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>

synapse/config/__main__.py

@@ -1,4 +1,5 @@
 # Copyright 2015, 2016 OpenMarket Ltd
+# 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.
@@ -11,25 +12,44 @@
 # 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 sys
+
 from synapse.config._base import ConfigError
+from synapse.config.homeserver import HomeServerConfig
 
-if __name__ == "__main__":
-    import sys
 
-    from synapse.config.homeserver import HomeServerConfig
+def main(args):
+    action = args[1] if len(args) > 1 and args[1] == "read" else None
+    # If we're reading a key in the config file, then `args[1]` will be `read`  and `args[2]`
+    # will be the key to read.
+    # We'll want to rework this code if we want to support more actions than just `read`.
+    load_config_args = args[3:] if action else args[1:]
 
-    action = sys.argv[1]
+    try:
+        config = HomeServerConfig.load_config("", load_config_args)
+    except ConfigError as e:
+        sys.stderr.write("\n" + str(e) + "\n")
+        sys.exit(1)
+
+    print("Config parses OK!")
 
     if action == "read":
-        key = sys.argv[2]
+        key = args[2]
+        key_parts = key.split(".")
+
+        value = config
         try:
-            config = HomeServerConfig.load_config("", sys.argv[3:])
-        except ConfigError as e:
-            sys.stderr.write("\n" + str(e) + "\n")
+            while len(key_parts):
+                value = getattr(value, key_parts[0])
+                key_parts.pop(0)
+
+            print(f"\n{key}: {value}")
+        except AttributeError:
+            print(
+                f"\nNo '{key}' key could be found in the provided configuration file."
+            )
             sys.exit(1)
 
-        print(getattr(config, key))
-        sys.exit(0)
-    else:
-        sys.stderr.write("Unknown command %r\n" % (action,))
-        sys.exit(1)
+
+if __name__ == "__main__":
+    main(sys.argv)