bots: Find external packaged bots via 'zulip_bots.registry' entry_point.

Now we will be able to execute `zulip-run-bot` with the `-r` argument
to search for and run bots from the `zulip_bots.registry` entry_point.

Each entry point should have the name correspond to the bot name,
and have the value be the bot module. E.g, an Python package for a
bot called "packaged_bot" should have an `entry_points` setup like
the following:

setup(
    ...
    entry_points={
        "zulip_bot.registry":[
            "packaged_bot=packaged_bot.packaged_bot"
        ]
    }
    ...
)

whose file structure may look like this:

packaged_bot/
├───packaged_bot/
|   ├───packaged_bot.py  # The bot module
|   ├───test_packaged_bot.py
|   ├───packaged_bot.conf
|   └───doc.md
└───setup.py  # Register the entry points here

Add test case.

zulip_bots/setup.py

@@ -66,6 +66,7 @@ setuptools_info = dict(
         "lxml",
         "BeautifulSoup4",
         "typing_extensions",
+        'importlib-metadata >= 3.6; python_version  < "3.10"',
     ],
 )
 

zulip_bots/zulip_bots/finder.py

@@ -3,10 +3,13 @@ import importlib.abc
 import importlib.util
 import os
 from pathlib import Path
+from types import ModuleType
 from typing import Any, Optional, Tuple
 
 current_dir = os.path.dirname(os.path.abspath(__file__))
 
+import importlib_metadata as metadata
+
 
 def import_module_from_source(path: str, name: str) -> Any:
     spec = importlib.util.spec_from_file_location(name, path)
@@ -25,6 +28,28 @@ def import_module_by_name(name: str) -> Any:
         return None
 
 
+class DuplicateRegisteredBotName(Exception):
+    pass
+
+
+def import_module_from_zulip_bot_registry(name: str) -> Optional[ModuleType]:
+    # Prior to Python 3.10, calling importlib.metadata.entry_points returns a
+    # SelectableGroups object when no parameters is given. Currently we use
+    # the importlib_metadata library for compatibility, but we need to migrate
+    # to the built-in library when we start to adapt Python 3.10.
+    # https://importlib-metadata.readthedocs.io/en/latest/using.html#entry-points
+    registered_bots = metadata.entry_points(group="zulip_bots.registry")
+    matching_bots = [bot for bot in registered_bots if bot.name == name]
+
+    if len(matching_bots) == 1:  # Unique matching entrypoint
+        return matching_bots[0].load()
+
+    if len(matching_bots) > 1:
+        raise DuplicateRegisteredBotName(name)
+
+    return None  # no matches in registry
+
+
 def resolve_bot_path(name: str) -> Optional[Tuple[Path, str]]:
     if os.path.isfile(name):
         bot_path = Path(name)

zulip_bots/zulip_bots/run.py

@@ -48,6 +48,13 @@ def parse_args() -> argparse.Namespace:
         help="try running the bot even if dependencies install fails",
     )
 
+    parser.add_argument(
+        "--registry",
+        "-r",
+        action="store_true",
+        help="run the bot via zulip_bots registry",
+    )
+
     parser.add_argument("--provision", action="store_true", help="install dependencies for the bot")
 
     args = parser.parse_args()
@@ -109,6 +116,18 @@ def exit_gracefully_if_bot_config_file_does_not_exist(bot_config_file: Optional[
 def main() -> None:
     args = parse_args()
 
+    if args.registry:
+        try:
+            lib_module = finder.import_module_from_zulip_bot_registry(args.bot)
+        except finder.DuplicateRegisteredBotName as error:
+            print(
+                f'ERROR: Found duplicate entries for "{error}" in zulip bots registry.\n'
+                "Make sure that you don't install bots using the same entry point. Exiting now."
+            )
+            sys.exit(1)
+        if lib_module:
+            bot_name = args.bot
+    else:
         result = finder.resolve_bot_path(args.bot)
         if result:
             bot_path, bot_name = result

zulip_bots/zulip_bots/tests/test_run.py

@@ -2,6 +2,7 @@
 import os
 import sys
 import unittest
+from importlib.metadata import EntryPoint
 from pathlib import Path
 from typing import Optional
 from unittest import TestCase, mock
@@ -15,6 +16,7 @@ class TestDefaultArguments(TestCase):
 
     our_dir = os.path.dirname(__file__)
     path_to_bot = os.path.abspath(os.path.join(our_dir, "../bots/giphy/giphy.py"))
+    packaged_bot_entrypoint = EntryPoint("packaged_bot", "module_name", "zulip_bots.registry")
 
     @patch("sys.argv", ["zulip-run-bot", "giphy", "--config-file", "/foo/bar/baz.conf"])
     @patch("zulip_bots.run.run_message_handler_for_bot")
@@ -48,6 +50,29 @@ class TestDefaultArguments(TestCase):
             quiet=False,
         )
 
+    @patch(
+        "sys.argv", ["zulip-run-bot", "packaged_bot", "--config-file", "/foo/bar/baz.conf", "-r"]
+    )
+    @patch("zulip_bots.run.run_message_handler_for_bot")
+    def test_argument_parsing_with_zulip_bot_registry(
+        self, mock_run_message_handler_for_bot: mock.Mock
+    ) -> None:
+        with patch("zulip_bots.run.exit_gracefully_if_zulip_config_is_missing"), patch(
+            "zulip_bots.finder.metadata.EntryPoint.load"
+        ), patch(
+            "zulip_bots.finder.metadata.entry_points",
+            return_value=(self.packaged_bot_entrypoint,),
+        ):
+            zulip_bots.run.main()
+
+        mock_run_message_handler_for_bot.assert_called_with(
+            bot_name="packaged_bot",
+            config_file="/foo/bar/baz.conf",
+            bot_config_file=None,
+            lib_module=mock.ANY,
+            quiet=False,
+        )
+
     def test_adding_bot_parent_dir_to_sys_path_when_bot_name_specified(self) -> None:
         bot_name = "helloworld"  # existing bot's name
         expected_bot_dir_path = Path(