr.get_actions_dict()
parsed_args = arguments.parse(
	arglist, "rdiff-backup {ver}".format(ver=Globals.version),
	actions_mgr.get_generic_parsers(),
	actions_mgr.get_parent_parsers_compat200(),
	discovered_actions)
action = discovered_actions[parsed_args.action](parsed_args, Log, ErrorLog)
action.pre_check()
# implicit context_manager.__enter__()
with action.connect() as conn_act:
	conn_act.check()
	conn_act.setup()
	conn_act.run()
# implicit context_manager.__exit__(exc_type, exc_val, exc_tb)
Tip
 the actual code can be found in rdiff_backup.Main._main_run().

5. Recommendations

  • save the repository as self.repo and the directory as self.dir. The _operate_regress method expects it, and it might help in future implementations.