Add a new plugin architecture.

This allows third parties to add initialization hooks, REST endpoints, 
and additional components.  Given by Jan Jancar.

50 files changed, 1236 insertions, 359 deletions

diff --git a/README.rst b/README.rst
index e58d18297..eae6b73cc 100644
--- a/README.rst
+++ b/README.rst
@@ -60,6 +60,7 @@ Table of Contents
     src/mailman/docs/mta
     src/mailman/docs/postorius
     src/mailman/docs/hyperkitty
+    src/mailman/plugins/docs/intro
     src/mailman/docs/contribute
     src/mailman/docs/STYLEGUIDE
     src/mailman/docs/internationalization
diff --git a/src/mailman/app/commands.py b/src/mailman/app/commands.py
index 7cf3bc4c5..32e5438ea 100644
--- a/src/mailman/app/commands.py
+++ b/src/mailman/app/commands.py
@@ -26,4 +26,4 @@ from public import public
 @public
 def initialize():
     """Initialize the email commands."""
-    add_components('mailman.commands', IEmailCommand, config.commands)
+    add_components('commands', IEmailCommand, config.commands)
diff --git a/src/mailman/app/docs/hooks.rst b/src/mailman/app/docs/hooks.rst
deleted file mode 100644
index 07d0ec33c..000000000
--- a/src/mailman/app/docs/hooks.rst
+++ /dev/null
@@ -1,121 +0,0 @@
-=====
-Hooks
-=====
-
-Mailman defines two initialization hooks, one which is run early in the
-initialization process and the other run late in the initialization process.
-Hooks name an importable callable so it must be accessible on ``sys.path``.
-::
-
-    >>> import os, sys
-    >>> from mailman.config import config
-    >>> config_directory = os.path.dirname(config.filename)
-    >>> sys.path.insert(0, config_directory)
-
-    >>> hook_path = os.path.join(config_directory, 'hooks.py')
-    >>> with open(hook_path, 'w') as fp:
-    ...     print("""\
-    ... counter = 1
-    ... def pre_hook():
-    ...     global counter
-    ...     print('pre-hook:', counter)
-    ...     counter += 1
-    ...
-    ... def post_hook():
-    ...     global counter
-    ...     print('post-hook:', counter)
-    ...     counter += 1
-    ... """, file=fp)
-    >>> fp.close()
-
-
-Pre-hook
-========
-
-We can set the pre-hook in the configuration file.
-
-    >>> config_path = os.path.join(config_directory, 'hooks.cfg')
-    >>> with open(config_path, 'w') as fp:
-    ...     print("""\
-    ... [meta]
-    ... extends: test.cfg
-    ...
-    ... [mailman]
-    ... pre_hook: hooks.pre_hook
-    ... """, file=fp)
-
-The hooks are run in the second and third steps of initialization.  However,
-we can't run those initialization steps in process, so call a command line
-script that will produce no output to force the hooks to run.
-::
-
-    >>> import subprocess
-    >>> from mailman.testing.layers import ConfigLayer
-    >>> def call():
-    ...     exe = os.path.join(os.path.dirname(sys.executable), 'mailman')
-    ...     env = os.environ.copy()
-    ...     env.update(
-    ...         MAILMAN_CONFIG_FILE=config_path,
-    ...         PYTHONPATH=config_directory,
-    ...         )
-    ...     test_cfg = os.environ.get('MAILMAN_EXTRA_TESTING_CFG')
-    ...     if test_cfg is not None:
-    ...         env['MAILMAN_EXTRA_TESTING_CFG'] = test_cfg
-    ...     proc = subprocess.Popen(
-    ...         [exe, 'lists', '--domain', 'ignore', '-q'],
-    ...         cwd=ConfigLayer.root_directory, env=env,
-    ...         universal_newlines=True,
-    ...         stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-    ...     stdout, stderr = proc.communicate()
-    ...     assert proc.returncode == 0, stderr
-    ...     print(stdout)
-
-    >>> call()
-    pre-hook: 1
-    <BLANKLINE>
-
-    >>> os.remove(config_path)
-
-
-Post-hook
-=========
-
-We can set the post-hook in the configuration file.
-::
-
-    >>> with open(config_path, 'w') as fp:
-    ...     print("""\
-    ... [meta]
-    ... extends: test.cfg
-    ...
-    ... [mailman]
-    ... post_hook: hooks.post_hook
-    ... """, file=fp)
-
-    >>> call()
-    post-hook: 1
-    <BLANKLINE>
-
-    >>> os.remove(config_path)
-
-
-Running both hooks
-==================
-
-We can set the pre- and post-hooks in the configuration file.
-::
-
-    >>> with open(config_path, 'w') as fp:
-    ...     print("""\
-    ... [meta]
-    ... extends: test.cfg
-    ...
-    ... [mailman]
-    ... pre_hook: hooks.pre_hook
-    ... post_hook: hooks.post_hook
-    ... """, file=fp)
-
-    >>> call()
-    pre-hook: 1
-    post-hook: 2
-    <BLANKLINE>
diff --git a/src/mailman/bin/mailman.py b/src/mailman/bin/mailman.py
index 6b8cb26da..0ea29edbb 100644
--- a/src/mailman/bin/mailman.py
+++ b/src/mailman/bin/mailman.py
@@ -16,7 +16,6 @@
 # GNU Mailman.  If not, see <http://www.gnu.org/licenses/>.
 
 """The 'mailman' command dispatcher."""
-
 import click
 
 from contextlib import ExitStack
@@ -35,17 +34,21 @@ class Subcommands(click.MultiCommand):
     def __init__(self, *args, **kws):
         super().__init__(*args, **kws)
         self._commands = {}
-        # Look at all modules in the mailman.bin package and if they are
-        # prepared to add a subcommand, let them do so.  I'm still undecided as
-        # to whether this should be pluggable or not.  If so, then we'll
-        # probably have to partially parse the arguments now, then initialize
-        # the system, then find the plugins.  Punt on this for now.
-        add_components('mailman.commands', ICLISubCommand, self._commands)
+        self._loaded = False
+
+    def _load(self):
+        # Load commands lazily as commands in plugins can only be found after
+        # the configuration file is loaded.
+        if not self._loaded:
+            add_components('commands', ICLISubCommand, self._commands)
+            self._loaded = True
 
-    def list_commands(self, ctx):
-        return sorted(self._commands)               # pragma: nocover
+    def list_commands(self, ctx):                    # pragma: nocover
+        self._load()
+        return sorted(self._commands)
 
     def get_command(self, ctx, name):
+        self._load()
         try:
             return self._commands[name].command
         except KeyError as error:
@@ -85,10 +88,11 @@ class Subcommands(click.MultiCommand):
         super().format_commands(ctx, formatter)
 
 
-@click.group(
-    cls=Subcommands,
-    context_settings=dict(help_option_names=['-h', '--help']))
-@click.pass_context
+def initialize_config(ctx, param, value):
+    if not ctx.resilient_parsing:
+        initialize(value)
+
+
 @click.option(
     '-C', '--config', 'config_file',
     envvar='MAILMAN_CONFIG_FILE',
@@ -96,7 +100,12 @@ class Subcommands(click.MultiCommand):
     help=_("""\
     Configuration file to use.  If not given, the environment variable
     MAILMAN_CONFIG_FILE is consulted and used if set.  If neither are given, a
-    default configuration file is loaded."""))
+    default configuration file is loaded."""),
+    is_eager=True, callback=initialize_config)
+@click.group(
+    cls=Subcommands,
+    context_settings=dict(help_option_names=['-h', '--help']))
+@click.pass_context
 @click.version_option(MAILMAN_VERSION_FULL, message='%(version)s')
 @public
 def main(ctx, config_file):
@@ -106,6 +115,4 @@ def main(ctx, config_file):
     Copyright 1998-2017 by the Free Software Foundation, Inc.
     http://www.list.org
     """
-    # Initialize the system.  Honor the -C flag if given.
-    initialize(config_file)
     # click handles dispatching to the subcommand via the Subcommands class.
diff --git a/src/mailman/bin/master.py b/src/mailman/bin/master.py
index 0b273d332..8339f0fc2 100644
--- a/src/mailman/bin/master.py
+++ b/src/mailman/bin/master.py
@@ -45,22 +45,23 @@ SUBPROC_START_WAIT = timedelta(seconds=20)
 # Environment variables to forward into subprocesses.
 PRESERVE_ENVS = (
     'COVERAGE_PROCESS_START',
-    'MAILMAN_EXTRA_TESTING_CFG',
     'LANG',
     'LANGUAGE',
-    'LC_CTYPE',
-    'LC_NUMERIC',
-    'LC_TIME',
+    'LC_ADDRESS',
+    'LC_ALL',
     'LC_COLLATE',
-    'LC_MONETARY',
+    'LC_CTYPE',
+    'LC_IDENTIFICATION',
+    'LC_MEASUREMENT',
     'LC_MESSAGES',
-    'LC_PAPER',
+    'LC_MONETARY',
     'LC_NAME',
-    'LC_ADDRESS',
+    'LC_NUMERIC',
+    'LC_PAPER',
     'LC_TELEPHONE',
-    'LC_MEASUREMENT',
-    'LC_IDENTIFICATION',
-    'LC_ALL',
+    'LC_TIME',
+    'MAILMAN_EXTRA_TESTING_CFG',
+    'PYTHONPATH',
     )
 
 
@@ -188,6 +189,9 @@ class PIDWatcher:
     def __init__(self):
         self._pids = {}
 
+    def __contains__(self, pid):
+        return pid in self._pids.keys()
+
     def __iter__(self):
         # Safely iterate over all the keys in the dictionary.  Because
         # asynchronous signals are involved, the dictionary's size could
@@ -312,16 +316,16 @@ class Loop:
         env = {'MAILMAN_UNDER_MASTER_CONTROL': '1'}
         # Craft the command line arguments for the exec() call.
         rswitch = '--runner=' + spec
-        # Wherever master lives, so too must live the runner script.
-        exe = os.path.join(config.BIN_DIR, 'runner')
-        # config.PYTHON, which is the absolute path to the Python interpreter,
-        # must be given as argv[0] due to Python's library search algorithm.
-        args = [sys.executable, sys.executable, exe, rswitch]
         # Always pass the explicit path to the configuration file to the
         # sub-runners.  This avoids any debate about which cfg file is used.
         config_file = (config.filename if self._config_file is None
                        else self._config_file)
-        args.extend(['-C', config_file])
+        # Wherever master lives, so too must live the runner script.
+        exe = os.path.join(config.BIN_DIR, 'runner')   # pragma: nocover
+        # config.PYTHON, which is the absolute path to the Python interpreter,
+        # must be given as argv[0] due to Python's library search algorithm.
+        args = [sys.executable, sys.executable, exe,   # pragma: nocover
+                '-C', config_file, rswitch]
         log = logging.getLogger('mailman.runner')
         log.debug('starting: %s', args)
         # We must pass this environment variable through if it's set,
@@ -399,9 +403,13 @@ class Loop:
             except ChildProcessError:
                 # No children?  We're done.
                 break
-            except InterruptedError:                # pragma: nocover
+            except InterruptedError:                         # pragma: nocover
                 # If the system call got interrupted, just restart it.
                 continue
+            if pid not in self._kids:                        # pragma: nocover
+                # This is not a runner subprocess that we own.  E.g. maybe a
+                # plugin started it.
+                continue
             # Find out why the subprocess exited by getting the signal
             # received or exit status.
             if os.WIFSIGNALED(status):
diff --git a/src/mailman/bin/runner.py b/src/mailman/bin/runner.py
index bf84bc3e5..b4780dcfb 100644
--- a/src/mailman/bin/runner.py
+++ b/src/mailman/bin/runner.py
@@ -151,14 +151,12 @@ def main(ctx, config_file, verbose, list_runners, once, runner_spec):
     run this way, the environment variable $MAILMAN_UNDER_MASTER_CONTROL
     will be set which subtly changes some error handling behavior.
     """
-
     global log
 
     if runner_spec is None and not list_runners:
         ctx.fail(_('No runner name given.'))
 
     # Initialize the system.  Honor the -C flag if given.
-
     initialize(config_file, verbose)
     log = logging.getLogger('mailman.runner')
     if verbose:
diff --git a/src/mailman/bin/tests/test_mailman.py b/src/mailman/bin/tests/test_mailman.py
index 73941d468..db13166c7 100644
--- a/src/mailman/bin/tests/test_mailman.py
+++ b/src/mailman/bin/tests/test_mailman.py
@@ -29,6 +29,7 @@ from mailman.interfaces.command import ICLISubCommand
 from mailman.testing.layers import ConfigLayer
 from mailman.utilities.datetime import now
 from mailman.utilities.modules import add_components
+from pkg_resources import resource_filename
 from unittest.mock import patch
 
 
@@ -38,7 +39,19 @@ class TestMailmanCommand(unittest.TestCase):
     def setUp(self):
         self._command = CliRunner()
 
-    def test_mailman_command_without_subcommand_prints_help(self):
+    def test_mailman_command_config(self):
+        config_path = resource_filename('mailman.testing', 'testing.cfg')
+        with patch('mailman.bin.mailman.initialize') as init:
+            self._command.invoke(main, ('-C', config_path, 'info'))
+        init.assert_called_once_with(config_path)
+
+    def test_mailman_command_no_config(self):
+        with patch('mailman.bin.mailman.initialize') as init:
+            self._command.invoke(main, ('info',))
+        init.assert_called_once_with(None)
+
+    @patch('mailman.bin.mailman.initialize')
+    def test_mailman_command_without_subcommand_prints_help(self, mock):
         # Issue #137: Running `mailman` without a subcommand raises an
         # AttributeError.
         result = self._command.invoke(main)
@@ -49,14 +62,15 @@ class TestMailmanCommand(unittest.TestCase):
         self.assertEqual(lines[0], 'Usage: main [OPTIONS] COMMAND [ARGS]...')
         # The help output includes a list of subcommands, in sorted order.
         commands = {}
-        add_components('mailman.commands', ICLISubCommand, commands)
+        add_components('commands', ICLISubCommand, commands)
         help_commands = list(
             line.split()[0].strip()
             for line in lines[-len(commands):]
             )
         self.assertEqual(sorted(commands), help_commands)
 
-    def test_mailman_command_with_bad_subcommand_prints_help(self):
+    @patch('mailman.bin.mailman.initialize')
+    def test_mailman_command_with_bad_subcommand_prints_help(self, mock):
         # Issue #137: Running `mailman` without a subcommand raises an
         # AttributeError.
         result = self._command.invoke(main, ('not-a-subcommand',))
diff --git a/src/mailman/commands/cli_withlist.py b/src/mailman/commands/cli_withlist.py
index d2b706e34..202b61584 100644
--- a/src/mailman/commands/cli_withlist.py
+++ b/src/mailman/commands/cli_withlist.py
@@ -178,9 +178,12 @@ def listaddr(mlist):
 def requestaddr(mlist):
     print(mlist.request_address)
 
-All run methods take at least one argument, the mailing list object to operate
+Run methods take at least one argument, the mailing list object to operate
 on.  Any additional arguments given on the command line are passed as
-positional arguments to the callable."""))
+positional arguments to the callable.
+
+If -l is not given then you can run a function that takes no arguments.
+"""))
     print()
     print(_("""\
 You can print the list's posting address by running the following from the
@@ -232,11 +235,16 @@ Mailman will do this for you (assuming no errors occured)."""))
 @click.option(
     '--run', '-r',
     help=_("""\
-    Run a script on a mailing list.  The argument is the module path to a
-    callable.  This callable will be imported and then called with the mailing
-    list as the first argument.  If additional arguments are given at the end
-    of the command line, they are passed as subsequent positional arguments to
-    the callable.  For additional help, see --details.
+
+    Run a script.  The argument is the module path to a callable.  This
+    callable will be imported and then, if --listspec/-l is also given, is
+    called with the mailing list as the first argument.  If additional
+    arguments are given at the end of the command line, they are passed as
+    subsequent positional arguments to the callable.  For additional help, see
+    --details.
+
+    If no --listspec/-l argument is given, the script function being called is
+    called with no arguments.
     """))
 @click.option(
     '--details',
@@ -270,9 +278,8 @@ def shell(ctx, interactive, run, listspec, run_args):
         # without the dot is allowed.
         dotted_name = (run if '.' in run else '{0}.{0}'.format(run))
         if listspec is None:
-            ctx.fail(_('--run requires a mailing list'))
-        # Parse the run arguments so we can pass them into the run method.
-        if listspec.startswith('^'):
+            r = call_name(dotted_name, *run_args)
+        elif listspec.startswith('^'):
             r = {}
             cre = re.compile(listspec, re.IGNORECASE)
             for mlist in list_manager.mailing_lists:
diff --git a/src/mailman/commands/docs/conf.rst b/src/mailman/commands/docs/conf.rst
index 1a6a4679d..5e548ec0b 100644
--- a/src/mailman/commands/docs/conf.rst
+++ b/src/mailman/commands/docs/conf.rst
@@ -45,6 +45,7 @@ key, along with the names of the corresponding sections.
     [logging.http] path: mailman.log
     [logging.locks] path: mailman.log
     [logging.mischief] path: mailman.log
+    [logging.plugins] path: plugins.log
     [logging.root] path: mailman.log
     [logging.runner] path: mailman.log
     [logging.smtp] path: smtp.log
diff --git a/src/mailman/commands/docs/info.rst b/src/mailman/commands/docs/info.rst
index 219143cab..40784a98b 100644
--- a/src/mailman/commands/docs/info.rst
+++ b/src/mailman/commands/docs/info.rst
@@ -59,7 +59,6 @@ definition.
         CFG_FILE        = .../test.cfg
         DATA_DIR        = /var/lib/mailman/data
         ETC_DIR         = /etc
-        EXT_DIR         = /etc/mailman.d
         LIST_DATA_DIR   = /var/lib/mailman/lists
         LOCK_DIR        = /var/lock/mailman
         LOCK_FILE       = /var/lock/mailman/master.lck
diff --git a/src/mailman/commands/tests/test_cli_shell.py b/src/mailman/commands/tests/test_cli_shell.py
index ffcf43803..74abd64a3 100644
--- a/src/mailman/commands/tests/test_cli_shell.py
+++ b/src/mailman/commands/tests/test_cli_shell.py
@@ -29,6 +29,7 @@ from mailman.interfaces.usermanager import IUserManager
 from mailman.testing.helpers import configuration
 from mailman.testing.layers import ConfigLayer
 from mailman.utilities.modules import hacked_sys_modules
+from types import ModuleType
 from unittest.mock import MagicMock, patch
 
 try:
@@ -40,6 +41,7 @@ except ImportError:
 
 class TestShell(unittest.TestCase):
     layer = ConfigLayer
+    maxDiff = None
 
     def setUp(self):
         self._command = CliRunner()
@@ -156,12 +158,12 @@ class TestShell(unittest.TestCase):
             'Error: No such list: ant.example.com\n')
 
     def test_run_without_listspec(self):
-        results = self._command.invoke(shell, ('--run', 'something'))
-        self.assertEqual(results.exit_code, 2)
-        self.assertEqual(
-            results.output,
-            'Usage: shell [OPTIONS] [RUN_ARGS]...\n\n'
-            'Error: --run requires a mailing list\n')
+        something = ModuleType('something')
+        something.something = lambda: print('I am a something!')
+        with hacked_sys_modules('something', something):
+            results = self._command.invoke(shell, ('--run', 'something'))
+        self.assertEqual(results.exit_code, 0)
+        self.assertEqual(results.output, 'I am a something!\n')
 
     def test_run_bogus_listspec(self):
         results = self._command.invoke(
diff --git a/src/mailman/config/config.py b/src/mailman/config/config.py
index 61c9fe6ed..c004d6e63 100644
--- a/src/mailman/config/config.py
+++ b/src/mailman/config/config.py
@@ -40,6 +40,19 @@ from zope.interface import implementer
 
 SPACE = ' '
 SPACERS = '\n'
+DIR_NAMES = (
+    'archive',
+    'bin',
+    'cache',
+    'data',
+    'etc',
+    'list_data',
+    'lock',
+    'log',
+    'messages',
+    'queue',
+    )
+
 
 MAILMAN_CFG_TEMPLATE = """\
 # AUTOMATICALLY GENERATED BY MAILMAN ON {} UTC
@@ -76,6 +89,7 @@ class Configuration:
         self.handlers = {}
         self.pipelines = {}
         self.commands = {}
+        self.plugins = {}
         self.password_context = None
         self.db = None
 
@@ -158,8 +172,7 @@ class Configuration:
                 else category.template_dir),
             )
         # Directories.
-        for name in ('archive', 'bin', 'cache', 'data', 'etc', 'ext',
-                     'list_data', 'lock', 'log', 'messages', 'queue'):
+        for name in DIR_NAMES:
             key = '{}_dir'.format(name)
             substitutions[key] = getattr(category, key)
         # Files.
@@ -248,6 +261,35 @@ class Configuration:
             yield archiver
 
     @property
+    def plugin_configs(self):
+        """Return all the plugin configuration sections."""
+        plugin_sections = self._config.getByCategory('plugin', [])
+        for section in plugin_sections:
+            # 2017-08-27 barry: There's a fundamental constraint imposed by
+            # lazr.config, namely that we have to use a .master section instead
+            # of a .template section in the schema.cfg, or user supplied
+            # configuration files cannot define new [plugin.*] sections.  See
+            # https://bugs.launchpad.net/lazr.config/+bug/310619 for
+            # additional details.
+            #
+            # However, this means that [plugin.master] will show up in the
+            # categories retrieved above.  But 'master' is not a real plugin,
+            # so we need to skip it (e.g. otherwise we'll get log warnings
+            # about plugin.master being disabled, etc.).  This imposes an
+            # additional limitation though in that users cannot define a
+            # plugin named 'master' because you can't override a master
+            # section with a real section.  There's no good way around this so
+            # we just have to live with this limitation.
+            if section.name == 'plugin.master':
+                continue
+            # The section.name will be something like 'plugin.example', but we
+            # only want the 'example' part as the name of the plugin.  We
+            # could split on dots, but lazr.config gives us a different way.
+            # `category_and_section_names` is a 2-tuple of e.g.
+            # ('plugin', 'example'), so just grab the last element.
+            yield section.category_and_section_names[1], section
+
+    @property
     def language_configs(self):
         """Iterate over all the language configuration sections."""
         yield from self._config.getByCategory('language', [])
diff --git a/src/mailman/config/schema.cfg b/src/mailman/config/schema.cfg
index 9568694be..b6f99c61d 100644
--- a/src/mailman/config/schema.cfg
+++ b/src/mailman/config/schema.cfg
@@ -54,14 +54,6 @@ pending_request_life: 3d
 # How long should files be saved before they are evicted from the cache?
 cache_life: 7d
 
-# A callable to run with no arguments early in the initialization process.
-# This runs before database initialization.
-pre_hook:
-
-# A callable to run with no arguments late in the initialization process.
-# This runs after adapters are initialized.
-post_hook:
-
 # Which paths.* file system layout to use.
 layout: here
 
@@ -82,6 +74,51 @@ html_to_plain_text_command: /usr/bin/lynx -dump $filename
 # unpredictable.
 listname_chars: [-_.0-9a-z]
 
+# These hooks are deprecated, but are kept here so as not to break existing
+# configuration files.  However, these hooks are not run.  Define a plugin
+# instead.
+pre_hook:
+post_hook:
+
+
+# Plugin configuration section template.
+#
+# To add a plugin, instantiate this section (changing `master` to whatever
+# your plugin's name is), and define at least a `path` and a `class`.  When
+# the plugin is loaded, its subpackages will be search for components matching
+# the following interfaces:
+#
+# - IChain for new chains
+# - ICliSubCommand - `mailman` subcommands
+# - IEmailCommand - new email commands
+# - IHandler for new handlers
+# - IPipeline for new pipelines
+# - IRule for new rules
+# - IStyle for new styles.
+#
+# See the IPlugin interface for more details.
+[plugin.master]
+
+# The full Python import path for you IPlugin implementing class.  It is
+# required to provide this.
+class:
+
+# Whether to enable this plugin or not.
+enabled: no
+
+# Additional configuration file for this plugin.  If the value starts with
+# `python:` it is a Python import path, in which case the value should not
+# include the trailing .cfg (although the file is required to have this
+# suffix).  Without `python:`, it is a file system path, and must be an
+# absolute path, since no guarantees are made about the current working
+# directory.
+configuration:
+
+# Package (as a dotted Python import path) to search for components that this
+# plugin wants to add, such as ISTyles, IRules, etc.  If not given, the
+# plugin's name is used.
+component_package:
+
 
 [shell]
 # `mailman shell` (also `withlist`) gives you an interactive prompt that you
@@ -138,8 +175,6 @@ data_dir: $var_dir/data
 cache_dir: $var_dir/cache
 # Directory for configuration files and such.
 etc_dir: $var_dir/etc
-# Directory containing Mailman plugins.
-ext_dir: $var_dir/ext
 # Directory where the default IMessageStore puts its messages.
 messages_dir: $var_dir/messages
 # Directory for archive backends to store their messages in.  Archivers should
@@ -261,6 +296,7 @@ debug: no
 # - http            --  Internal wsgi-based web interface
 # - locks           --  Lock state changes
 # - mischief        --  Various types of hostile activity
+# - plugins         --  Plugin logs
 # - runner          --  Runner process start/stops
 # - smtp            --  Successful SMTP activity
 # - smtp-failure    --  Unsuccessful SMTP activity
@@ -298,6 +334,9 @@ level: info
 
 [logging.mischief]
 
+[logging.plugins]
+path: plugins.log
+
 [logging.runner]
 
 [logging.smtp]
@@ -797,11 +836,6 @@ class: mailman.archiving.prototype.Prototype
 
 
 [styles]
-# Python import paths inside which components are searched for which implement
-# the IStyle interface.  Use one path per line.
-paths:
-    mailman.styles
-
 # The default style to apply if nothing else was requested.  The value is the
 # name of an existing style.  If no such style exists, no style will be
 # applied.
diff --git a/src/mailman/core/chains.py b/src/mailman/core/chains.py
index c251d38bc..ebd0a5739 100644
--- a/src/mailman/core/chains.py
+++ b/src/mailman/core/chains.py
@@ -89,5 +89,4 @@ def process(mlist, msg, msgdata, start_chain='default-posting-chain'):
 @public
 def initialize():
     """Set up chains, both built-in and from the database."""
-    add_components('mailman.chains', IChain, config.chains)
-    # XXX Read chains from the database and initialize them.
+    add_components('chains', IChain, config.chains)
diff --git a/src/mailman/core/initialize.py b/src/mailman/core/initialize.py
index dc67e9a68..5696cc3bb 100644
--- a/src/mailman/core/initialize.py
+++ b/src/mailman/core/initialize.py
@@ -26,11 +26,11 @@ by the command line arguments.
 
 import os
 import sys
+import logging
 import mailman.config.config
 import mailman.core.logging
 
 from mailman.interfaces.database import IDatabaseFactory
-from mailman.utilities.modules import call_name
 from pkg_resources import resource_string as resource_bytes
 from public import public
 from zope.component import getUtility
@@ -133,7 +133,7 @@ def initialize_2(debug=False, propagate_logs=None, testing=False):
 
     * Database
     * Logging
-    * Pre-hook
+    * Plugin pre_hook()'s
     * Rules
     * Chains
     * Pipelines
@@ -146,10 +146,28 @@ def initialize_2(debug=False, propagate_logs=None, testing=False):
     """
     # Create the queue and log directories if they don't already exist.
     mailman.core.logging.initialize(propagate_logs)
-    # Run the pre-hook if there is one.
+    # Initialize plugins
+    from mailman.plugins.initialize import initialize as initialize_plugins
+    initialize_plugins()
+    # Check for deprecated features in config.
     config = mailman.config.config
-    if config.mailman.pre_hook:
-        call_name(config.mailman.pre_hook)
+    if len(config.mailman.pre_hook) > 0:            # pragma: nocover
+        log = logging.getLogger('mailman.plugins')
+        log.warn(
+            'The [mailman]pre_hook configuration value has been replaced '
+            "by the plugins infrastructure, and won't be called.")
+    # Run the plugin pre_hooks, if one fails, disable the offending plugin.
+    for name in config.plugins:                     # pragma: nocover
+        plugin = config.plugins[name]
+        if hasattr(plugin, 'pre_hook'):
+            try:
+                plugin.pre_hook()
+            except Exception:                                # pragma: nocover
+                log = logging.getLogger('mailman.plugins')
+                log.exception('Plugin failed to run its pre_hook: {}'
+                              'It will be disabled and its components '
+                              "won't be loaded.".format(name))
+                del config.plugins[name]
     # Instantiate the database class, ensure that it's of the right type, and
     # initialize it.  Then stash the object on our configuration object.
     utility_name = ('testing' if testing else 'production')
@@ -171,12 +189,24 @@ def initialize_2(debug=False, propagate_logs=None, testing=False):
 def initialize_3():
     """Third initialization step.
 
-    * Post-hook
+    * Plugin post_hook()'s.
     """
-    # Run the post-hook if there is one.
+    # Run the plugin post_hooks.
     config = mailman.config.config
-    if config.mailman.post_hook:
-        call_name(config.mailman.post_hook)
+    log = logging.getLogger('mailman.plugins')
+    if len(config.mailman.post_hook) > 0:           # pragma: nocover
+        log.warn(
+            'The [mailman]post_hook configuration value has been replaced '
+            "by the plugins infrastructure, and won't be called.")
+    for plugin in config.plugins.values():          # pragma: nocover
+        try:
+            plugin.post_hook()
+        except Exception:                         # pragma: nocover
+            # A post_hook may fail, here we just hope for the best that the
+            # plugin can work even if it post_hook failed as it's components
+            # are already loaded.
+            log.exception(
+                'Plugin failed to run its post_hook: {}'.format(plugin.name))
 
 
 @public
diff --git a/src/mailman/core/pipelines.py b/src/mailman/core/pipelines.py
index 4265d24d1..c62c7dacd 100644
--- a/src/mailman/core/pipelines.py
+++ b/src/mailman/core/pipelines.py
@@ -63,6 +63,6 @@ def process(mlist, msg, msgdata, pipeline_name='built-in'):
 def initialize():
     """Initialize the pipelines."""
     # Find all handlers in the registered plugins.
-    add_components('mailman.handlers', IHandler, config.handlers)
+    add_components('handlers', IHandler, config.handlers)
     # Set up some pipelines.
-    add_components('mailman.pipelines', IPipeline, config.pipelines)
+    add_components('pipelines', IPipeline, config.pipelines)
diff --git a/src/mailman/core/rules.py b/src/mailman/core/rules.py
index 8e0d9197c..78dd13971 100644
--- a/src/mailman/core/rules.py
+++ b/src/mailman/core/rules.py
@@ -27,4 +27,4 @@ from public import public
 def initialize():
     """Find and register all rules in all plugins."""
     # Find rules in plugins.
-    add_components('mailman.rules', IRule, config.rules)
+    add_components('rules', IRule, config.rules)
diff --git a/src/mailman/docs/NEWS.rst b/src/mailman/docs/NEWS.rst
index 4000c135b..328699371 100644
--- a/src/mailman/docs/NEWS.rst
+++ b/src/mailman/docs/NEWS.rst
@@ -32,8 +32,20 @@ Command line
   "fqdn list name" (i.e. the posting address of a mailing list), now also
   accept a ``List-ID``.  Every attempt has been made to keep the CLI backward
   compatible, but there may be subtle differences.  (Closes #346)
+* If no listname is given, running ``mailman withlist -r`` must name a
+  function taking no arguments.  This can be used to introspect Mailman
+  outside of the context of a mailing list.
 * Fix ``mailman withlist`` command parsing.  (Closes #319)
 
+Configuration
+-------------
+* The ``[mailman]pre_hook`` and ``[mailman]post_hook`` variables are
+  deprecated.  They can still be specified but they will not be run.
+* The ``[paths.*]ext_dir`` variable has been removed.
+* A new logger has been added called ``logging.plugins``.
+* The ``[styles]paths`` variable has been removed; you can now specify
+  additional styles using the new plugin architecture.
+
 Interfaces
 ----------
 * Broaden the semantics for ``IListManager.get()``.  This API now accepts
@@ -45,6 +57,9 @@ Interfaces
 
 Other
 -----
+* Add a new plugin architecture, which allows third parties to add
+  initialization hooks, REST endpoints, and additional components.  Given by
+  Jan Jancar.
 * Drop support for Python 3.4.  (Closes #373)
 * Bump minimum requirements for aiosmtpd (>= 1.1) and flufl.lock (>= 3.1).
 
diff --git a/src/mailman/interfaces/plugin.py b/src/mailman/interfaces/plugin.py
new file mode 100644
index 000000000..10c26d4c5
--- /dev/null
+++ b/src/mailman/interfaces/plugin.py
@@ -0,0 +1,51 @@
+# Copyright (C) 2017 by the Free Software Foundation, Inc.
+#
+# This file is part of GNU Mailman.
+#
+# GNU Mailman is free software: you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation, either version 3 of the License, or (at your option)
+# any later version.
+#
+# GNU Mailman is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+# more details.
+#
+# You should have received a copy of the GNU General Public License along with
+# GNU Mailman.  If not, see <http://www.gnu.org/licenses/>.
+
+"""Interfaces for plugins."""
+
+from public import public
+from zope.interface import Attribute, Interface
+
+
+@public
+class IPlugin(Interface):
+    """A plugin providing components and hooks."""
+
+    def pre_hook():
+        """A plugin hook called in the first initialization step.
+
+        This is called before the database is initialized.
+        """
+
+    def post_hook():
+        """A plugin hook called in the second initialization step.
+
+        This is called after the database is initialized.
+        """
+
+    resource = Attribute("""\
+        The object for use as the root of this plugin's REST resources.
+
+        This is the resource which will be hooked up to the REST API, and
+        served at the /<api>/plugins/<plugin.name>/ location.  All parsing
+        below that location is up to the plugin.
+
+        This attribute should be None if the plugin doesn't provide a REST
+        resource.
+
+        The resource must support getattr() and dir().
+        """)
diff --git a/src/mailman/model/docs/listmanager.rst b/src/mailman/model/docs/listmanager.rst
index 234394ac6..d5d3a5dbe 100644
--- a/src/mailman/model/docs/listmanager.rst
+++ b/src/mailman/model/docs/listmanager.rst
@@ -60,7 +60,7 @@ always get the same object back.
     >>> list_manager.get('ant@example.com')
     <mailing list "ant@example.com" at ...>
 
-The ``.get()`` method is ambidextrous, so it also accepts ``List-ID``s.
+The ``.get()`` method is ambidextrous, so it also accepts ``List-ID``'s.
 
     >>> list_manager.get('ant.example.com')
     <mailing list "ant@example.com" at ...>
diff --git a/src/mailman/plugins/__init__.py b/src/mailman/plugins/__init__.py
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/src/mailman/plugins/__init__.py
diff --git a/src/mailman/plugins/docs/__init__.py b/src/mailman/plugins/docs/__init__.py
new file mode 100644
index 000000000..0b3957648
--- /dev/null
+++ b/src/mailman/plugins/docs/__init__.py
@@ -0,0 +1,25 @@
+# Copyright (C) 2017 by the Free Software Foundation, Inc.
+#
+# This file is part of GNU Mailman.
+#
+# GNU Mailman is free software: you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation, either version 3 of the License, or (at your option)
+# any later version.
+#
+# GNU Mailman is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+# more details.
+#
+# You should have received a copy of the GNU General Public License along with
+# GNU Mailman.  If not, see <http://www.gnu.org/licenses/>.
+
+"""REST layer for plugins."""
+
+
+from mailman.plugins.testing.layer import PluginRESTLayer
+from public import public
+
+# For flufl.testing.
+public(layer=PluginRESTLayer)
diff --git a/src/mailman/plugins/docs/intro.rst b/src/mailman/plugins/docs/intro.rst
new file mode 100644
index 000000000..be6fb365b
--- /dev/null
+++ b/src/mailman/plugins/docs/intro.rst
@@ -0,0 +1,207 @@
+=========
+ Plugins
+=========
+
+Mailman defines a plugin as a Python package on ``sys.path`` that provides
+components matching the ``IPlugin`` interface.  ``IPlugin`` implementations
+can define a *pre-hook*, a *post-hook*, and a *REST resource*.  Plugins are
+enabled by adding a section to your ``mailman.cfg`` file, such as:
+
+.. literalinclude:: ../testing/hooks.cfg
+
+.. note::
+   Because of a `design limitation`_ in the underlying configuration library,
+   you cannot name a plugin "master".  Specifically you cannot define a
+   section in your ``mailman.cfg`` file named ``[plugin.master]``.
+
+We have such a configuration file handy.
+
+    >>> from pkg_resources import resource_filename
+    >>> config_file = resource_filename('mailman.plugins.testing', 'hooks.cfg')
+
+The section must at least define the class implementing the ``IPlugin``
+interface, using a Python dotted-name import path.  For the import to work,
+you must include the top-level directory on ``sys.path``.
+
+    >>> import os
+    >>> from pkg_resources import resource_filename
+    >>> plugin_path = os.path.join(os.path.dirname(
+    ...     resource_filename('mailman.plugins', '__init__.py')),
+    ...     'testing')
+
+
+Hooks
+=====
+
+Plugins can add initialization hooks, which will be run at two stages in the
+initialization process - one before the database is initialized and one after.
+These correspond to methods the plugin defines, a ``pre_hook()`` method and a
+``post_hook()`` method.  Each of these methods are optional.
+
+Here is a plugin that defines these hooks:
+
+.. literalinclude:: ../testing/example/hooks.py
+
+To illustrate how the hooks work, we'll invoke a simple Mailman command to be
+run in a subprocess.  The plugin itself supports debugging hooking invocation
+when an environment variable is set.
+
+    >>> proc = run(['-C', config_file, 'info'],
+    ...            DEBUG_HOOKS='1',
+    ...            PYTHONPATH=plugin_path)
+    >>> print(proc.stdout)
+    I'm in my pre-hook
+    I'm in my post-hook
+    ...
+
+
+Components
+==========
+
+Plugins can also add components such as rules, chains, list styles, etc.  By
+default, components are searched for in the package matching the plugin's
+name.  So in the case above, the plugin is named ``example`` (because the
+section is called ``[plugin.example]``, and there is a subpackage called
+``rules`` under the ``example`` package.  The file system layout looks like
+this::
+
+    example/
+        __init__.py
+        hooks.py
+        rules/
+            __init__.py
+            rules.py
+
+And the contents of ``rules.py`` looks like:
+
+.. literalinclude:: ../testing/example/rules/rules.py
+
+To see that the plugin's rule get added, we invoke Mailman as an external
+process, running a script that prints out all the defined rule names,
+including our plugin's ``example-rule``.
+
+    >>> proc = run(['-C', config_file, 'withlist', '-r', 'showrules'],
+    ...            PYTHONPATH=plugin_path)
+    >>> print(proc.stdout)
+    administrivia
+    ...
+    example-rule
+    ...
+
+Component directories can live under any importable path, not just one named
+after the plugin.  By adding a ``component_package`` section to your plugin's
+configuration, you can name an alternative location to search for components.
+
+.. literalinclude:: ../testing/alternate.cfg
+
+We use this configuration file and the following file system layout::
+
+    example/
+        __init__.py
+        hooks.py
+    alternate/
+        rules/
+            __init__.py
+            rules.py
+
+Here, ``rules.py`` likes like:
+
+.. literalinclude:: ../testing/alternate/rules/rules.py
+
+You can see that this rule has a different name.  If we use the
+``alternate.cfg`` configuration file from above::
+
+    >>> config_file = resource_filename(
+    ...     'mailman.plugins.testing', 'alternate.cfg')
+
+we'll pick up the alternate rule when we print them out.
+
+    >>> proc = run(['-C', config_file, 'withlist', '-r', 'showrules'],
+    ...            PYTHONPATH=plugin_path)
+    >>> print(proc.stdout)
+    administrivia
+    alternate-rule
+    ...
+
+
+REST
+====
+
+Plugins can also supply REST routes.  Let's say we have a plugin defined like
+so:
+
+.. literalinclude:: ../testing/example/rest.py
+
+which we can enable with the following configuration file:
+
+.. literalinclude:: ../testing/rest.cfg
+
+The plugin defines a ``resource`` attribute that exposes the root of the
+plugin's resource tree.  The plugin will show up when we navigate to the
+``plugin`` resource.
+::
+
+    >>> dump_json('http://localhost:9001/3.1/plugins')
+    entry 0:
+        class: example.rest.ExamplePlugin
+        enabled: True
+        http_etag: "..."
+        name: example
+    http_etag: "..."
+    start: 0
+    total_size: 1
+
+The plugin may provide a ``GET`` on the resource itself.
+::
+
+    >>> dump_json('http://localhost:9001/3.1/plugins/example')
+    http_etag: "..."
+    my-child-resources: yes, no, echo
+    my-name: example-plugin
+
+And it may provide child resources.
+::
+
+    >>> dump_json('http://localhost:9001/3.1/plugins/example/yes')
+    http_etag: "..."
+    yes: True
+
+Plugins and their child resources can support any HTTP method, such as
+``GET``...
+::
+
+    >>> dump_json('http://localhost:9001/3.1/plugins/example/echo')
+    http_etag: "..."
+    number: 0
+
+... or ``POST`` ...
+::
+
+    >>> dump_json('http://localhost:9001/3.1/plugins/example/echo',
+    ...           dict(number=7))
+    content-length: 0
+    date: ...
+    server: ...
+    status: 204
+
+    >>> dump_json('http://localhost:9001/3.1/plugins/example/echo')
+    http_etag: "..."
+    number: 7
+
+... or ``DELETE``.
+
+    >>> dump_json('http://localhost:9001/3.1/plugins/example/echo',
+    ...           method='DELETE')
+    content-length: 0
+    date: ...
+    server: ...
+    status: 204
+
+    >>> dump_json('http://localhost:9001/3.1/plugins/example/echo')
+    http_etag: "..."
+    number: 0
+
+It's up to the plugin of course.
+
+
+.. _`design limitation`: https://bugs.launchpad.net/lazr.config/+bug/310619
diff --git a/src/mailman/plugins/initialize.py b/src/mailman/plugins/initialize.py
new file mode 100644
index 000000000..431f69a6f
--- /dev/null
+++ b/src/mailman/plugins/initialize.py
@@ -0,0 +1,54 @@
+# Copyright (C) 2017 by the Free Software Foundation, Inc.
+#
+# This file is part of GNU Mailman.
+#
+# GNU Mailman is free software: you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation, either version 3 of the License, or (at your option)
+# any later version.
+#
+# GNU Mailman is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+# more details.
+#
+# You should have received a copy of the GNU General Public License along with
+# GNU Mailman.  If not, see <http://www.gnu.org/licenses/>.
+
+"""Initialize the plugins."""
+
+import logging
+
+from lazr.config import as_boolean
+from mailman.config import config
+from mailman.interfaces.plugin import IPlugin
+from mailman.utilities.modules import call_name
+from public import public
+from zope.interface.exceptions import DoesNotImplement
+from zope.interface.verify import verifyObject
+
+
+log = logging.getLogger('mailman.plugins')
+
+
+@public
+def initialize():
+    """Initialize all enabled plugins."""
+    for name, plugin_config in config.plugin_configs:
+        class_path = plugin_config['class'].strip()
+        if not as_boolean(plugin_config['enabled']) or len(class_path) == 0:
+            log.info('Plugin not enabled, or empty class path: {}'.format(
+                name))
+            continue
+        if name in config.plugins:
+            log.error('Duplicate plugin name: {}'.format(name))
+            continue
+        plugin = call_name(class_path)
+        try:
+            verifyObject(IPlugin, plugin)
+        except DoesNotImplement:
+            log.error('Plugin class does not implement IPlugin: {}'.format(
+                class_path))
+            continue
+        plugin.name = name
+        config.plugins[name] = plugin
diff --git a/src/mailman/plugins/testing/__init__.py b/src/mailman/plugins/testing/__init__.py
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/src/mailman/plugins/testing/__init__.py
diff --git a/src/mailman/plugins/testing/alternate.cfg b/src/mailman/plugins/testing/alternate.cfg
new file mode 100644
index 000000000..6cf6ea4f3
--- /dev/null
+++ b/src/mailman/plugins/testing/alternate.cfg
@@ -0,0 +1,7 @@
+[plugin.example]
+class: example.hooks.ExamplePlugin
+enabled: yes
+component_package: alternate
+
+[logging.plugins]
+propagate: yes
diff --git a/src/mailman/plugins/testing/alternate/__init__.py b/src/mailman/plugins/testing/alternate/__init__.py
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/src/mailman/plugins/testing/alternate/__init__.py
diff --git a/src/mailman/plugins/testing/alternate/rules/__init__.py b/src/mailman/plugins/testing/alternate/rules/__init__.py
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/src/mailman/plugins/testing/alternate/rules/__init__.py
diff --git a/src/mailman/plugins/testing/alternate/rules/rules.py b/src/mailman/plugins/testing/alternate/rules/rules.py
new file mode 100644
index 000000000..f8b8d725c
--- /dev/null
+++ b/src/mailman/plugins/testing/alternate/rules/rules.py
@@ -0,0 +1,14 @@
+from mailman.interfaces.rules import IRule
+from public import public
+from zope.interface import implementer
+
+
+@public
+@implementer(IRule)
+class AlternateRule:
+    name = 'alternate-rule'
+    description = 'An alternate rule.'
+    record = True
+
+    def check(self, mlist, msg, msgdata):
+        return 'alternate' in msgdata
diff --git a/src/mailman/plugins/testing/example/__init__.py b/src/mailman/plugins/testing/example/__init__.py
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/src/mailman/plugins/testing/example/__init__.py
diff --git a/src/mailman/plugins/testing/example/hooks.py b/src/mailman/plugins/testing/example/hooks.py
new file mode 100644
index 000000000..55c09c4a5
--- /dev/null
+++ b/src/mailman/plugins/testing/example/hooks.py
@@ -0,0 +1,21 @@
+import os
+
+from mailman.interfaces.plugin import IPlugin
+from public import public
+from zope.interface import implementer
+
+
+@public
+@implementer(IPlugin)
+class ExamplePlugin:
+    def pre_hook(self):
+        if os.environ.get('DEBUG_HOOKS'):
+            print("I'm in my pre-hook")
+
+    def post_hook(self):
+        if os.environ.get('DEBUG_HOOKS'):
+            print("I'm in my post-hook")
+
+    @property
+    def resource(self):
+        return None
diff --git a/src/mailman/plugins/testing/example/rest.py b/src/mailman/plugins/testing/example/rest.py
new file mode 100644
index 000000000..1d83ce2cd
--- /dev/null
+++ b/src/mailman/plugins/testing/example/rest.py
@@ -0,0 +1,79 @@
+from mailman.config import config
+from mailman.interfaces.plugin import IPlugin
+from mailman.rest.helpers import bad_request, child, etag, no_content, okay
+from mailman.rest.validator import Validator
+from public import public
+from zope.interface import implementer
+
+
+@public
+class Yes:
+    def on_get(self, request, response):
+        okay(response, etag(dict(yes=True)))
+
+
+@public
+class No:
+    def on_get(self, request, response):
+        bad_request(response, etag(dict(no=False)))
+
+
+@public
+class NumberEcho:
+    def __init__(self):
+        self._plugin = config.plugins['example']
+
+    def on_get(self, request, response):
+        okay(response, etag(dict(number=self._plugin.number)))
+
+    def on_post(self, request, response):
+        try:
+            resource = Validator(number=int)(request)
+            self._plugin.number = resource['number']
+        except ValueError as error:
+            bad_request(response, str(error))
+        else:
+            no_content(response)
+
+    def on_delete(self, request, response):
+        self._plugin.number = 0
+        no_content(response)
+
+
+@public
+class RESTExample:
+    def on_get(self, request, response):
+        resource = {
+            'my-name': 'example-plugin',
+            'my-child-resources': 'yes, no, echo',
+            }
+        okay(response, etag(resource))
+
+    @child()
+    def yes(self, context, segments):
+        return Yes(), []
+
+    @child()
+    def no(self, context, segments):
+        return No(), []
+
+    @child()
+    def echo(self, context, segments):
+        return NumberEcho(), []
+
+
+@public
+@implementer(IPlugin)
+class ExamplePlugin:
+    def __init__(self):
+        self.number = 0
+
+    def pre_hook(self):
+        pass
+
+    def post_hook(self):
+        pass
+
+    @property
+    def resource(self):
+        return RESTExample()
diff --git a/src/mailman/plugins/testing/example/rules/__init__.py b/src/mailman/plugins/testing/example/rules/__init__.py
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/src/mailman/plugins/testing/example/rules/__init__.py
diff --git a/src/mailman/plugins/testing/example/rules/rules.py b/src/mailman/plugins/testing/example/rules/rules.py
new file mode 100644
index 000000000..7e9ba6ce7
--- /dev/null
+++ b/src/mailman/plugins/testing/example/rules/rules.py
@@ -0,0 +1,14 @@
+from mailman.interfaces.rules import IRule
+from public import public
+from zope.interface import implementer
+
+
+@public
+@implementer(IRule)
+class ExampleRule:
+    name = 'example-rule'
+    description = 'An example rule.'
+    record = True
+
+    def check(self, mlist, msg, msgdata):
+        return 'example' in msgdata
diff --git a/src/mailman/plugins/testing/hooks.cfg b/src/mailman/plugins/testing/hooks.cfg
new file mode 100644
index 000000000..15fb31815
--- /dev/null
+++ b/src/mailman/plugins/testing/hooks.cfg
@@ -0,0 +1,3 @@
+[plugin.example]
+class: example.hooks.ExamplePlugin
+enabled: yes
diff --git a/src/mailman/plugins/testing/layer.py b/src/mailman/plugins/testing/layer.py
new file mode 100644
index 000000000..311a419f4
--- /dev/null
+++ b/src/mailman/plugins/testing/layer.py
@@ -0,0 +1,53 @@
+# Copyright (C) 2017 by the Free Software Foundation, Inc.
+#
+# This file is part of GNU Mailman.
+#
+# GNU Mailman is free software: you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation, either version 3 of the License, or (at your option)
+# any later version.
+#
+# GNU Mailman is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+# more details.
+#
+# You should have received a copy of the GNU General Public License along with
+# GNU Mailman.  If not, see <http://www.gnu.org/licenses/>.
+
+"""REST layer for plugins."""
+
+import os
+
+from contextlib import ExitStack
+from mailman.testing.helpers import (
+    TestableMaster, hackenv, wait_for_webservice)
+from mailman.testing.layers import SMTPLayer
+from pkg_resources import resource_filename
+from public import public
+
+
+# Don't inherit from RESTLayer since layers get run in bottom up order,
+# meaning RESTLayer will get setUp() before this layer does, and that won't
+# use the configuration file we need it to use.
+@public
+class PluginRESTLayer(SMTPLayer):
+    @classmethod
+    def setUp(cls):
+        cls.resources = ExitStack()
+        plugin_path = os.path.join(
+            os.path.dirname(
+                resource_filename('mailman.plugins', '__init__.py')),
+            'testing')
+        config_file = resource_filename('mailman.plugins.testing', 'rest.cfg')
+        cls.resources.enter_context(
+            hackenv('MAILMAN_CONFIG_FILE', config_file))
+        cls.resources.enter_context(
+            hackenv('PYTHONPATH', plugin_path))
+        cls.server = TestableMaster(wait_for_webservice)
+        cls.server.start('rest')
+        cls.resources.callback(cls.server.stop)
+
+    @classmethod
+    def tearDown(cls):
+        cls.resources.close()
diff --git a/src/mailman/plugins/testing/rest.cfg b/src/mailman/plugins/testing/rest.cfg
new file mode 100644
index 000000000..a9b4b9bd9
--- /dev/null
+++ b/src/mailman/plugins/testing/rest.cfg
@@ -0,0 +1,6 @@
+[plugin.example]
+class: example.rest.ExamplePlugin
+enabled: yes
+
+[webservice]
+port: 9001
diff --git a/src/mailman/plugins/testing/showrules.py b/src/mailman/plugins/testing/showrules.py
new file mode 100644
index 000000000..926ef0f70
--- /dev/null
+++ b/src/mailman/plugins/testing/showrules.py
@@ -0,0 +1,6 @@
+from mailman.config import config
+
+
+def showrules():
+    for name in sorted(config.rules):
+        print(name)
diff --git a/src/mailman/plugins/tests/__init__.py b/src/mailman/plugins/tests/__init__.py
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/src/mailman/plugins/tests/__init__.py
diff --git a/src/mailman/plugins/tests/test_plugins.py b/src/mailman/plugins/tests/test_plugins.py
new file mode 100644
index 000000000..4b432f603
--- /dev/null
+++ b/src/mailman/plugins/tests/test_plugins.py
@@ -0,0 +1,146 @@
+# Copyright (C) 2017 by the Free Software Foundation, Inc.
+#
+# This file is part of GNU Mailman.
+#
+# GNU Mailman is free software: you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation, either version 3 of the License, or (at your option)
+# any later version.
+#
+# GNU Mailman is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+# more details.
+#
+# You should have received a copy of the GNU General Public License along with
+# GNU Mailman.  If not, see <http://www.gnu.org/licenses/>.
+
+"""Test some additional plugin stuff."""
+
+import sys
+import unittest
+
+from contextlib import ExitStack
+from mailman.interfaces.plugin import IPlugin
+from mailman.plugins.initialize import initialize
+from mailman.plugins.testing.layer import PluginRESTLayer
+from mailman.testing.helpers import call_api
+from mailman.testing.layers import ConfigLayer
+from tempfile import TemporaryDirectory
+from types import SimpleNamespace
+from unittest.mock import patch
+from urllib.error import HTTPError
+from zope.interface import implementer
+
+
+class TestRESTPlugin(unittest.TestCase):
+    layer = PluginRESTLayer
+
+    def test_plugin_raises_exception(self):
+        with self.assertRaises(HTTPError) as cm:
+            call_api('http://localhost:9001/3.1/plugins/example/no')
+        self.assertEqual(cm.exception.code, 400)
+
+
+@implementer(IPlugin)
+class TestablePlugin:
+    def pre_hook(self):
+        pass
+
+    def post_hook(self):
+        pass
+
+    resource = None
+
+
+class TestInitializePlugins(unittest.TestCase):
+    layer = ConfigLayer
+
+    def test_duplicate_plugin_name(self):
+        with ExitStack() as resources:
+            system_path = resources.enter_context(TemporaryDirectory())
+            fake_plugin_config = {
+                'path': system_path,
+                'enabled': 'yes',
+                'class': 'ExamplePlugin',
+                }
+            log_mock = resources.enter_context(
+                patch('mailman.plugins.initialize.log'))
+            fake_mailman_config = SimpleNamespace(
+                plugin_configs=[('example', fake_plugin_config)],
+                plugins=['example'],
+                )
+            resources.enter_context(patch(
+                'mailman.plugins.initialize.config', fake_mailman_config))
+            initialize()
+            log_mock.error.assert_called_once_with(
+                'Duplicate plugin name: example')
+
+    def test_does_not_implement(self):
+        with ExitStack() as resources:
+            system_path = resources.enter_context(TemporaryDirectory())
+            fake_plugin_config = {
+                'path': system_path,
+                'enabled': 'yes',
+                'class': 'ExamplePlugin',
+                }
+            log_mock = resources.enter_context(
+                patch('mailman.plugins.initialize.log'))
+            fake_mailman_config = SimpleNamespace(
+                plugin_configs=[('example', fake_plugin_config)],
+                plugins=[],
+                )
+            resources.enter_context(patch(
+                'mailman.plugins.initialize.config', fake_mailman_config))
+            resources.enter_context(patch(
+                'mailman.plugins.initialize.call_name',
+                # object() does not implement IPlugin.
+                return_value=object()))
+            initialize()
+            log_mock.error.assert_called_once_with(
+                'Plugin class does not implement IPlugin: ExamplePlugin')
+            self.assertNotIn(system_path, sys.path)
+
+    def test_adds_plugins_to_config(self):
+        with ExitStack() as resources:
+            system_path = resources.enter_context(TemporaryDirectory())
+            fake_plugin_config = {
+                'path': system_path,
+                'enabled': 'yes',
+                'class': 'ExamplePlugin',
+                }
+            fake_mailman_config = SimpleNamespace(
+                plugin_configs=[('example', fake_plugin_config)],
+                plugins={},
+                )
+            resources.enter_context(patch(
+                'mailman.plugins.initialize.config', fake_mailman_config))
+            testable_plugin = TestablePlugin()
+            resources.enter_context(patch(
+                'mailman.plugins.initialize.call_name',
+                # object() does not implement IPlugin.
+                return_value=testable_plugin))
+            initialize()
+            self.assertIn('example', fake_mailman_config.plugins)
+            self.assertEqual(
+                fake_mailman_config.plugins['example'],
+                testable_plugin)
+
+    def test_not_enabled(self):
+        with ExitStack() as resources:
+            fake_plugin_config = {
+                'path': '/does/not/exist',
+                'enabled': 'no',
+                'class': 'ExamplePlugin',
+                }
+            log_mock = resources.enter_context(
+                patch('mailman.plugins.initialize.log'))
+            fake_mailman_config = SimpleNamespace(
+                plugin_configs=[('example', fake_plugin_config)],
+                plugins={},
+                )
+            resources.enter_context(patch(
+                'mailman.plugins.initialize.config', fake_mailman_config))
+            initialize()
+            log_mock.info.assert_called_once_with(
+                'Plugin not enabled, or empty class path: example')
diff --git a/src/mailman/rest/docs/__init__.py b/src/mailman/rest/docs/__init__.py
index 350a6b83e..671ea38d6 100644
--- a/src/mailman/rest/docs/__init__.py
+++ b/src/mailman/rest/docs/__init__.py
@@ -19,22 +19,13 @@
 
 import threading
 
+from http import HTTPStatus
 from http.server import BaseHTTPRequestHandler, HTTPServer
 from mailman.testing.helpers import wait_for_webservice
 from mailman.testing.layers import RESTLayer
 from public import public
 
 
-# New in Python 3.5.
-try:
-    from http import HTTPStatus
-except ImportError:                                 # pragma: nocover
-    class HTTPStatus:
-        FORBIDDEN = 403
-        NOT_FOUND = 404
-        OK = 200
-
-
 # We need a web server to vend non-mailman: urls.
 class TestableHandler(BaseHTTPRequestHandler):
     # Be quiet.
@@ -78,6 +69,7 @@ class HTTPLayer(RESTLayer):
         cls._thread.join()
 
 
+# For flufl.testing -- define this doctest's layer.
 public(layer=HTTPLayer)
 
 
diff --git a/src/mailman/rest/plugins.py b/src/mailman/rest/plugins.py
new file mode 100644
index 000000000..2d61d881c
--- /dev/null
+++ b/src/mailman/rest/plugins.py
@@ -0,0 +1,76 @@
+# Copyright (C) 2010-2017 by the Free Software Foundation, Inc.
+#
+# This file is part of GNU Mailman.
+#
+# GNU Mailman is free software: you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation, either version 3 of the License, or (at your option)
+# any later version.
+#
+# GNU Mailman is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+# more details.
+#
+# You should have received a copy of the GNU General Public License along with
+# GNU Mailman.  If not, see <http://www.gnu.org/licenses/>.
+
+"""REST for plugins, dynamically proxies requests to plugin's rest_object."""
+
+from lazr.config import as_boolean
+from mailman.config import config
+from mailman.rest.helpers import CollectionMixin, NotFound, etag, okay
+from operator import itemgetter
+from public import public
+
+
+@public
+class AllPlugins(CollectionMixin):
+    """Read-only list of all plugin configs."""
+
+    def _resource_as_dict(self, plugin_config):
+        """See `CollectionMixin`."""
+        name, plugin_section = plugin_config
+        resource = {
+            'name': name,
+            'class': plugin_section['class'],
+            'enabled': as_boolean(plugin_section['enabled']),
+            }
+        # Add the path to the plugin's own configuration file, if one was
+        # given.
+        plugin_config = plugin_section['configuration'].strip()
+        if len(plugin_config) > 0:
+            resource['configuration'] = plugin_config
+        return resource
+
+    def _get_collection(self, request):
+        """See `CollectionMixin`."""
+        # plugin_configs returns a 2-tuple of (name, section), so sort
+        # alphabetically on the plugin name.
+        return sorted(config.plugin_configs, key=itemgetter(0))
+
+    def on_get(self, request, response):
+        """/plugins"""
+        resource = self._make_collection(request)
+        okay(response, etag(resource))
+
+
+@public
+class APlugin:
+    """REST proxy to the plugin's rest_object."""
+
+    def __init__(self, plugin_name):
+        self._resource = None
+        if plugin_name in config.plugins:
+            plugin = config.plugins[plugin_name]
+            self._resource = plugin.resource
+        # If the plugin doesn't exist or doesn't provide a resource, just proxy
+        # to NotFound.
+        if self._resource is None:
+            self._resource = NotFound()
+
+    def __getattr__(self, attrib):
+        return getattr(self._resource, attrib)
+
+    def __dir__(self):
+        return dir(self._resource)
diff --git a/src/mailman/rest/root.py b/src/mailman/rest/root.py
index 76289078b..46116aba6 100644
--- a/src/mailman/rest/root.py
+++ b/src/mailman/rest/root.py
@@ -30,6 +30,7 @@ from mailman.rest.helpers import (
     BadRequest, NotFound, child, etag, no_content, not_found, okay)
 from mailman.rest.lists import AList, AllLists, Styles
 from mailman.rest.members import AMember, AllMembers, FindMembers
+from mailman.rest.plugins import APlugin, AllPlugins
 from mailman.rest.preferences import ReadOnlyPreferences
 from mailman.rest.queues import AQueue, AQueueFile, AllQueues
 from mailman.rest.templates import TemplateFinder
@@ -308,6 +309,20 @@ class TopLevel:
             return BadRequest(), []
 
     @child()
+    def plugins(self, context, segments):
+        """/<api>/plugins
+           /<api>/plugins/<plugin_name>
+           /<api>/plugins/<plugin_name>/...
+        """
+        if self.api.version_info < (3, 1):
+            return NotFound(), []
+        if len(segments) == 0:
+            return AllPlugins(), []
+        else:
+            plugin_name = segments.pop(0)
+            return APlugin(plugin_name), segments
+
+    @child()
     def bans(self, context, segments):
         """/<api>/bans
            /<api>/bans/<email>
diff --git a/src/mailman/rest/tests/test_systemconf.py b/src/mailman/rest/tests/test_systemconf.py
index f9fe9caa0..91ee9befa 100644
--- a/src/mailman/rest/tests/test_systemconf.py
+++ b/src/mailman/rest/tests/test_systemconf.py
@@ -168,6 +168,7 @@ class TestSystemConfiguration(unittest.TestCase):
             'logging.http',
             'logging.locks',
             'logging.mischief',
+            'logging.plugins',
             'logging.root',
             'logging.runner',
             'logging.smtp',
@@ -182,6 +183,7 @@ class TestSystemConfiguration(unittest.TestCase):
             'paths.here',
             'paths.local',
             'paths.testing',
+            'plugin.master',
             'runner.archive',
             'runner.bad',
             'runner.bounces',
diff --git a/src/mailman/styles/manager.py b/src/mailman/styles/manager.py
index 1ec83b2a9..a7da45fba 100644
--- a/src/mailman/styles/manager.py
+++ b/src/mailman/styles/manager.py
@@ -38,13 +38,7 @@ class StyleManager:
 
     def populate(self):
         self._styles.clear()
-        # Avoid circular imports.
-        from mailman.config import config
-        # Calculate the Python import paths to search.
-        paths = filter(None, (path.strip()
-                              for path in config.styles.paths.splitlines()))
-        for path in paths:
-            add_components(path, IStyle, self._styles)
+        add_components('styles', IStyle, self._styles)
 
     def get(self, name):
         """See `IStyleManager`."""
diff --git a/src/mailman/testing/documentation.py b/src/mailman/testing/documentation.py
index d460be89f..ee37aa1ed 100644
--- a/src/mailman/testing/documentation.py
+++ b/src/mailman/testing/documentation.py
@@ -21,6 +21,9 @@ Note that doctest extraction does not currently work for zip file
 distributions.  doctest discovery currently requires file system traversal.
 """
 
+import os
+import sys
+
 from click.testing import CliRunner
 from contextlib import ExitStack
 from importlib import import_module
@@ -30,6 +33,7 @@ from mailman.testing.helpers import (
     call_api, get_queue_messages, specialized_message_from_string, subscribe)
 from mailman.testing.layers import SMTPLayer
 from public import public
+from subprocess import PIPE, STDOUT, run
 
 
 DOT = '.'
@@ -109,6 +113,13 @@ def call_http(url, data=None, method=None, username=None, password=None):
     return content
 
 
+def _print_dict(data, depth=0):
+    for item, value in sorted(data.items()):
+        if isinstance(value, dict):
+            _print_dict(value, depth+1)
+        print('    ' * depth + '{}: {}'.format(item, value))
+
+
 def dump_json(url, data=None, method=None, username=None, password=None):
     """Print the JSON dictionary read from a URL.
 
@@ -140,6 +151,9 @@ def dump_json(url, data=None, method=None, username=None, password=None):
             printable_value = COMMASPACE.join(
                 "'{}'".format(s) for s in sorted(value))
             print('{}: [{}]'.format(key, printable_value))
+        elif isinstance(value, dict):
+            print('{}:'.format(key))
+            _print_dict(value, 1)
         else:
             print('{}: {}'.format(key, value))
 
@@ -169,6 +183,18 @@ def cli(command_path):
 
 
 @public
+def run_mailman(args, **overrides):
+    exe = os.path.join(os.path.dirname(sys.executable), 'mailman')
+    env = os.environ.copy()
+    env.update(overrides)
+    run_args = [exe]
+    run_args.extend(args)
+    proc = run(
+        run_args, env=env, stdout=PIPE, stderr=STDOUT, universal_newlines=True)
+    return proc
+
+
+@public
 def setup(testobj):
     """Test setup."""
     # In general, I don't like adding convenience functions, since I think
@@ -176,18 +202,19 @@ def setup(testobj):
     # documentation that way.  However, a few are really useful, or help to
     # hide some icky test implementation details.
     testobj.globs['call_http'] = call_http
+    testobj.globs['cli'] = cli
     testobj.globs['config'] = config
     testobj.globs['create_list'] = create_list
     testobj.globs['dump_json'] = dump_json
-    testobj.globs['dump_msgdata'] = dump_msgdata
     testobj.globs['dump_list'] = dump_list
+    testobj.globs['dump_msgdata'] = dump_msgdata
     testobj.globs['get_queue_messages'] = get_queue_messages
     testobj.globs['message_from_string'] = specialized_message_from_string
+    testobj.globs['run'] = run_mailman
     testobj.globs['smtpd'] = SMTPLayer.smtpd
     testobj.globs['stop'] = stop
     testobj.globs['subscribe'] = subscribe
     testobj.globs['transaction'] = config.db
-    testobj.globs['cli'] = cli
     # Add this so that cleanups can be automatically added by the doctest.
     testobj.globs['cleanups'] = ExitStack()
 
diff --git a/src/mailman/testing/helpers.py b/src/mailman/testing/helpers.py
index 8bb8e8bdd..f3f6b11aa 100644
--- a/src/mailman/testing/helpers.py
+++ b/src/mailman/testing/helpers.py
@@ -151,7 +151,9 @@ class TestableMaster(Master):
             until the pass condition is set.
         :type start_check: Callable taking no arguments, returning nothing.
         """
-        super().__init__(restartable=False, config_file=config.filename)
+        super().__init__(
+            restartable=False,
+            config_file=os.environ.get('MAILMAN_CONFIG_FILE', config.filename))
         self.start_check = start_check
         self.event = threading.Event()
         self.thread = threading.Thread(target=self.loop)
diff --git a/src/mailman/tests/test_hook_deprecations.py b/src/mailman/tests/test_hook_deprecations.py
new file mode 100644
index 000000000..1e15e0d3a
--- /dev/null
+++ b/src/mailman/tests/test_hook_deprecations.py
@@ -0,0 +1,69 @@
+# Copyright (C) 2017 by the Free Software Foundation, Inc.
+#
+# This file is part of GNU Mailman.
+#
+# GNU Mailman is free software: you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation, either version 3 of the License, or (at your option)
+# any later version.
+#
+# GNU Mailman is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+# more details.
+#
+# You should have received a copy of the GNU General Public License along with
+# GNU Mailman.  If not, see <http://www.gnu.org/licenses/>.
+
+"""Test some plugin behavior."""
+
+import unittest
+
+from contextlib import ExitStack
+from mailman.testing.documentation import run_mailman
+from mailman.testing.layers import ConfigLayer
+from tempfile import NamedTemporaryFile
+
+
+class TestExternalHooks(unittest.TestCase):
+    layer = ConfigLayer
+    maxDiff = None
+
+    def setUp(self):
+        self.resources = ExitStack()
+        self.addCleanup(self.resources.close)
+        self.config_file = self.resources.enter_context(NamedTemporaryFile())
+
+    def test_pre_hook_deprecated(self):
+        with open(self.config_file.name, 'w', encoding='utf-8') as fp:
+            print("""\
+[mailman]
+pre_hook: sys.exit
+
+[logging.plugins]
+propagate: yes
+""", file=fp)
+        proc = run_mailman(['-C', self.config_file.name, 'info'])
+        # We only care about the log warning printed to stdout.
+        warning = proc.stdout.splitlines()[0]
+        self.assertEqual(
+            warning[-111:],
+            'The [mailman]pre_hook configuration value has been replaced '
+            "by the plugins infrastructure, and won't be called.")
+
+    def test_post_hook_deprecated(self):
+        with open(self.config_file.name, 'w', encoding='utf-8') as fp:
+            print("""\
+[mailman]
+post_hook: sys.exit
+
+[logging.plugins]
+propagate: yes
+""", file=fp)
+        proc = run_mailman(['-C', self.config_file.name, 'info'])
+        # We only care about the log warning printed to stdout.
+        warning = proc.stdout.splitlines()[0]
+        self.assertEqual(
+            warning[-112:],
+            'The [mailman]post_hook configuration value has been replaced '
+            "by the plugins infrastructure, and won't be called.")
diff --git a/src/mailman/utilities/modules.py b/src/mailman/utilities/modules.py
index 155a34ed2..48cee1a0e 100644
--- a/src/mailman/utilities/modules.py
+++ b/src/mailman/utilities/modules.py
@@ -22,7 +22,7 @@ import sys
 
 from contextlib import contextmanager
 from importlib import import_module
-from pkg_resources import resource_filename, resource_listdir
+from pkg_resources import resource_filename, resource_isdir, resource_listdir
 from public import public
 
 
@@ -70,6 +70,32 @@ def call_name(dotted_name, *args, **kws):
     return named_callable(*args, **kws)
 
 
+@public
+def expand_path(url):
+    """Expand a python: path, returning the absolute file system path."""
+    # Is the context coming from a file system or Python path?
+    if url.startswith('python:'):
+        resource_path = url[7:]
+        package, dot, resource = resource_path.rpartition('.')
+        return resource_filename(package, resource + '.cfg')
+    else:
+        return url
+
+
+@public
+@contextmanager
+def hacked_sys_modules(name, module):
+    old_module = sys.modules.get(name)
+    sys.modules[name] = module
+    try:
+        yield
+    finally:
+        if old_module is None:
+            del sys.modules[name]
+        else:
+            sys.modules[name] = old_module
+
+
 def scan_module(module, interface):
     """Return all the object in a module that conform to an interface.
 
@@ -102,7 +128,6 @@ def scan_module(module, interface):
             yield component()
 
 
-@public
 def find_components(package, interface):
     """Find components which conform to a given interface.
 
@@ -122,25 +147,63 @@ def find_components(package, interface):
         if extension != '.py' or basename.startswith('.'):
             continue
         module_name = '{}.{}'.format(package, basename)
-        __import__(module_name, fromlist='*')
-        module = sys.modules[module_name]
+        module = import_module(module_name)
         if not hasattr(module, '__all__'):
             continue
         yield from scan_module(module, interface)
 
 
+def find_pluggable_components(subpackage, interface):
+    """Find components which conform to a given interface.
+
+    This finds components which can be implemented in a plugin.  It will
+    search for the interface in the named subpackage, where the Python import
+    path of the subpackage will be prepended by `mailman` for system
+    components, and the various plugin names for any external components.
+
+    :param subpackage: The subpackage to search.  This is prepended by
+        'mailman' to search for system components, and each enabled plugin for
+        external components.
+    :type subpackage: str
+    :param interface: The interface that returned objects must conform to.
+    :type interface: `Interface`
+    :return: The sequence of matching components.
+    :rtype: Objects implementing `interface`
+    """
+    # This can't be imported at module level because of circular imports.
+    from mailman.config import config
+    # Return the system components first.
+    yield from find_components('mailman.' + subpackage, interface)
+    # Return all the matching components in all the subpackages of all enabled
+    # plugins.  Only enabled and existing plugins will appear in this
+    # dictionary.
+    for name, plugin_config in config.plugin_configs:
+        # If the plugin's configuration defines a components package, use
+        # that, falling back to the plugin's name.
+        package = plugin_config['component_package'].strip()
+        if len(package) == 0:
+            package = name
+        # It's possible that the plugin doesn't include the directory for this
+        # subpackage.  That's fine.
+        if resource_isdir(package, subpackage):
+            plugin_package = '{}.{}'.format(package, subpackage)
+            yield from find_components(plugin_package, interface)
+
+
 @public
-def add_components(package, interface, mapping):
+def add_components(subpackage, interface, mapping):
     """Add components to a given mapping.
 
-    Similarly to `find_components()` this inspects all modules in a given
-    package looking for objects that conform to a given interface.  All such
-    found objects (unless decorated with `@abstract_component`) are added to
-    the given mapping, keyed by the object's `.name` attribute, which is
-    required.  It is a fatal error if that key already exists in the mapping.
+    Similarly to `find_pluggable_components()` this inspects all modules
+    in the given subpackage, relative to the 'mailman' parent package,
+    and all the plugin names, that match the given interface.  All such
+    found objects (unless decorated with `@abstract_component`) are
+    added to the given mapping, keyed by the object's `.name` attribute,
+    which is required.  It is a fatal error if that key already exists
+    in the mapping.
 
-    :param package: The package path to search.
-    :type package: string
+    :param subpackage: The subpackage path to search.
+    :type subpackage: str
     :param interface: The interface that returned objects must conform to.
         Objects found must have a `.name` attribute containing a unique
         string.
@@ -150,35 +213,9 @@ def add_components(package, interface, mapping):
         containment tests (e.g. `in` and `not in`) and `__setitem__()`.
     :raises RuntimeError: when a duplicate key is found.
     """
-    for component in find_components(package, interface):
+    for component in find_pluggable_components(subpackage, interface):
         if component.name in mapping:
-            raise RuntimeError(
+            raise RuntimeError(     # pragma: nocover
                 'Duplicate key "{}" found in {}; previously {}'.format(
                     component.name, component, mapping[component.name]))
         mapping[component.name] = component
-
-
-@public
-def expand_path(url):
-    """Expand a python: path, returning the absolute file system path."""
-    # Is the context coming from a file system or Python path?
-    if url.startswith('python:'):
-        resource_path = url[7:]
-        package, dot, resource = resource_path.rpartition('.')
-        return resource_filename(package, resource + '.cfg')
-    else:
-        return url
-
-
-@public
-@contextmanager
-def hacked_sys_modules(name, module):
-    old_module = sys.modules.get(name)
-    sys.modules[name] = module
-    try:
-        yield
-    finally:
-        if old_module is None:
-            del sys.modules[name]
-        else:
-            sys.modules[name] = old_module
diff --git a/src/mailman/utilities/tests/test_modules.py b/src/mailman/utilities/tests/test_modules.py
index 82d44cff4..5ea2e5d23 100644
--- a/src/mailman/utilities/tests/test_modules.py
+++ b/src/mailman/utilities/tests/test_modules.py
@@ -22,10 +22,14 @@ import sys
 import unittest
 
 from contextlib import ExitStack, contextmanager
+from mailman.interfaces.rules import IRule
 from mailman.interfaces.styles import IStyle
+from mailman.testing.helpers import configuration
+from mailman.testing.layers import ConfigLayer
 from mailman.utilities.modules import (
-    add_components, find_components, hacked_sys_modules)
+    find_components, find_pluggable_components, hacked_sys_modules)
 from pathlib import Path
+from pkg_resources import resource_filename
 from tempfile import TemporaryDirectory
 
 
@@ -48,6 +52,8 @@ def clean_mypackage():
 
 
 class TestModuleImports(unittest.TestCase):
+    layer = ConfigLayer
+
     def test_find_modules_with_dotfiles(self):
         # Emacs creates lock files when a single file is opened by more than
         # one user. These files look like .#<filename>.py because of which
@@ -141,95 +147,6 @@ class AbstractStyle:
                      in find_components('mypackage', IStyle)]
             self.assertEqual(names, ['concrete-style'])
 
-    def test_add_components(self):
-        with ExitStack() as resources:
-            # Creating a temporary directory and adding it to sys.path.
-            temp_package = resources.enter_context(TemporaryDirectory())
-            resources.enter_context(hack_syspath(0, temp_package))
-            resources.callback(clean_mypackage)
-            # Create a module inside the above package along with an
-            # __init__.py file so that we can import from it.
-            module_path = os.path.join(temp_package, 'mypackage')
-            os.mkdir(module_path)
-            init_file = os.path.join(module_path, '__init__.py')
-            Path(init_file).touch()
-            component_file = os.path.join(module_path, 'components.py')
-            with open(component_file, 'w', encoding='utf-8') as fp:
-                print("""\
-from mailman.interfaces.styles import IStyle
-from mailman.utilities.modules import abstract_component
-from public import public
-from zope.interface import implementer
-
-@public
-@implementer(IStyle)
-class StyleA:
-    name = 'styleA'
-    def apply(self):
-        pass
-
-@public
-@implementer(IStyle)
-@abstract_component
-class StyleB:
-    name = 'styleB'
-    def apply(self):
-        pass
-
-@public
-@implementer(IStyle)
-class StyleC:
-    name = 'styleC'
-    def apply(self):
-        pass
-""", file=fp)
-            styles = {}
-            add_components('mypackage', IStyle, styles)
-            self.assertEqual(set(styles), {'styleA', 'styleC'})
-
-    def test_add_components_duplicates(self):
-        # A duplicate name exists, so a RuntimeError is raised.
-        with ExitStack() as resources:
-            # Creating a temporary directory and adding it to sys.path.
-            temp_package = resources.enter_context(TemporaryDirectory())
-            resources.enter_context(hack_syspath(0, temp_package))
-            resources.callback(clean_mypackage)
-            # Create a module inside the above package along with an
-            # __init__.py file so that we can import from it.
-            module_path = os.path.join(temp_package, 'mypackage')
-            os.mkdir(module_path)
-            init_file = os.path.join(module_path, '__init__.py')
-            Path(init_file).touch()
-            component_file = os.path.join(module_path, 'components.py')
-            with open(component_file, 'w', encoding='utf-8') as fp:
-                print("""\
-from mailman.interfaces.styles import IStyle
-from mailman.utilities.modules import abstract_component
-from public import public
-from zope.interface import implementer
-
-@public
-@implementer(IStyle)
-class StyleA1:
-    name = 'styleA'
-    def apply(self):
-        pass
-
-@public
-@implementer(IStyle)
-class StyleA2:
-    name = 'styleA'
-    def apply(self):
-        pass
-""", file=fp)
-            with self.assertRaisesRegex(
-                    RuntimeError,
-                    'Duplicate key "styleA" found in '
-                    '<mypackage.components.StyleA2 object at .*>; '
-                    'previously <mypackage.components.StyleA1 object at '
-                    '.*>'):
-                add_components('mypackage', IStyle, {})
-
     def test_hacked_sys_modules(self):
         self.assertIsNone(sys.modules.get('mailman.not_a_module'))
         with hacked_sys_modules('mailman.not_a_module', object()):
@@ -241,3 +158,27 @@ class StyleA2:
         with hacked_sys_modules('email', sentinel):
             self.assertEqual(sys.modules.get('email'), sentinel)
         self.assertEqual(sys.modules.get('email'), email_package)
+
+    def test_find_pluggable_components_by_plugin_name(self):
+        path = resource_filename('mailman.plugins.testing', '')
+        with ExitStack() as resources:
+            resources.enter_context(hack_syspath(0, path))
+            resources.enter_context(configuration('plugin.example', **{
+                'class': 'example.hooks.ExamplePlugin',
+                'enabled': 'yes',
+                }))
+            components = list(find_pluggable_components('rules', IRule))
+        self.assertIn('example-rule', {rule.name for rule in components})
+
+    def test_find_pluggable_components_by_component_package(self):
+        path = resource_filename('mailman.plugins.testing', '')
+        with ExitStack() as resources:
+            resources.enter_context(hack_syspath(0, path))
+            resources.enter_context(configuration('plugin.example', **{
+                'class': 'example.hooks.ExamplePlugin',
+                'enabled': 'yes',
+                'component_package': 'alternate',
+                }))
+            components = list(find_pluggable_components('rules', IRule))
+        self.assertNotIn('example-rule', {rule.name for rule in components})
+        self.assertIn('alternate-rule', {rule.name for rule in components})