Minor optimization to reduce number of RPCs sent.

If multiple log sync are requested, only do the last one at the
earliest time.

src/lib.rs

@@ -7,7 +7,7 @@ extern crate serde_derive;
 extern crate tokio;
 
 use std::convert::TryFrom;
-use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
 use std::sync::Arc;
 use std::time::{Duration, Instant};
 
@@ -600,6 +600,10 @@ impl Raft {
         // Clone everything that the thread needs.
         let this = self.clone();
         std::thread::spawn(move || {
+            let mut openings = vec![];
+            openings.resize_with(this.peers.len(), || AtomicUsize::new(0));
+            let openings = Arc::new(openings); // Not mutable beyond this point.
+
             while let Ok(peer) = rx.recv() {
                 if !this.keep_running.load(Ordering::SeqCst) {
                     break;
@@ -610,11 +614,13 @@ impl Raft {
                 for (i, rpc_client) in this.peers.iter().enumerate() {
                     if i != this.me.0 && peer.map(|p| p.0 == i).unwrap_or(true)
                     {
+                        openings[i].fetch_add(1, Ordering::SeqCst);
                         this.thread_pool.spawn(Self::sync_log_entry(
                             this.inner_state.clone(),
                             rpc_client.clone(),
                             i,
                             this.new_log_entry.clone().unwrap(),
+                            openings.clone(),
                             this.apply_command_signal.clone(),
                         ));
                     }
@@ -633,8 +639,14 @@ impl Raft {
         rpc_client: RpcClient,
         peer_index: usize,
         rerun: std::sync::mpsc::Sender<Option<Peer>>,
+        openings: Arc<Vec<AtomicUsize>>,
         apply_command_signal: Arc<Condvar>,
     ) {
+        let opening = &openings[peer_index];
+        if opening.swap(0, Ordering::SeqCst) == 0 {
+            return;
+        }
+
         // TODO: cancel in flight changes?
         let args = match Self::build_append_entries(&rf, peer_index) {
             Some(args) => args,