Add an option to disable search for homeservers which may not be interested in it (#4230)

This is useful for homeservers not intended for users, such as bot-only homeservers or ones that only process IoT data.

changelog.d/4230.feature

@@ -0,0 +1 @@
+Add an option to disable search for homeservers that may not be interested in it.

synapse/config/server.py

@@ -62,6 +62,11 @@ class ServerConfig(Config):
         # master, potentially causing inconsistency.
         self.enable_media_repo = config.get("enable_media_repo", True)
 
+        # whether to enable search. If disabled, new entries will not be inserted
+        # into the search tables and they will not be indexed. Users will receive
+        # errors when attempting to search for messages.
+        self.enable_search = config.get("enable_search", True)
+
         self.filter_timeline_limit = config.get("filter_timeline_limit", -1)
 
         # Whether we should block invites sent to users on this server
@@ -384,7 +389,12 @@ class ServerConfig(Config):
           # mau_limit_reserved_threepids:
           # - medium: 'email'
           #   address: 'reserved_user@example.com'
-
+          #
+          # Room searching
+          #
+          # If disabled, new messages will not be indexed for searching and users
+          # will receive errors when searching for messages. Defaults to enabled.
+          # enable_search: true
         """ % locals()
 
     def read_arguments(self, args):

synapse/handlers/search.py

@@ -50,6 +50,9 @@ class SearchHandler(BaseHandler):
             dict to be returned to the client with results of search
         """
 
+        if not self.hs.config.enable_search:
+            raise SynapseError(400, "Search is disabled on this homeserver")
+
         batch_group = None
         batch_group_key = None
         batch_token = None

synapse/storage/search.py

@@ -45,6 +45,10 @@ class SearchStore(BackgroundUpdateStore):
 
     def __init__(self, db_conn, hs):
         super(SearchStore, self).__init__(db_conn, hs)
+
+        if not hs.config.enable_search:
+            return
+
         self.register_background_update_handler(
             self.EVENT_SEARCH_UPDATE_NAME, self._background_reindex_search
         )
@@ -316,6 +320,8 @@ class SearchStore(BackgroundUpdateStore):
             entries (iterable[SearchEntry]):
                 entries to be added to the table
         """
+        if not self.hs.config.enable_search:
+            return
         if isinstance(self.database_engine, PostgresEngine):
             sql = (
                 "INSERT INTO event_search"