create support user (#4141)

Allow for the creation of a support user.

A support user can access the server, join rooms, interact with other users, but does not appear in the user directory nor does it contribute to monthly active user limits.

synapse/storage/monthly_active_users.py

@@ -55,9 +55,12 @@ class MonthlyActiveUsersStore(SQLBaseStore):
                 txn,
                 tp["medium"], tp["address"]
             )
+
             if user_id:
-                self.upsert_monthly_active_user_txn(txn, user_id)
-                reserved_user_list.append(user_id)
+                is_support = self.is_support_user_txn(txn, user_id)
+                if not is_support:
+                    self.upsert_monthly_active_user_txn(txn, user_id)
+                    reserved_user_list.append(user_id)
             else:
                 logger.warning(
                     "mau limit reserved threepid %s not found in db" % tp
@@ -182,6 +185,18 @@ class MonthlyActiveUsersStore(SQLBaseStore):
         Args:
             user_id (str): user to add/update
         """
+        # Support user never to be included in MAU stats. Note I can't easily call this
+        # from upsert_monthly_active_user_txn because then I need a _txn form of
+        # is_support_user which is complicated because I want to cache the result.
+        # Therefore I call it here and ignore the case where
+        # upsert_monthly_active_user_txn is called directly from
+        # _initialise_reserved_users reasoning that it would be very strange to
+        #  include a support user in this context.
+
+        is_support = yield self.is_support_user(user_id)
+        if is_support:
+            return
+
         is_insert = yield self.runInteraction(
             "upsert_monthly_active_user", self.upsert_monthly_active_user_txn,
             user_id
@@ -200,6 +215,16 @@ class MonthlyActiveUsersStore(SQLBaseStore):
         in a database thread rather than the main thread, and we can't call
         txn.call_after because txn may not be a LoggingTransaction.
 
+        We consciously do not call is_support_txn from this method because it
+        is not possible to cache the response. is_support_txn will be false in
+        almost all cases, so it seems reasonable to call it only for
+        upsert_monthly_active_user and to call is_support_txn manually
+        for cases where upsert_monthly_active_user_txn is called directly,
+        like _initialise_reserved_users
+
+        In short, don't call this method with support users. (Support users
+        should not appear in the MAU stats).
+
         Args:
             txn (cursor):
             user_id (str): user to add/update
@@ -208,6 +233,7 @@ class MonthlyActiveUsersStore(SQLBaseStore):
             bool: True if a new entry was created, False if an
             existing one was updated.
         """
+
         # Am consciously deciding to lock the table on the basis that is ought
         # never be a big table and alternative approaches (batching multiple
         # upserts into a single txn) introduced a lot of extra complexity.