Enable experimental spaces by default. (#10011)

The previous spaces_enabled flag now defaults to true and
is exposed in the sample config.

changelog.d/10011.feature

+Enable experimental support for [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946) (spaces summary API) and [MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083) (restricted join rules) by default.

docs/sample_config.yaml

@@ -2943,3 +2943,18 @@ redis:
   # Optional password if configured on the Redis instance
   #
   #password: <secret_password>
+
+
+# Enable experimental features in Synapse.
+#
+# Experimental features might break or be removed without a deprecation
+# period.
+#
+experimental_features:
+  # Support for Spaces (MSC1772), it enables the following:
+  #
+  # * The Spaces Summary API (MSC2946).
+  # * Restricting room membership based on space membership (MSC3083).
+  #
+  # Uncomment to disable support for Spaces.
+  #spaces_enabled: false

synapse/config/experimental.py

@@ -29,9 +29,26 @@ class ExperimentalConfig(Config):
         self.msc2858_enabled = experimental.get("msc2858_enabled", False)  # type: bool
 
         # Spaces (MSC1772, MSC2946, MSC3083, etc)
-        self.spaces_enabled = experimental.get("spaces_enabled", False)  # type: bool
+        self.spaces_enabled = experimental.get("spaces_enabled", True)  # type: bool
         if self.spaces_enabled:
             KNOWN_ROOM_VERSIONS[RoomVersions.MSC3083.identifier] = RoomVersions.MSC3083
 
         # MSC3026 (busy presence state)
         self.msc3026_enabled = experimental.get("msc3026_enabled", False)  # type: bool
+
+    def generate_config_section(self, **kwargs):
+        return """\
+        # Enable experimental features in Synapse.
+        #
+        # Experimental features might break or be removed without a deprecation
+        # period.
+        #
+        experimental_features:
+          # Support for Spaces (MSC1772), it enables the following:
+          #
+          # * The Spaces Summary API (MSC2946).
+          # * Restricting room membership based on space membership (MSC3083).
+          #
+          # Uncomment to disable support for Spaces.
+          #spaces_enabled: false
+        """

synapse/config/homeserver.py

@@ -57,7 +57,6 @@ class HomeServerConfig(RootConfig):
 
     config_classes = [
         ServerConfig,
-        ExperimentalConfig,
         TlsConfig,
         FederationConfig,
         CacheConfig,
@@ -94,4 +93,5 @@ class HomeServerConfig(RootConfig):
         TracerConfig,
         WorkerConfig,
         RedisConfig,
+        ExperimentalConfig,
     ]