26 files changed, 882 insertions, 181 deletions

diff --git a/src/mailman/app/commands.py b/src/mailman/app/commands.py
index 7cf3bc4c5..ca257ffda 100644
--- a/src/mailman/app/commands.py
+++ b/src/mailman/app/commands.py
@@ -19,11 +19,11 @@
 
 from mailman.config import config
 from mailman.interfaces.command import IEmailCommand
-from mailman.utilities.modules import add_components
+from mailman.utilities.plugins import add_pluggable_components
 from public import public
 
 
 @public
 def initialize():
     """Initialize the email commands."""
-    add_components('mailman.commands', IEmailCommand, config.commands)
+    add_pluggable_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/app/docs/plugins.rst b/src/mailman/app/docs/plugins.rst
new file mode 100644
index 000000000..4430179ae
--- /dev/null
+++ b/src/mailman/app/docs/plugins.rst
@@ -0,0 +1,210 @@
+=======
+Plugins
+=======
+
+Mailman defines a plugin as a package accessible on ``sys.path`` that provides
+components Mailman will use. Such add handlers, rules, chains, etc...
+First we create an example plugin that provides one additional rule:
+::
+
+    >>> import os, sys
+    >>> from mailman.config import config
+    >>> config_directory = os.path.dirname(config.filename)
+    >>> sys.path.insert(0, config_directory)
+
+    >>> example_plugin_path = os.path.join(config_directory, 'example_plugin')
+    >>> rules_path = os.path.join(example_plugin_path, 'rules')
+    >>> os.makedirs(rules_path)
+    >>> open(os.path.join(example_plugin_path, '__init__.py'), 'a').close()
+    >>> open(os.path.join(rules_path, '__init__.py'), 'a').close()
+    >>> rule_path = os.path.join(rules_path, 'example_rule.py')
+    >>> with open(rule_path, 'w') as fp:
+    ...     print("""\
+    ... from mailman.interfaces.rules import IRule
+    ... from public import public
+    ... from zope.interface import implementer
+    ...
+    ... @public
+    ... @implementer(IRule)
+    ... class ExampleRule:
+    ...
+    ...     name = 'example'
+    ...     description = 'Example rule to show pluggable components.'
+    ...     record = True
+    ...
+    ...     def check(self, mlist, msg, msgdata):
+    ...         return msg.original_size > 1024
+    ... """, file=fp)
+    >>> fp.close()
+
+Then enable the example plugin in config.
+::
+
+    >>> example_config = """\
+    ... [plugin.example]
+    ... path: example_plugin
+    ... enable: yes
+    ... """
+    >>> config.push('example_cfg', example_config)
+
+Now the `example` rule can be seen as an ``IRule`` component and will be used
+when any chain uses a link called `example`.
+::
+
+    >>> from mailman.interfaces.rules import IRule
+    >>> from mailman.utilities.plugins import find_pluggable_components
+    >>> rules = sorted([rule.name
+    ...                 for rule in find_pluggable_components('rules', IRule)])
+    >>> for rule in rules:
+    ...     print(rule)
+    administrivia
+    any
+    approved
+    banned-address
+    dmarc-mitigation
+    emergency
+    example
+    implicit-dest
+    loop
+    max-recipients
+    max-size
+    member-moderation
+    news-moderation
+    no-senders
+    no-subject
+    nonmember-moderation
+    suspicious-header
+    truth
+
+
+    >>> config.pop('example_cfg')
+    >>> from shutil import rmtree
+    >>> rmtree(example_plugin_path)
+
+
+Hooks
+=====
+
+Plugins can also add initialization hooks, which will be run during the
+initialization process. By creating a class implementing the IPlugin interface.
+One which is run early in the process and the other run late in the
+initialization process.
+::
+
+    >>> hook_path = os.path.join(config_directory, 'hooks.py')
+    >>> with open(hook_path, 'w') as fp:
+    ...     print("""\
+    ... from mailman.interfaces.plugin import IPlugin
+    ... from public import public
+    ... from zope.interface import implementer
+    ...
+    ... counter = 1
+    ...
+    ... @public
+    ... @implementer(IPlugin)
+    ... class ExamplePlugin:
+    ...
+    ...     def pre_hook(self):
+    ...         global counter
+    ...         print('pre-hook:', counter)
+    ...         counter += 1
+    ...
+    ...     def post_hook(self):
+    ...         global counter
+    ...         print('post-hook:', counter)
+    ...         counter += 1
+    ...
+    ...     def rest_object(self):
+    ...         pass
+    ... """, file=fp)
+
+Running the hooks
+-----------------
+
+We can set the plugin class in the config file.
+::
+
+    >>> config_path = os.path.join(config_directory, 'hooks.cfg')
+    >>> with open(config_path, 'w') as fp:
+    ...     print("""\
+    ... [meta]
+    ... extends: test.cfg
+    ...
+    ... [plugin.hook_example]
+    ... class: hooks.ExamplePlugin
+    ... enable: yes
+    ... """, 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(cfg_path, python_path):
+    ...     exe = os.path.join(os.path.dirname(sys.executable), 'mailman')
+    ...     env = os.environ.copy()
+    ...     env.update(
+    ...         MAILMAN_CONFIG_FILE=cfg_path,
+    ...         PYTHONPATH=python_path,
+    ...         )
+    ...     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)
+    ...     print(stderr)
+
+    >>> call(config_path, config_directory)
+    pre-hook: 1
+    post-hook: 2
+    <BLANKLINE>
+    <BLANKLINE>
+
+    >>> os.remove(config_path)
+
+Deprecated hooks
+----------------
+
+The old-style `pre_hook` and `post_hook` callables are deprecated and are no
+longer called upon startup.
+::
+
+    >>> deprecated_hook_path = os.path.join(config_directory, 'deprecated_hooks.py')
+    >>> with open(deprecated_hook_path, 'w') as fp:
+    ...     print("""\
+    ... def do_something():
+    ...     print("does something")
+    ...
+    ... def do_something_else():
+    ...     print("does something else")
+    ...
+    ... """, file=fp)
+
+    >>> deprecated_config_path = os.path.join(config_directory, 'deprecated.cfg')
+    >>> with open(deprecated_config_path, 'w') as fp:
+    ...     print("""\
+    ... [meta]
+    ... extends: test.cfg
+    ...
+    ... [mailman]
+    ... pre_hook: deprecated_hooks.do_something
+    ... post_hook: deprecated_hooks.do_something_else
+    ... """, file=fp)
+
+    >>> call(deprecated_config_path, config_directory)
+    does something
+    does something else
+    <BLANKLINE>
+    ... UserWarning: The pre_hook configuration value has been replaced by the plugins infrastructure. ...
+    ... UserWarning: The post_hook configuration value has been replaced by the plugins infrastructure. ...
+    <BLANKLINE>
+
+    >>> os.remove(deprecated_config_path)
diff --git a/src/mailman/bin/mailman.py b/src/mailman/bin/mailman.py
index e012237f3..f8006218d 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
@@ -25,7 +24,7 @@ from mailman.core.i18n import _
 from mailman.core.initialize import initialize
 from mailman.database.transaction import transaction
 from mailman.interfaces.command import ICLISubCommand
-from mailman.utilities.modules import add_components
+from mailman.utilities.plugins import add_pluggable_components
 from mailman.version import MAILMAN_VERSION_FULL
 from public import public
 
@@ -35,17 +34,22 @@ 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 pluggable commands need a parsed config to
+        # find plugins.
+        if not self._loaded:
+            add_pluggable_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:
@@ -83,10 +87,12 @@ class Subcommands(click.MultiCommand):
                 formatter.write_dl(opts)
 
 
-@click.group(
-    cls=Subcommands,
-    context_settings=dict(help_option_names=['-h', '--help']))
-@click.pass_context
+def initialize_config(ctx, param, value):
+    if ctx.resilient_parsing:  # pragma: nocover
+        return
+    initialize(value)
+
+
 @click.option(
     '-C', '--config', 'config_file',
     envvar='MAILMAN_CONFIG_FILE',
@@ -94,7 +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):
@@ -104,6 +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..ccaa16398 100644
--- a/src/mailman/bin/master.py
+++ b/src/mailman/bin/master.py
@@ -188,6 +188,9 @@ 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
@@ -402,6 +405,10 @@ class Loop:
             except InterruptedError:                # pragma: nocover
                 # If the system call got interrupted, just restart it.
                 continue
+            if pid not in self._kids:  # pragma: nocover
+                # Not a runner subprocess, maybe a plugin started one
+                # ignore 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/tests/test_mailman.py b/src/mailman/bin/tests/test_mailman.py
index 54ee54bce..196437291 100644
--- a/src/mailman/bin/tests/test_mailman.py
+++ b/src/mailman/bin/tests/test_mailman.py
@@ -27,6 +27,7 @@ from mailman.config import config
 from mailman.database.transaction import transaction
 from mailman.testing.layers import ConfigLayer
 from mailman.utilities.datetime import now
+from pkg_resources import resource_filename
 from unittest.mock import patch
 
 
@@ -36,7 +37,19 @@ 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)
@@ -46,7 +59,8 @@ class TestMailmanCommand(unittest.TestCase):
         # command line.
         self.assertEqual(lines[0], 'Usage: main [OPTIONS] COMMAND [ARGS]...')
 
-    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/docs/conf.rst b/src/mailman/commands/docs/conf.rst
index 1a6a4679d..9981b71ed 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.plugin] path: plugin.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/config/config.py b/src/mailman/config/config.py
index 61c9fe6ed..a5e3ed3a7 100644
--- a/src/mailman/config/config.py
+++ b/src/mailman/config/config.py
@@ -76,6 +76,7 @@ class Configuration:
         self.handlers = {}
         self.pipelines = {}
         self.commands = {}
+        self.plugins = {}
         self.password_context = None
         self.db = None
 
@@ -158,7 +159,7 @@ class Configuration:
                 else category.template_dir),
             )
         # Directories.
-        for name in ('archive', 'bin', 'cache', 'data', 'etc', 'ext',
+        for name in ('archive', 'bin', 'cache', 'data', 'etc',
                      'list_data', 'lock', 'log', 'messages', 'queue'):
             key = '{}_dir'.format(name)
             substitutions[key] = getattr(category, key)
@@ -248,6 +249,13 @@ 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:
+            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..742bceae1 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 +74,38 @@ html_to_plain_text_command: /usr/bin/lynx -dump $filename
 # unpredictable.
 listname_chars: [-_.0-9a-z]
 
+# Deprecated, callable run before DB initialization.
+pre_hook:
+
+# Deprecated, callable run after DB initialization.
+post_hook:
+
+
+[plugin.master]
+# Plugin package
+# It's sub packages will be searched for components.
+#   - commands for IEmailCommand and ICliSubCommand
+#   - chains for IChain
+#   - rules for IRule
+#   - pipelines for IPipeline
+#   - handlers for IHandler
+#   - styles for IStyle
+path:
+
+# The full import path for the class implementing IPlugin.
+class:
+
+# Whether to enable this plugin or not.
+enable: no
+
+# Additional config for this plugin. The path can be either a file system path
+# or a Python import path.  If the value starts with python: then it is
+# a Python import path, otherwise it is a file system path.  File system paths
+# must be absolute since no guarantees are made about the current working
+# directory.  Python paths should not include the trailing .cfg, which the file
+# must end with.
+configuration:
+
 
 [shell]
 # `mailman shell` (also `withlist`) gives you an interactive prompt that you
@@ -138,8 +162,6 @@ 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 +283,7 @@ debug: no
 # - http            --  Internal wsgi-based web interface
 # - locks           --  Lock state changes
 # - mischief        --  Various types of hostile activity
+# - plugin          --  Plugin logs
 # - runner          --  Runner process start/stops
 # - smtp            --  Successful SMTP activity
 # - smtp-failure    --  Unsuccessful SMTP activity
@@ -298,6 +321,9 @@ level: info
 
 [logging.mischief]
 
+[logging.plugin]
+path: plugin.log
+
 [logging.runner]
 
 [logging.smtp]
@@ -797,11 +823,6 @@ 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..48ed82d63 100644
--- a/src/mailman/core/chains.py
+++ b/src/mailman/core/chains.py
@@ -19,7 +19,7 @@
 
 from mailman.config import config
 from mailman.interfaces.chain import IChain, LinkAction
-from mailman.utilities.modules import add_components
+from mailman.utilities.plugins import add_pluggable_components
 from public import public
 
 
@@ -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_pluggable_components('chains', IChain, config.chains)
diff --git a/src/mailman/core/initialize.py b/src/mailman/core/initialize.py
index dc67e9a68..830bd9b84 100644
--- a/src/mailman/core/initialize.py
+++ b/src/mailman/core/initialize.py
@@ -26,9 +26,12 @@ by the command line arguments.
 
 import os
 import sys
+import warnings
+import traceback
 import mailman.config.config
 import mailman.core.logging
 
+from mailman.core.i18n import _
 from mailman.interfaces.database import IDatabaseFactory
 from mailman.utilities.modules import call_name
 from pkg_resources import resource_string as resource_bytes
@@ -133,7 +136,7 @@ def initialize_2(debug=False, propagate_logs=None, testing=False):
 
     * Database
     * Logging
-    * Pre-hook
+    * Plugin's pre_hook
     * Rules
     * Chains
     * Pipelines
@@ -146,10 +149,34 @@ 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.core.plugins import initialize as initialize_plugins
+    initialize_plugins()
+    # Check for deprecated features in config.
     config = mailman.config.config
-    if config.mailman.pre_hook:
+    if config.mailman.pre_hook:  # pragma: nocover
+        warnings.warn(
+            _('The pre_hook configuration value has been replaced by the '
+              'plugins infrastructure.'), UserWarning)
         call_name(config.mailman.pre_hook)
+    # Run the plugin pre_hooks, if one fails, disable the offending plugin.
+    for name in list(config.plugins.keys()):
+        plugin = config.plugins[name]
+        try:
+            plugin.pre_hook()
+        except:   # pragma: nocover
+            traceback.print_exc()
+            warnings.warn(_('Plugin $name failed to run its pre_hook,'
+                            'it will be disabled and its components'
+                            'wont be loaded.'), RuntimeWarning)
+            # It failed, push disabling overlay to config. This will stop
+            # components from being loaded.
+            config.push(name + '_error', """\
+[plugin.{}]
+enable: no
+            """.format(name))
+            # And forget about it. This will stop running its post_hook.
+            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 +198,23 @@ def initialize_2(debug=False, propagate_logs=None, testing=False):
 def initialize_3():
     """Third initialization step.
 
-    * Post-hook
+    * Plugin's post_hook
     """
-    # Run the post-hook if there is one.
+    # Run the plugin post_hooks
     config = mailman.config.config
-    if config.mailman.post_hook:
+    if config.mailman.post_hook:  # pragma: nocover
+        warnings.warn(
+            _('The post_hook configuration value has been replaced by the '
+              'plugins infrastructure.'), UserWarning)
         call_name(config.mailman.post_hook)
+    for plugin in config.plugins.values():
+        try:
+            plugin.post_hook()
+        except:   # 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.
+            pass
 
 
 @public
diff --git a/src/mailman/core/pipelines.py b/src/mailman/core/pipelines.py
index 4265d24d1..6e2ad4a27 100644
--- a/src/mailman/core/pipelines.py
+++ b/src/mailman/core/pipelines.py
@@ -24,7 +24,7 @@ from mailman.config import config
 from mailman.interfaces.handler import IHandler
 from mailman.interfaces.pipeline import (
     DiscardMessage, IPipeline, RejectMessage)
-from mailman.utilities.modules import add_components
+from mailman.utilities.plugins import add_pluggable_components
 from public import public
 
 
@@ -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_pluggable_components('handlers', IHandler, config.handlers)
     # Set up some pipelines.
-    add_components('mailman.pipelines', IPipeline, config.pipelines)
+    add_pluggable_components('pipelines', IPipeline, config.pipelines)
diff --git a/src/mailman/core/plugins.py b/src/mailman/core/plugins.py
new file mode 100644
index 000000000..e891f380d
--- /dev/null
+++ b/src/mailman/core/plugins.py
@@ -0,0 +1,41 @@
+# Copyright (C) 2008-2017 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/>.
+
+"""Various plugin helpers."""
+
+from lazr.config import as_boolean
+from mailman.config import config
+from mailman.interfaces.plugin import IPlugin
+from mailman.utilities.modules import find_name
+from public import public
+from zope.interface.verify import verifyObject
+
+
+@public
+def initialize():
+    """Initialize all enabled plugins."""
+    for name, plugin_config in config.plugin_configs:
+        plugin_class_path = plugin_config['class']
+        if as_boolean(plugin_config.enable) and plugin_class_path:
+            plugin_class = find_name(plugin_class_path)
+            plugin = plugin_class()
+            verifyObject(IPlugin, plugin)
+            plugin.name = name
+            assert plugin.name not in config.plugins, (
+                'Duplicate plugin "{}" found in {}'.format(
+                    plugin.name, plugin_class))
+            config.plugins[plugin.name] = plugin
diff --git a/src/mailman/core/rules.py b/src/mailman/core/rules.py
index 8e0d9197c..74934eb99 100644
--- a/src/mailman/core/rules.py
+++ b/src/mailman/core/rules.py
@@ -19,7 +19,7 @@
 
 from mailman.config import config
 from mailman.interfaces.rules import IRule
-from mailman.utilities.modules import add_components
+from mailman.utilities.plugins import add_pluggable_components
 from public import public
 
 
@@ -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_pluggable_components('rules', IRule, config.rules)
diff --git a/src/mailman/interfaces/plugin.py b/src/mailman/interfaces/plugin.py
new file mode 100644
index 000000000..f1b7212f0
--- /dev/null
+++ b/src/mailman/interfaces/plugin.py
@@ -0,0 +1,38 @@
+# Copyright (C) 2007-2017 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 Interface
+
+
+@public
+class IPlugin(Interface):
+    """A plugin providing components and hooks."""
+
+    def pre_hook():
+        """A plugin hook called in the first initialization step. Before DB."""
+
+    def post_hook():
+        """A plugin hook called in the second initialization step. After DB."""
+
+    def rest_object():
+        """Return the REST object which will be hooked up to the REST api.
+
+        This object will be served at the /<api>/plugins/<plugin.name>/ root.
+        """
diff --git a/src/mailman/rest/docs/plugins.rst b/src/mailman/rest/docs/plugins.rst
new file mode 100644
index 000000000..737a911c0
--- /dev/null
+++ b/src/mailman/rest/docs/plugins.rst
@@ -0,0 +1,60 @@
+=========
+ Plugins
+=========
+
+Plugins can supply REST routes.
+::
+
+    >>> dump_json('http://localhost:9001/3.1/plugins')
+    entry 0:
+        class: mailman.testing.plugin.ExamplePlugin
+        configuration:
+        enable: True
+        http_etag: "..."
+        name: example
+        path:
+    http_etag: "..."
+    start: 0
+    total_size: 1
+
+    >>> dump_json('http://localhost:9001/3.1/plugins/example')
+    example-plugin-reply: yes
+    http_etag: "..."
+
+    >>> dump_json('http://localhost:9001/3.1/plugins/example/good')
+    good: True
+    http_etag: "..."
+
+    >>> dump_json('http://localhost:9001/3.1/plugins/example/bad')
+    Traceback (most recent call last):
+    ...
+    urllib.error.HTTPError: HTTP Error 400: ...
+
+For example this route that presents a counter.
+::
+
+    >>> dump_json('http://localhost:9001/3.1/plugins/example/count')
+    count: 0
+    http_etag: "..."
+
+    >>> call_http('http://localhost:9001/3.1/plugins/example/count',
+    ...           {'count':3}, method='POST')
+    content-length: 0
+    date: ...
+    server: ...
+    status: 204
+
+    >>> dump_json('http://localhost:9001/3.1/plugins/example/count')
+    count: 3
+    http_etag: "..."
+
+    >>> call_http('http://localhost:9001/3.1/plugins/example/count',
+    ...           method='DELETE')
+    content-length: 0
+    date: ...
+    server: ...
+    status: 204
+
+    >>> dump_json('http://localhost:9001/3.1/plugins/example/count')
+    count: 0
+    http_etag: "..."
diff --git a/src/mailman/rest/plugins.py b/src/mailman/rest/plugins.py
new file mode 100644
index 000000000..fa2605f24
--- /dev/null
+++ b/src/mailman/rest/plugins.py
@@ -0,0 +1,70 @@
+# 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 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'],
+            'enable': as_boolean(plugin_section['enable']),
+            'path': plugin_section['path'],
+            'configuration': plugin_section['configuration']
+                    }
+        return resource
+
+    def _get_collection(self, request):
+        """See `CollectionMixin`."""
+        return sorted(config.plugin_configs)
+
+    def on_get(self, request, response):
+        """/plugins"""
+        resource = self._make_collection(request)
+        okay(response, etag(resource))
+
+
+@public
+class APlugin:
+    """REST proxy to the plugins rest_object."""
+
+    def __init__(self, plugin_name):
+        self.__rest_object = None
+        if plugin_name in config.plugins.keys():
+            plugin = config.plugins[plugin_name]
+            self.__rest_object = plugin.rest_object()
+        # If the plugin doesn't exist or doesn't provide a rest_object,
+        # just proxy to NotFound
+        if self.__rest_object is None:
+            self.__rest_object = NotFound()
+
+    def __getattr__(self, attrib):
+        return getattr(self.__rest_object, attrib)
+
+    def __dir__(self):
+        return dir(self.__rest_object)
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_plugins.py b/src/mailman/rest/tests/test_plugins.py
new file mode 100644
index 000000000..9c12e1160
--- /dev/null
+++ b/src/mailman/rest/tests/test_plugins.py
@@ -0,0 +1,105 @@
+# 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/>.
+
+"""Test RESTability of plugins."""
+
+import unittest
+
+from mailman.testing.helpers import call_api
+from mailman.testing.layers import RESTLayer
+from urllib.error import HTTPError
+
+
+class TestAPI31Plugins(unittest.TestCase):
+    layer = RESTLayer
+
+    def test_list_plugins(self):
+        json, response = call_api('http://localhost:9001/3.1/plugins')
+        self.assertEqual(response.status_code, 200)
+        json.pop('http_etag')
+        self.assertEqual(len(json.keys()), 3)
+        self.assertIn('entries', json.keys())
+        entries = json['entries']
+        self.assertEqual(len(entries), 1)
+        plugin_conf = entries[0]
+        self.assertEqual(plugin_conf['name'], 'example')
+        self.assertEqual(plugin_conf['enable'], True)
+        self.assertEqual(plugin_conf['class'],
+                         'mailman.testing.plugin.ExamplePlugin')
+        self.assertEqual(plugin_conf['path'], '')
+        self.assertEqual(plugin_conf['configuration'], '')
+
+    def test_non_existent_plugin(self):
+        with self.assertRaises(HTTPError) as cm:
+            call_api('http://localhost:9001/3.1/plugins/does-not-exist')
+        self.assertEqual(cm.exception.code, 404)
+
+    def test_example_plugin(self):
+        json, response = call_api('http://localhost:9001/3.1/plugins/example')
+        self.assertEqual(response.status_code, 200)
+        json.pop('http_etag')
+        self.assertEqual(len(json.keys()), 1)
+        self.assertEqual(json['example-plugin-reply'], 'yes')
+
+        with self.assertRaises(HTTPError) as cm:
+            call_api('http://localhost:9001/3.1/plugins/example',
+                     method='POST')
+        self.assertEqual(cm.exception.code, 400)
+
+    def test_example_plugin_child(self):
+        json, response = \
+            call_api('http://localhost:9001/3.1/plugins/example/good')
+        self.assertEqual(response.status_code, 200)
+        json.pop('http_etag')
+        self.assertEqual(len(json.keys()), 1)
+        self.assertEqual(json['good'], True)
+
+        with self.assertRaises(HTTPError) as cm:
+            call_api('http://localhost:9001/3.1/plugins/example/bad')
+        self.assertEqual(cm.exception.code, 400)
+
+    def test_example_plugin_counter(self):
+        json, response = \
+            call_api('http://localhost:9001/3.1/plugins/example/count')
+        self.assertEqual(response.status_code, 200)
+        json.pop('http_etag')
+        self.assertEqual(len(json.keys()), 1)
+        self.assertEqual(json['count'], 0)
+
+        json, response = \
+            call_api('http://localhost:9001/3.1/plugins/example/count',
+                     method='POST', data={'count': 5})
+        self.assertEqual(response.status_code, 204)
+
+        json, response = \
+            call_api('http://localhost:9001/3.1/plugins/example/count')
+        self.assertEqual(response.status_code, 200)
+        json.pop('http_etag')
+        self.assertEqual(len(json.keys()), 1)
+        self.assertEqual(json['count'], 5)
+
+        json, response = \
+            call_api('http://localhost:9001/3.1/plugins/example/count',
+                     method='DELETE')
+        self.assertEqual(response.status_code, 204)
+
+        json, response = \
+            call_api('http://localhost:9001/3.1/plugins/example/count')
+        self.assertEqual(response.status_code, 200)
+        json.pop('http_etag')
+        self.assertEqual(len(json.keys()), 1)
+        self.assertEqual(json['count'], 0)
diff --git a/src/mailman/rest/tests/test_systemconf.py b/src/mailman/rest/tests/test_systemconf.py
index f9fe9caa0..59f0619a3 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.plugin',
             'logging.root',
             'logging.runner',
             'logging.smtp',
@@ -182,6 +183,7 @@ class TestSystemConfiguration(unittest.TestCase):
             'paths.here',
             'paths.local',
             'paths.testing',
+            'plugin.example',
             'runner.archive',
             'runner.bad',
             'runner.bounces',
diff --git a/src/mailman/styles/manager.py b/src/mailman/styles/manager.py
index 1ec83b2a9..1969d4484 100644
--- a/src/mailman/styles/manager.py
+++ b/src/mailman/styles/manager.py
@@ -20,7 +20,7 @@
 from mailman.interfaces.configuration import ConfigurationUpdatedEvent
 from mailman.interfaces.styles import (
     DuplicateStyleError, IStyle, IStyleManager)
-from mailman.utilities.modules import add_components
+from mailman.utilities.plugins import add_pluggable_components
 from public import public
 from zope.component import getUtility
 from zope.interface import implementer
@@ -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_pluggable_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..421a272f4 100644
--- a/src/mailman/testing/documentation.py
+++ b/src/mailman/testing/documentation.py
@@ -109,6 +109,13 @@ 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 +147,9 @@ 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))
 
diff --git a/src/mailman/testing/plugin.py b/src/mailman/testing/plugin.py
new file mode 100644
index 000000000..a8b001406
--- /dev/null
+++ b/src/mailman/testing/plugin.py
@@ -0,0 +1,101 @@
+# Copyright (C) 2009-2017 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/>.
+
+"""Example plugin with hooks and REST for testing purposes."""
+
+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 GoodREST:
+
+    def on_get(self, request, response):
+        okay(response, etag({'good': True}))
+
+
+@public
+class BadREST:
+
+    def on_get(self, request, response):
+        bad_request(response, etag({'good': False}))
+
+
+@public
+class CountingREST:
+
+    def __init__(self):
+        self._plugin = config.plugins['example']
+
+    def on_get(self, request, response):
+        okay(response, etag({'count': self._plugin.counter}))
+
+    def on_post(self, request, response):
+        try:
+            counter = Validator(count=int)(request)
+            self._plugin.counter = counter['count']
+        except ValueError as error:
+            bad_request(response, str(error))
+        else:
+            no_content(response)
+
+    def on_delete(self, request, response):
+        self._plugin.counter = 0
+        no_content(response)
+
+
+@public
+class RESTExample:
+
+    def on_get(self, request, response):
+        okay(response, etag({'example-plugin-reply': 'yes'}))
+
+    def on_post(self, request, response):
+        bad_request(response)
+
+    @child()
+    def good(self, context, segments):
+        return GoodREST(), []
+
+    @child()
+    def bad(self, context, segments):
+        return BadREST(), []
+
+    @child()
+    def count(self, context, segments):
+        return CountingREST(), []
+
+
+@public
+@implementer(IPlugin)
+class ExamplePlugin:
+
+    def __init__(self):
+        self.counter = 0
+
+    def pre_hook(self):
+        pass
+
+    def post_hook(self):
+        pass
+
+    def rest_object(self):
+        return RESTExample()
diff --git a/src/mailman/testing/testing.cfg b/src/mailman/testing/testing.cfg
index 94b4c07b9..9e17015ef 100644
--- a/src/mailman/testing/testing.cfg
+++ b/src/mailman/testing/testing.cfg
@@ -28,6 +28,10 @@ incoming: mailman.testing.mta.FakeMTA
 [passwords]
 configuration: python:mailman.testing.passlib
 
+[plugin.example]
+enable: yes
+class: mailman.testing.plugin.ExamplePlugin
+
 [webservice]
 port: 9001
 
diff --git a/src/mailman/utilities/plugins.py b/src/mailman/utilities/plugins.py
new file mode 100644
index 000000000..aad909056
--- /dev/null
+++ b/src/mailman/utilities/plugins.py
@@ -0,0 +1,76 @@
+# Copyright (C) 2009-2017 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/>.
+
+"""Plugin utilities."""
+
+from lazr.config import as_boolean
+from mailman.config import config
+from mailman.utilities.modules import find_components
+from pkg_resources import resource_isdir
+from public import public
+
+
+@public
+def find_pluggable_components(subpackage, interface):
+    """Find components which conform to a given interface, in subpackage.
+
+    :param subpackage: Mailman subpackage to search. The search is also
+    done inside the corresponding plugins subpackages.
+    :type subpackage: string
+    :param interface: The interface that returned objects must conform to.
+    :type interface: `Interface`
+    :return: The sequence of matching components.
+    :rtype: objects implementing `interface`
+    """
+    yield from find_components('mailman.' + subpackage, interface)
+    package_roots = [plugin.path
+                     for name, plugin in config.plugin_configs
+                     if as_boolean(plugin.enable) and plugin.path]
+    for package_root in package_roots:
+        package = package_root + '.' + subpackage
+        if resource_isdir(package_root, subpackage):
+            yield from find_components(package, interface)
+
+
+@public
+def add_pluggable_components(subpackage, interface, mapping):
+    """Add components to a given mapping.
+
+    Similarly to `find_pluggable_components()` this inspects all modules in a
+    given mailman and plugin's subpackage 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.
+
+    :param subpackage: The subpackage path to search.
+    :type subpackage: string
+    :param interface: The interface that returned objects must conform to.
+        Objects found must have a `.name` attribute containing a unique
+        string.
+    :type interface: `Interface`
+    :param mapping: The mapping to add the found components to.
+    :type mapping: A dict-like mapping.  This only needs to support
+        containment tests (e.g. `in` and `not in`) and `__setitem__()`.
+    :raises RuntimeError: when a duplicate key is found.
+    """
+    for component in find_pluggable_components(subpackage, interface):
+        if component.name in mapping:
+            raise RuntimeError(     # pragma: nocover
+                'Duplicate key "{}" found in {}; previously {}'.format(
+                    component.name, component, mapping[component.name]))
+        mapping[component.name] = component