Merge pull request apache#31 from richardod/trunk

Bug fix where 2 candidates can become leaders

src/dotnet/ZooKeeperNet.Recipes.Tests/LeaderElectionTests.cs

@@ -37,7 +37,7 @@ public void Teardown() {
 		}
 
 		private class TestLeaderWatcher : ILeaderWatcher {
-			public static byte Leader;
+			public byte Leader;
 			private readonly byte b;
 
 			public TestLeaderWatcher(byte b) {
@@ -53,7 +53,6 @@ public void TakeLeadership() {
 		[Test]
 		public void testElection() {
 			String dir = "/test";
-			String testString = "Hello World";
 			int num_clients = 10;
 			clients = new ZooKeeper[num_clients];
 			LeaderElection[] elections = new LeaderElection[num_clients];
@@ -71,5 +70,36 @@ public void testElection() {
 			}
 			Assert.Pass();
 		}
+
+		[Test]
+		public void testNode4DoesNotBecomeLeaderWhenNonLeader3Closes()
+		{
+			var dir = "/test";
+			var num_clients = 4;
+			clients = new ZooKeeper[num_clients];
+			var elections = new LeaderElection[num_clients];
+			var leaderWatchers = new TestLeaderWatcher[num_clients];
+
+			for (byte i = 0; i < clients.Length; i++)
+			{
+				clients[i] = CreateClient();
+				leaderWatchers[i] = new TestLeaderWatcher((byte)(i + 1)); // Start at 1 so we can check it is set
+				elections[i] = new LeaderElection(clients[i], dir, leaderWatchers[i], new[] { i });
+				elections[i].Start();
+			}
+
+			// Kill 2
+			elections[2].Close();
+			// First one should still be leader
+			Thread.Sleep(3000);
+			Assert.AreEqual(1, leaderWatchers[0].Leader);
+			Assert.AreEqual(0, leaderWatchers[1].Leader);
+			Assert.AreEqual(0, leaderWatchers[2].Leader);
+			Assert.AreEqual(0, leaderWatchers[3].Leader);
+			elections[0].Close();
+			elections[1].Close();
+			elections[3].Close();
+		}
 	}
 }
+

src/dotnet/ZooKeeperNet.Recipes/LeaderElection.cs

@@ -17,7 +17,6 @@
  */
 
 using System;
-using System.Diagnostics;
 using System.Linq;
 
 namespace ZooKeeperNet.Recipes {
@@ -100,8 +99,8 @@ public bool RunForLeader() {
 			if (penultimate == null) {
 				throw new InvalidOperationException("Penultimate value in priors is null, but count shoudl have been at least 2.");
 			}
-			var watchPath = path.Combine(penultimate.Name);
-			if (Zookeeper.Exists(watchPath, new LeaderWatcher(this, watchPath, watcher)) == null) {
+			var penultimatePath = path.Combine(penultimate.Name);
+			if (Zookeeper.Exists(penultimatePath, new LeaderWatcher(Zookeeper, this, penultimatePath, watcher, path, id)) == null) {
 				IsOwner = true;
 				watcher.TakeLeadership();
 				return true;
@@ -111,21 +110,44 @@ public bool RunForLeader() {
 
 		private class LeaderWatcher : IWatcher {
 			private readonly LeaderElection election;
+			private readonly string penultimatePath;
 			private readonly string path;
 			private readonly ILeaderWatcher watcher;
+			private readonly IZooKeeper zooKeeper;
+			private readonly string id;
 
-			public LeaderWatcher(LeaderElection election, string path, ILeaderWatcher watcher) {
+			public LeaderWatcher(IZooKeeper zooKeeper, LeaderElection election, string penultimatePath, ILeaderWatcher watcher, string path, string id) { 
+				this.zooKeeper = zooKeeper;
 				this.election = election;
-				this.path = path;
+				this.penultimatePath = penultimatePath;
 				this.watcher = watcher;
+				this.path = path;
+				this.id = id;
 			}
 
 			public void Process(WatchedEvent @event) {
-				if (@event.Type == EventType.NodeDeleted && @event.Path == path) {
-					election.IsOwner = true;
-					watcher.TakeLeadership();
+				if (@event.Type == EventType.NodeDeleted && @event.Path == penultimatePath) {
+					var names = zooKeeper.GetChildren(path, false);
+					var sortedNames = new SortedSet<ZNodeName>();
+
+					foreach (var name in names)
+					{
+						sortedNames.Add(new ZNodeName(name));
+					}
+
+					if (SmallestZNode(sortedNames))
+					{
+						election.IsOwner = true;
+						watcher.TakeLeadership();
+					}
 				}
 			}
+
+			private bool SmallestZNode(SortedSet<ZNodeName> sortedNames)
+			{
+				// If a client receives a notification that the znode it is watching is gone, then it becomes the new leader in the case that there is no smaller znode		
+				return sortedNames.Any() && id.EndsWith(sortedNames.First().Name, StringComparison.InvariantCulture);
+			}
 		}
 
 		public void Start() {