Introduce RootConfig.validate_config() which can be subclassed in HomeServerConfig to do cross-config class validation (#19027)

This means we
can move the open registration config validation from `setup()` to
`HomeServerConfig.validate_config()` (much more sane).

Spawning from looking at this area of code in
https://github.com/element-hq/synapse/pull/19015

tests/config/test_load.py

@@ -99,7 +99,14 @@ class ConfigLoadingFileTestCase(ConfigFileTestCase):
     def test_disable_registration(self) -> None:
         self.generate_config()
         self.add_lines_to_config(
-            ["enable_registration: true", "disable_registration: true"]
+            [
+                "enable_registration: true",
+                "disable_registration: true",
+                # We're not worried about open registration in this test. This test is
+                # focused on making sure that enable/disable_registration properly
+                # override each other.
+                "enable_registration_without_verification: true",
+            ]
         )
         # Check that disable_registration clobbers enable_registration.
         config = HomeServerConfig.load_config("", ["-c", self.config_file])

tests/config/test_registration_config.py

@@ -19,6 +19,8 @@
 #
 #
 
+import argparse
+
 import synapse.app.homeserver
 from synapse.config import ConfigError
 from synapse.config.homeserver import HomeServerConfig
@@ -99,6 +101,39 @@ class RegistrationConfigTestCase(ConfigFileTestCase):
         )
 
     def test_refuse_to_start_if_open_registration_and_no_verification(self) -> None:
+        """
+        Test that our utilities to start the main Synapse homeserver process refuses
+        to start if we detect open registration.
+        """
+        self.generate_config()
+        self.add_lines_to_config(
+            [
+                " ",
+                "enable_registration: true",
+                "registrations_require_3pid: []",
+                "enable_registration_captcha: false",
+                "registration_requires_token: false",
+            ]
+        )
+
+        # Test that allowing open registration without verification raises an error
+        with self.assertRaises(SystemExit):
+            # Do a normal homeserver creation and setup
+            homeserver_config = synapse.app.homeserver.load_or_generate_config(
+                ["-c", self.config_file]
+            )
+            # XXX: The error will be raised at this point
+            hs = synapse.app.homeserver.create_homeserver(homeserver_config)
+            # Continue with the setup. We don't expect this to run because we raised
+            # earlier, but in the future, the code could be refactored to raise the
+            # error in a different place.
+            synapse.app.homeserver.setup(hs)
+
+    def test_load_config_error_if_open_registration_and_no_verification(self) -> None:
+        """
+        Test that `HomeServerConfig.load_config(...)` raises an exception when we detect open
+        registration.
+        """
         self.generate_config()
         self.add_lines_to_config(
             [
@@ -112,13 +147,57 @@ class RegistrationConfigTestCase(ConfigFileTestCase):
 
         # Test that allowing open registration without verification raises an error
         with self.assertRaises(ConfigError):
-            # Do a normal homeserver creation and setup
-            homeserver_config = synapse.app.homeserver.load_or_generate_config(
-                ["-c", self.config_file]
+            _homeserver_config = HomeServerConfig.load_config(
+                description="test", argv_options=["-c", self.config_file]
+            )
+
+    def test_load_or_generate_config_error_if_open_registration_and_no_verification(
+        self,
+    ) -> None:
+        """
+        Test that `HomeServerConfig.load_or_generate_config(...)` raises an exception when we detect open
+        registration.
+        """
+        self.generate_config()
+        self.add_lines_to_config(
+            [
+                " ",
+                "enable_registration: true",
+                "registrations_require_3pid: []",
+                "enable_registration_captcha: false",
+                "registration_requires_token: false",
+            ]
+        )
+
+        # Test that allowing open registration without verification raises an error
+        with self.assertRaises(ConfigError):
+            _homeserver_config = HomeServerConfig.load_or_generate_config(
+                description="test", argv_options=["-c", self.config_file]
+            )
+
+    def test_load_config_with_parser_error_if_open_registration_and_no_verification(
+        self,
+    ) -> None:
+        """
+        Test that `HomeServerConfig.load_config_with_parser(...)` raises an exception when we detect open
+        registration.
+        """
+        self.generate_config()
+        self.add_lines_to_config(
+            [
+                " ",
+                "enable_registration: true",
+                "registrations_require_3pid: []",
+                "enable_registration_captcha: false",
+                "registration_requires_token: false",
+            ]
+        )
+
+        # Test that allowing open registration without verification raises an error
+        with self.assertRaises(ConfigError):
+            config_parser = argparse.ArgumentParser(description="test")
+            HomeServerConfig.add_arguments_to_parser(config_parser)
+
+            _homeserver_config = HomeServerConfig.load_config_with_parser(
+                parser=config_parser, argv_options=["-c", self.config_file]
             )
-            # XXX: The error will be raised at this point
-            hs = synapse.app.homeserver.create_homeserver(homeserver_config)
-            # Continue with the setup. We don't expect this to run because we raised
-            # earlier, but in the future, the code could be refactored to raise the
-            # error in a different place.
-            synapse.app.homeserver.setup(hs)