TestService_WithDrops and TestService_SubmitVerificationWithDrops
submitted three items into a size-1 queue and asserted at least one was
dropped, relying on the single consumer not draining the queue between
submits. synctest does not fully pin this because the MockDest send path
does real logging I/O outside the bubble, so under CI's -race scheduling
the consumer occasionally drained all three, delivering everything and
failing the "<= 2" assertion (~0.2% of CI runs).
Add an optional gate channel to MockDest so a destination blocks in
Send/SendVerification until released. The tests now submit one item (the
consumer picks it up and blocks on the gate), fill the size-1 queue,
submit an overflow item that is dropped, then release the gate — the
drop is deterministic regardless of scheduling. Assert exactly two
delivered instead of "at most two".
Dmitry VerkhoturovandUmputun
95f59213e5