When implementing the fix for progress notifications
(https://github.com/etcd-io/etcd/pull/15237) we made a incorrect
assumption that that unsynched watches will always get at least one event.
Unsynched watches include not only slow watchers, but also newly created
watches that requested current or older revision. In case that non of the events
match watch filter, those newly created watches might become synched
without any event going through.
Signed-off-by: Marek Siarkowicz <siarkowicz@google.com>
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