Add --required-plugins and --allowed-pluginsplugin-loading

Closes #283
Closes #488

6 files changed, 172 insertions, 7 deletions

diff --git a/src/flake8/api/legacy.py b/src/flake8/api/legacy.py
index f80cb3d..3f1a0f7 100644
--- a/src/flake8/api/legacy.py
+++ b/src/flake8/api/legacy.py
@@ -38,7 +38,7 @@ def get_style_guide(**kwargs):
         ignore_config_files=prelim_opts.isolated,
     )
 
-    application.find_plugins(config_finder)
+    application.find_plugins(config_finder, prelim_opts)
     application.register_plugin_options()
     application.parse_configuration_and_cli(
         config_finder,
diff --git a/src/flake8/exceptions.py b/src/flake8/exceptions.py
index 45db94d..14b50c9 100644
--- a/src/flake8/exceptions.py
+++ b/src/flake8/exceptions.py
@@ -1,5 +1,6 @@
 """Exception classes for all of Flake8."""
 from typing import Dict
+from typing import List
 
 
 class Flake8Exception(Exception):
@@ -69,3 +70,24 @@ class PluginExecutionFailed(Flake8Exception):
             "name": self.plugin["plugin_name"],
             "exc": self.original_exception,
         }
+
+
+class PluginMissingError(Flake8Exception):
+    """A plugin that was required was not found."""
+
+    FORMAT = "User required %(plugins)s but %(missing)s was not found."
+
+    def __init__(
+        self, required_plugins: List[str], missing_plugins: List[str]
+    ) -> None:
+        """Store the information passed in to format the exception message."""
+        self.required_plugins = required_plugins
+        self.missing_plugins = missing_plugins
+        super().__init__(required_plugins, missing_plugins)
+
+    def __str__(self) -> str:
+        """Format our exception message."""
+        return self.FORMAT % {
+            "plugins": ", ".join(self.required_plugins),
+            "missing": ", ".join(self.missing_plugins),
+        }
diff --git a/src/flake8/main/application.py b/src/flake8/main/application.py
index 44a5524..c7912d7 100644
--- a/src/flake8/main/application.py
+++ b/src/flake8/main/application.py
@@ -141,7 +141,11 @@ class Application:
                 (self.result_count > 0) or self.catastrophic_failure
             )
 
-    def find_plugins(self, config_finder: config.ConfigFileFinder) -> None:
+    def find_plugins(
+        self,
+        config_finder: config.ConfigFileFinder,
+        prelim_opts: argparse.Namespace,
+    ) -> None:
         """Find and load the plugins for this application.
 
         Set the :attr:`check_plugins` and :attr:`formatting_plugins` attributes
@@ -149,8 +153,16 @@ class Application:
 
         :param config.ConfigFileFinder config_finder:
             The finder for finding and reading configuration files.
+        :param argparse.Namespace prelim_opts:
+            The options parsed preliminarily from the CLI
         """
         local_plugins = config.get_local_plugins(config_finder)
+        (
+            allowed_plugins,
+            required_plugins,
+        ) = config.get_plugin_allowlist_and_requirements(
+            config_finder, prelim_opts
+        )
 
         sys.path.extend(local_plugins.paths)
 
@@ -160,7 +172,11 @@ class Application:
             local_plugins.report
         )
 
-        self.check_plugins.load_plugins()
+        try:
+            self.check_plugins.load_plugins(allowed_plugins, required_plugins)
+        except exceptions.PluginMissingError as e:
+            print(f"Error: {e!s}")
+            raise SystemExit(2)
         self.formatting_plugins.load_plugins()
 
     def register_plugin_options(self) -> None:
@@ -340,7 +356,7 @@ class Application:
             config_file=prelim_opts.config,
             ignore_config_files=prelim_opts.isolated,
         )
-        self.find_plugins(config_finder)
+        self.find_plugins(config_finder, prelim_opts)
         self.register_plugin_options()
         self.parse_configuration_and_cli(
             config_finder,
diff --git a/src/flake8/main/options.py b/src/flake8/main/options.py
index c35dbc6..bc2464c 100644
--- a/src/flake8/main/options.py
+++ b/src/flake8/main/options.py
@@ -59,6 +59,23 @@ def register_preliminary_options(parser: argparse.ArgumentParser) -> None:
         help="Ignore all configuration files.",
     )
 
+    add_argument(
+        "--allowed-plugins",
+        default=None,
+        # parse_from_config=True,
+        # comma_separated_list=True,
+        help="Which plugins are allowed to run from the environment",
+    )
+
+    add_argument(
+        "--required-plugins",
+        default=None,
+        # parse_from_config=True,
+        # comma_separated_list=True,
+        help="Which plugins are required for linting. Exits if not all are "
+        "present.",
+    )
+
 
 class JobsArgument:
     """Type callback for the --jobs argument."""
diff --git a/src/flake8/options/config.py b/src/flake8/options/config.py
index fc3b205..a2365c5 100644
--- a/src/flake8/options/config.py
+++ b/src/flake8/options/config.py
@@ -1,4 +1,5 @@
 """Config handling logic for Flake8."""
+import argparse
 import collections
 import configparser
 import logging
@@ -273,7 +274,7 @@ def get_local_plugins(config_finder):
     if config_finder.ignore_config_files:
         LOG.debug(
             "Refusing to look for local plugins in configuration"
-            "files due to user-requested isolation"
+            " files due to user-requested isolation"
         )
         return local_plugins
 
@@ -315,4 +316,87 @@ def get_local_plugins(config_finder):
     return local_plugins
 
 
+def get_plugin_allowlist_and_requirements(
+    config_finder: ConfigFileFinder, preliminary_opts: argparse.Namespace
+) -> Tuple[List[str], List[str]]:
+    """Get allowed and required plugin lists from config.
+
+    :param config_finder:
+        The config file finder to use.
+    :type config_finder:
+        :class:`~flake8.options.config.ConfigFileFinder`
+    :param preliminary_opts:
+        The config file finder to use.
+    :type preliminary_opts:
+        :class:`~argparse.Namespace`
+    :returns:
+        tuple of the allowed and required plugin lists
+    """
+    read_allowed_plugins_from_config = True
+    read_required_plugins_from_config = True
+    allowed_plugins: List[str] = []
+    required_plugins: List[str] = []
+    return_tuple: Tuple[List[str], List[str]] = (
+        allowed_plugins,
+        required_plugins,
+    )
+
+    if preliminary_opts.allowed_plugins is not None:
+        allowed_plugins.extend(
+            utils.parse_comma_separated_list(preliminary_opts.allowed_plugins)
+        )
+        read_allowed_plugins_from_config = False
+
+    if preliminary_opts.required_plugins is not None:
+        required_plugins.extend(
+            utils.parse_comma_separated_list(preliminary_opts.required_plugins)
+        )
+        read_required_plugins_from_config = False
+
+    if config_finder.ignore_config_files:
+        LOG.debug(
+            "Refusing to look for plugin configuration in configuration"
+            " files due to user-requested isolation"
+        )
+        return return_tuple
+
+    if (
+        not read_allowed_plugins_from_config
+        and not read_required_plugins_from_config
+    ):
+        LOG.debug("Found --allowed-plugins and --required-plugins")
+        return return_tuple
+
+    if config_finder.config_file:
+        LOG.debug(
+            'Reading local plugins only from "%s" specified via '
+            "--config by the user",
+            config_finder.config_file,
+        )
+        config = config_finder.cli_config(config_finder.config_file)
+        config_files = [config_finder.config_file]
+    else:
+        config, config_files = config_finder.local_configs_with_files()
+
+    section = f"{config_finder.program_name}"
+    if (
+        config.has_option(section, "allowed-plugins")
+        and read_allowed_plugins_from_config
+    ):
+        allowed_plugins_str = config.get(section, "allowed-plugins").strip()
+        allowed_plugins.extend(
+            utils.parse_comma_separated_list(allowed_plugins_str)
+        )
+    if (
+        config.has_option(section, "required-plugins")
+        and read_required_plugins_from_config
+    ):
+        required_plugins_str = config.get(section, "required-plugins").strip()
+        required_plugins.extend(
+            utils.parse_comma_separated_list(required_plugins_str)
+        )
+
+    return return_tuple
+
+
 LocalPlugins = collections.namedtuple("LocalPlugins", "extension report paths")
diff --git a/src/flake8/plugins/manager.py b/src/flake8/plugins/manager.py
index 840bf65..eba259b 100644
--- a/src/flake8/plugins/manager.py
+++ b/src/flake8/plugins/manager.py
@@ -1,6 +1,7 @@
 """Plugin loading and management logic and classes."""
 import logging
 from typing import Any
+from typing import cast
 from typing import Dict
 from typing import List
 from typing import Optional
@@ -410,13 +411,38 @@ class PluginTypeManager:
 
         return generated_function
 
-    def load_plugins(self):
-        """Load all plugins of this type that are managed by this manager."""
+    def load_plugins(
+        self,
+        allowed_plugins: Optional[List[str]] = None,
+        required_plugins: Optional[List[str]] = None,
+    ) -> None:
+        """Load all plugins of this type that are managed by this manager.
+
+        :param allowed_plugins:
+            This is the list of plugins allowed to be loaded
+        :param required_plugins:
+            This is the list of plugins required by the user
+        :raises PluginMissingError:
+            If a required plugin is missing
+        """
         if self.plugins_loaded:
             return
 
+        requireset = set(required_plugins or [])
+        allowset = set(allowed_plugins or [])
+        loaded = set()
         for plugin in self.plugins.values():
+            if allowset and plugin.name not in allowset:
+                continue
             plugin.load_plugin()
+            loaded.add(plugin.name)
+
+        missing = requireset.difference(loaded)
+        if requireset and missing:
+            required_plugins = cast(List[str], required_plugins)
+            raise exceptions.PluginMissingError(
+                required_plugins, sorted(missing)
+            )
 
         # Do not set plugins_loaded if we run into an exception
         self.plugins_loaded = True