add default_power_level_content_override config option.

changelog.d/12618.feature

@@ -0,0 +1 @@
+Add default_power_level_content_override config option.

docs/sample_config.yaml

@@ -2455,6 +2455,22 @@ push:
 #
 #encryption_enabled_by_default_for_room_type: invite
 
+# Override the default power levels for rooms created on this server, per
+# room creation preset.
+#
+# Useful if you know that your users need special permissions in rooms
+# that they create (e.g. to send particular types of state events without
+# needing an elevated power level).  This takes the same shape as the
+# `power_level_content_override` parameter in the /createRoom API, but
+# is applied before that parameter.
+#
+# This is something of a workaround in the absence of MSC3779 or MSC3761.
+#
+#default_power_level_content_override:
+#   private_chat: { "events": { "com.example.foo" : 0 } }
+#   trusted_private_chat: null
+#   public_chat: null
+
 
 # Uncomment to allow non-server-admin users to create groups on this server
 #

docs/usage/configuration/config_documentation.md

@@ -3284,6 +3284,32 @@ room_list_publication_rules:
     room_id: "*"
     action: allow
 ```
+
+---
+Config option: `default_power_level_content_override`
+
+The `default_power_level_content_override` option controls the default power
+levels for rooms.
+
+Useful if you know that your users need special permissions in rooms
+that they create (e.g. to send particular types of state events without
+needing an elevated power level).  This takes the same shape as the
+`power_level_content_override` parameter in the /createRoom API, but
+is applied before that parameter.
+
+Note that each key provided inside a preset (for example `events` in the example
+below) will overwrite all existing defaults inside that key. So in the example
+below, newly-created private_chat rooms will have no rules for any event types
+except `com.example.foo`.
+
+Example configuration:
+```yaml
+default_power_level_content_override:
+   private_chat: { "events": { "com.example.foo" : 0 } }
+   trusted_private_chat: null
+   public_chat: null
+```
+
 ---
 ## Opentracing ##
 Configuration options related to Opentracing support.

synapse/config/room.py

@@ -63,6 +63,19 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
                 "Invalid value for encryption_enabled_by_default_for_room_type"
             )
 
+        self.default_power_level_content_override = config.get(
+            "default_power_level_content_override",
+            None,
+        )
+        if self.default_power_level_content_override is not None:
+            for preset in self.default_power_level_content_override:
+                if preset not in vars(RoomCreationPreset).values():
+                    raise ConfigError(
+                        "Unrecognised room preset %s in default_power_level_content_override"
+                        % preset
+                    )
+                # We validate the actual overrides when we try to apply them.
+
     def generate_config_section(self, **kwargs: Any) -> str:
         return """\
         ## Rooms ##
@@ -83,4 +96,20 @@ def generate_config_section(self, **kwargs: Any) -> str:
         # will also not affect rooms created by other servers.
         #
         #encryption_enabled_by_default_for_room_type: invite
+
+        # Override the default power levels for rooms created on this server, per
+        # room creation preset.
+        #
+        # Useful if you know that your users need special permissions in rooms
+        # that they create (e.g. to send particular types of state events without
+        # needing an elevated power level).  This takes the same shape as the
+        # `power_level_content_override` parameter in the /createRoom API, but
+        # is applied before that parameter.
+        #
+        # This is something of a workaround in the absence of MSC3779 or MSC3761.
+        #
+        #default_power_level_content_override:
+        #   private_chat: { "events": { "com.example.foo" : 0 } }
+        #   trusted_private_chat: null
+        #   public_chat: null
         """

synapse/handlers/room.py

@@ -1040,9 +1040,25 @@ async def send(etype: str, content: JsonDict, **kwargs: Any) -> int:
                 for invitee in invite_list:
                     power_level_content["users"][invitee] = 100
 
-            # Power levels overrides are defined per chat preset
+            # If the user supplied a preset name e.g. "private_chat",
+            # we apply that preset
             power_level_content.update(config["power_level_content_override"])
 
+            # If the server config contains default_power_level_content_override,
+            # and that contains information for this room preset, apply it.
+            default_power_level_content_override = (
+                self.config.room.default_power_level_content_override
+            )
+            if (
+                default_power_level_content_override
+                and default_power_level_content_override.get(preset_config)
+            ):
+                power_level_content.update(
+                    default_power_level_content_override.get(preset_config)
+                )
+
+            # Finally, if the user supplied specific permissions for this room,
+            # apply those.
             if power_level_content_override:
                 power_level_content.update(power_level_content_override)