upstream: https://github.com/dtolnay/cxx/issues/1331

• Delete trailing newline

Handle the case where find_if returns localEchoes.end(), and update related test

In D143#2771, @tusooa wrote:

otherwise looks good

otherwise looks good

add a test to verify it does not crash when the txn id is not found.

• Use MockHelper and deepEqual for test

• Use eventId instead of timeline index

We need a way to stop and restart the sync immediately, and at the same time ensure there is only one sync job running.