Move to mutable state

Raft.Test/TestServer.fs

@@ -1,5 +1,6 @@
 namespace Raft.Test
 
+open System.Threading
 open Raft
 open NUnit.Framework
 open FsUnitTyped
@@ -22,7 +23,7 @@ module TestServer =
 
         let sendMessage = cluster.Servers.[0].OutboundMessageChannel
 
-        // Candidate 1 asks to be elected.
+        // Candidate 1 asks server 0 to vote for it.
 
         {
             CandidateTerm = 0<Term>
@@ -34,3 +35,32 @@ module TestServer =
         |> sendMessage 0<ServerId>
 
         logs () |> shouldEqual [ "Received message for term 0" ]
+
+        // Candidate 1 asks to be elected again! This is fine, maybe the network is replaying requests
+        // and the network swallowed our reply, so we should reply in the same way.
+        {
+            CandidateTerm = 0<Term>
+            CandidateId = 1<ServerId>
+            ReplyChannel = fun message -> logger (sprintf "Received message for term %i" message.VoterTerm)
+            CandidateLastLogEntry = 0<LogIndex>, 0<Term>
+        }
+        |> Message.RequestVote
+        |> sendMessage 0<ServerId>
+
+        logs ()
+        |> shouldEqual [ "Received message for term 0" ; "Received message for term 0" ]
+
+        // Candidate 2 asks to be elected. We won't vote for them, because we've already voted.
+        // and the network swallowed our reply, so we should reply in the same way.
+        let calls = ref 0
+
+        {
+            CandidateTerm = 0<Term>
+            CandidateId = 2<ServerId>
+            ReplyChannel = fun _ -> Interlocked.Increment calls |> ignore
+            CandidateLastLogEntry = 0<LogIndex>, 0<Term>
+        }
+        |> Message.RequestVote
+        |> sendMessage 0<ServerId>
+
+        calls.Value |> shouldEqual 0