Fix synapse.config module "read" command (#11145)

`synapse.config.__main__` has the possibility to read a config item. This can be used to conveniently also validate the config is valid before trying to start Synapse.

 The "read" command broke in https://github.com/matrix-org/synapse/pull/10916 as it now requires passing in "server.server_name" for example.

 Also made the read command optional so one can just call this with just the confirm file reference and get a "Config parses OK" if things are ok.

Signed-off-by: Jason Robinson <jasonr@matrix.org>
Co-authored-by: Brendan Abolivier <babolivier@matrix.org>

tests/config/utils.py

@@ -0,0 +1,58 @@
+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import os
+import shutil
+import tempfile
+import unittest
+from contextlib import redirect_stdout
+from io import StringIO
+
+from synapse.config.homeserver import HomeServerConfig
+
+
+class ConfigFileTestCase(unittest.TestCase):
+    def setUp(self):
+        self.dir = tempfile.mkdtemp()
+        self.config_file = os.path.join(self.dir, "homeserver.yaml")
+
+    def tearDown(self):
+        shutil.rmtree(self.dir)
+
+    def generate_config(self):
+        with redirect_stdout(StringIO()):
+            HomeServerConfig.load_or_generate_config(
+                "",
+                [
+                    "--generate-config",
+                    "-c",
+                    self.config_file,
+                    "--report-stats=yes",
+                    "-H",
+                    "lemurs.win",
+                ],
+            )
+
+    def generate_config_and_remove_lines_containing(self, needle):
+        self.generate_config()
+
+        with open(self.config_file) as f:
+            contents = f.readlines()
+        contents = [line for line in contents if needle not in line]
+        with open(self.config_file, "w") as f:
+            f.write("".join(contents))
+
+    def add_lines_to_config(self, lines):
+        with open(self.config_file, "a") as f:
+            for line in lines:
+                f.write(line + "\n")