Fix MAU reaping where reserved users are specified. (#6168)

2 files changed, 65 insertions, 42 deletions

diff --git a/synapse/app/homeserver.py b/synapse/app/homeserver.py
index 774326dff9..eb54f56853 100644
--- a/synapse/app/homeserver.py
+++ b/synapse/app/homeserver.py
@@ -605,13 +605,13 @@ def run(hs):
     @defer.inlineCallbacks
     def generate_monthly_active_users():
         current_mau_count = 0
-        reserved_count = 0
+        reserved_users = ()
         store = hs.get_datastore()
         if hs.config.limit_usage_by_mau or hs.config.mau_stats_only:
             current_mau_count = yield store.get_monthly_active_count()
-            reserved_count = yield store.get_registered_reserved_users_count()
+            reserved_users = yield store.get_registered_reserved_users()
         current_mau_gauge.set(float(current_mau_count))
-        registered_reserved_users_mau_gauge.set(float(reserved_count))
+        registered_reserved_users_mau_gauge.set(float(len(reserved_users)))
         max_mau_gauge.set(float(hs.config.max_mau_value))
 
     def start_generate_monthly_active_users():
diff --git a/synapse/storage/monthly_active_users.py b/synapse/storage/monthly_active_users.py
index 752e9788a2..3803604be7 100644
--- a/synapse/storage/monthly_active_users.py
+++ b/synapse/storage/monthly_active_users.py
@@ -32,7 +32,6 @@ class MonthlyActiveUsersStore(SQLBaseStore):
         super(MonthlyActiveUsersStore, self).__init__(None, hs)
         self._clock = hs.get_clock()
         self.hs = hs
-        self.reserved_users = ()
         # Do not add more reserved users than the total allowable number
         self._new_transaction(
             dbconn,
@@ -51,7 +50,6 @@ class MonthlyActiveUsersStore(SQLBaseStore):
             txn (cursor):
             threepids (list[dict]): List of threepid dicts to reserve
         """
-        reserved_user_list = []
 
         for tp in threepids:
             user_id = self.get_user_id_by_threepid_txn(txn, tp["medium"], tp["address"])
@@ -60,10 +58,8 @@ class MonthlyActiveUsersStore(SQLBaseStore):
                 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)
-        self.reserved_users = tuple(reserved_user_list)
 
     @defer.inlineCallbacks
     def reap_monthly_active_users(self):
@@ -74,8 +70,11 @@ class MonthlyActiveUsersStore(SQLBaseStore):
             Deferred[]
         """
 
-        def _reap_users(txn):
-            # Purge stale users
+        def _reap_users(txn, reserved_users):
+            """
+            Args:
+                reserved_users (tuple): reserved users to preserve
+            """
 
             thirty_days_ago = int(self._clock.time_msec()) - (1000 * 60 * 60 * 24 * 30)
             query_args = [thirty_days_ago]
@@ -83,20 +82,19 @@ class MonthlyActiveUsersStore(SQLBaseStore):
 
             # Need if/else since 'AND user_id NOT IN ({})' fails on Postgres
             # when len(reserved_users) == 0. Works fine on sqlite.
-            if len(self.reserved_users) > 0:
+            if len(reserved_users) > 0:
                 # questionmarks is a hack to overcome sqlite not supporting
                 # tuples in 'WHERE IN %s'
-                questionmarks = "?" * len(self.reserved_users)
+                question_marks = ",".join("?" * len(reserved_users))
 
-                query_args.extend(self.reserved_users)
-                sql = base_sql + """ AND user_id NOT IN ({})""".format(
-                    ",".join(questionmarks)
-                )
+                query_args.extend(reserved_users)
+                sql = base_sql + " AND user_id NOT IN ({})".format(question_marks)
             else:
                 sql = base_sql
 
             txn.execute(sql, query_args)
 
+            max_mau_value = self.hs.config.max_mau_value
             if self.hs.config.limit_usage_by_mau:
                 # If MAU user count still exceeds the MAU threshold, then delete on
                 # a least recently active basis.
@@ -106,31 +104,52 @@ class MonthlyActiveUsersStore(SQLBaseStore):
                 # While Postgres does not require 'LIMIT', but also does not support
                 # negative LIMIT values. So there is no way to write it that both can
                 # support
-                safe_guard = self.hs.config.max_mau_value - len(self.reserved_users)
-                # Must be greater than zero for postgres
-                safe_guard = safe_guard if safe_guard > 0 else 0
-                query_args = [safe_guard]
-
-                base_sql = """
-                    DELETE FROM monthly_active_users
-                    WHERE user_id NOT IN (
-                        SELECT user_id FROM monthly_active_users
-                        ORDER BY timestamp DESC
-                        LIMIT ?
+                if len(reserved_users) == 0:
+                    sql = """
+                        DELETE FROM monthly_active_users
+                        WHERE user_id NOT IN (
+                            SELECT user_id FROM monthly_active_users
+                            ORDER BY timestamp DESC
+                            LIMIT ?
                         )
-                    """
+                        """
+                    txn.execute(sql, (max_mau_value,))
                 # Need if/else since 'AND user_id NOT IN ({})' fails on Postgres
                 # when len(reserved_users) == 0. Works fine on sqlite.
-                if len(self.reserved_users) > 0:
-                    query_args.extend(self.reserved_users)
-                    sql = base_sql + """ AND user_id NOT IN ({})""".format(
-                        ",".join(questionmarks)
-                    )
                 else:
-                    sql = base_sql
-                txn.execute(sql, query_args)
+                    # Must be >= 0 for postgres
+                    num_of_non_reserved_users_to_remove = max(
+                        max_mau_value - len(reserved_users), 0
+                    )
+
+                    # It is important to filter reserved users twice to guard
+                    # against the case where the reserved user is present in the
+                    # SELECT, meaning that a legitmate mau is deleted.
+                    sql = """
+                        DELETE FROM monthly_active_users
+                        WHERE user_id NOT IN (
+                            SELECT user_id FROM monthly_active_users
+                            WHERE user_id NOT IN ({})
+                            ORDER BY timestamp DESC
+                            LIMIT ?
+                        )
+                        AND user_id NOT IN ({})
+                    """.format(
+                        question_marks, question_marks
+                    )
+
+                    query_args = [
+                        *reserved_users,
+                        num_of_non_reserved_users_to_remove,
+                        *reserved_users,
+                    ]
 
-        yield self.runInteraction("reap_monthly_active_users", _reap_users)
+                    txn.execute(sql, query_args)
+
+        reserved_users = yield self.get_registered_reserved_users()
+        yield self.runInteraction(
+            "reap_monthly_active_users", _reap_users, reserved_users
+        )
         # It seems poor to invalidate the whole cache, Postgres supports
         # 'Returning' which would allow me to invalidate only the
         # specific users, but sqlite has no way to do this and instead
@@ -159,21 +178,25 @@ class MonthlyActiveUsersStore(SQLBaseStore):
         return self.runInteraction("count_users", _count_users)
 
     @defer.inlineCallbacks
-    def get_registered_reserved_users_count(self):
-        """Of the reserved threepids defined in config, how many are associated
+    def get_registered_reserved_users(self):
+        """Of the reserved threepids defined in config, which are associated
         with registered users?
 
         Returns:
-            Defered[int]: Number of real reserved users
+            Defered[list]: Real reserved users
         """
-        count = 0
-        for tp in self.hs.config.mau_limits_reserved_threepids:
+        users = []
+
+        for tp in self.hs.config.mau_limits_reserved_threepids[
+            : self.hs.config.max_mau_value
+        ]:
             user_id = yield self.hs.get_datastore().get_user_id_by_threepid(
                 tp["medium"], tp["address"]
             )
             if user_id:
-                count = count + 1
-        return count
+                users.append(user_id)
+
+        return users
 
     @defer.inlineCallbacks
     def upsert_monthly_active_user(self, user_id):