Grandfather in existing user threepids

We assume, as we did before, that users bound their threepid to one of
the trusted identity servers. So we simply fill the new table with all
threepids in `user_threepids` joined with the trusted identity servers.

synapse/storage/registration.py

@@ -433,6 +433,10 @@ class RegistrationStore(RegistrationWorkerStore,
         # clear the background update.
         self.register_noop_background_update("refresh_tokens_device_index")
 
+        self.register_background_update_handler(
+            "user_threepids_grandfather", self._bg_user_threepids_grandfather,
+        )
+
     @defer.inlineCallbacks
     def add_access_token_to_user(self, user_id, token, device_id=None):
         """Adds an access token for the given user.
@@ -815,3 +819,34 @@ class RegistrationStore(RegistrationWorkerStore,
             allow_none=True,
             desc="get_users_pending_deactivation",
         )
+
+    @defer.inlineCallbacks
+    def _bg_user_threepids_grandfather(self, progress, batch_size):
+        """We now track which identity servers a user binds their 3PID to, so
+        we need to handle the case of existing bindings where we didn't track
+        this.
+
+        We do this by grandfathering in existing user threepids assuming that
+        they used one of the server configured trusted identity servers.
+        """
+
+        id_servers = set(self.config.trusted_third_party_id_servers)
+
+        def _bg_user_threepids_grandfather_txn(txn):
+            sql = """
+                INSERT INTO user_threepid_id_server
+                    (user_id, medium, address, id_server)
+                SELECT user_id, medium, address, ?
+                FROM user_threepids
+            """
+
+            txn.executemany(sql, [(id_server,) for id_server in id_servers])
+
+        if id_servers:
+            yield self.runInteraction(
+                "_bg_user_threepids_grandfather", _bg_user_threepids_grandfather_txn,
+            )
+
+        yield self._end_background_update("user_threepids_grandfather")
+
+        defer.returnValue(1)

synapse/storage/schema/delta/53/user_threepid_id.sql

@@ -25,3 +25,5 @@ CREATE UNIQUE INDEX user_threepid_id_server_idx ON user_threepid_id_server(
     user_id, medium, address, id_server
 );
 
+INSERT INTO background_updates (update_name, progress_json) VALUES
+  ('user_threepids_grandfather', '{}');