The single watcher / group watcher distinction limited and
complicated watcher coalescing more than necessary. Reworked:
Each server watcher is represented by a WatchBroadcast, each
client "Watcher" attaches to some WatchBroadcast. WatchBroadcasts
hold all WatchBroadcast instances for a range. WatchRanges holds
all WatchBroadcasts for the proxy.
WatchProxyStreams represent a grpc watch stream between the proxy and
a client. When a client requests a new watcher through its grpc stream,
the ProxyStream will allocate a Watcher and WatchRanges assigns it to
some WatchBroadcast based on its range.
Coalescing is done by WatchBroadcasts when it receives an update
notification from a WatchBroadcast.
Supports leader failure detection so watches on a bad member
can migrate to other members. Coincidentally, Fixes#6303.
If the txn comparison block makes claims about a key's current
state, then it may say a key has been updated. Future range/txn
operations may expect this update to eventually be propagated through
the cluster and show up in serialized requests. To avoid spinning
forever on txn/serialized range loops, invalidate the comparison keys.
Couldn't find watcher group from rid on server stream close, leading to
the watcher group sending on a closed channel.
Also got rid of send closing the watcher stream if the buffer is full,
this could lead to a send after close while broadcasting to all receivers.
Instead, if a send times out then the server stream is canceled.
Fixes#6739
We do not wait for the cancellation from actual etcd server,
but generate it at the proxy side. The rule is to return the
latest rev that the watcher has seen. This should be good
enough for most use cases if not all.
