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ab5df57ecf
When snapshot store requests raft snapshot from etcdserver apply loop, it may block on the channel for some time, or wait some time for KV to snapshot. This is unexpected because raft state machine should be unblocked. Even worse, this block may lead to deadlock: 1. raft state machine waits on getting snapshot from raft memory storage 2. raft memory storage waits snapshot store to get snapshot 3. snapshot store requests raft snapshot from apply loop 4. apply loop is applying entries, and waits raftNode loop to finish messages sending 5. raftNode loop waits peer loop in Transport to send out messages 6. peer loop in Transport waits for raft state machine to process message Fix it by changing the logic of getSnap to be asynchronously creation.