in raftNode start(), why we should 'finish processing incoming messages before we signal notifyc chan'?

[chen-zhuohan]

in raftNode start(), I see the code(removed some comment and unimportant code):

if !islead {
					// finish processing incoming messages before we signal notifyc chan
					msgs := r.processMessages(rd.Messages)

					// now unblocks 'applyAll' that waits on Raft log disk writes before triggering snapshots
					notifyc <- struct{}{}

					if confChanged {
						// blocks until 'applyAll' calls 'applyWait.Trigger'
						// to be in sync with scheduled config-change job
						// (assume notifyc has cap of 1)
						select {
						case notifyc <- struct{}{}:
						case <-r.stopped:
							return
						}
					}

					r.transport.Send(msgs)
				} else {
					// leader already processed 'MsgSnap' and signaled
					notifyc <- struct{}{}

in order to know the purpose, I found the pr: #7706 , and I see the original comment:

but I still don't know why the code is that. Could example the purpose?

[chen-zhuohan]

I known, processMessages() handled snapshot msg and send snapshot-msg to channel, then applyAll() can get snapshot-msg from channel.