Handle failures of individual dht routers

If a single dht router is offline/unavailable, we should still be able
to successfully resolve the remainder and bootstrap.

Additionally, tweak how initialisation is handled. The list of initial
nodes and the DHT bootstrap nodes must be provided at the start. If both
are provided then the initial nodes will be used for bootstrapping prior
to attempting to resolve/use the DHT bootstrap nodes.

If an empty list of bootstrap nodes is provided, then no lookup will be
performed and only the 'initial nodes' will be used for bootstrapping.

If an empty list of initial nodes and an empty list of bootstrap nodes
are provided, then the engine will fail to initialise.

src/MonoTorrent.Connections/MonoTorrent.Connections/SocketConnector.cs

@@ -59,8 +59,6 @@ static SocketAsyncEventArgs GetSocketAsyncEventArgs ()
 
         static void HandleOperationCompleted (object? sender, SocketAsyncEventArgs e)
         {
-            // Don't retain the TCS forever. Note we do not want to null out the byte[] buffer
-            // as we *do* want to retain that so that we can avoid the expensive SetBuffer calls.
             var tcs = (ReusableTaskCompletionSource<int>) e.UserToken!;
             SocketError error = e.SocketError;
 

src/MonoTorrent.Dht/MonoTorrent.Dht.Tasks/InitialiseTask.cs

@@ -38,12 +38,6 @@ namespace MonoTorrent.Dht.Tasks
 {
     class InitialiseTask
     {
-        static readonly string[] DefaultBootstrapRouters = new[] {
-            "router.bittorrent.com",
-            "router.utorrent.com",
-            "dht.transmissionbt.com"
-        };
-
         // Choose a completely arbitrary value here. If we have at least this many
         // nodes in the routing table we can consider it 'healthy' enough to allow
         // the state to change to 'Ready' so torrents can begin searching for peers
@@ -57,7 +51,7 @@ class InitialiseTask
         string[] BootstrapRouters { get; set; }
 
         public InitialiseTask (DhtEngine engine)
-            : this (engine, Enumerable.Empty<Node> (), DefaultBootstrapRouters)
+            : this (engine, Enumerable.Empty<Node> (), DhtEngine.DefaultBootstrapRouters.ToArray ())
         {
 
         }
@@ -66,7 +60,7 @@ public InitialiseTask (DhtEngine engine, IEnumerable<Node> nodes, string[] boots
         {
             this.engine = engine;
             initialNodes = nodes.ToList ();
-            BootstrapRouters = bootstrapRouters.Length == 0 ? DefaultBootstrapRouters : bootstrapRouters.ToArray ();
+            BootstrapRouters = bootstrapRouters;
             initializationComplete = new TaskCompletionSource<object?> ();
         }
 
@@ -100,13 +94,24 @@ async void BeginAsyncInit ()
 
         async Task<Node[]> GenerateBootstrapNodes ()
         {
-            var addresses = await Task.WhenAll (BootstrapRouters.Select (Dns.GetHostAddressesAsync));
-            return addresses
-                .SelectMany (t => t)
-                .Distinct ()
-                .Select (t => new IPEndPoint (t, 6881))
-                .Select (t => new Node (NodeId.Create (), t))
-                .ToArray ();
+            // Handle when one, or more, of the bootstrap nodes are offline
+            var results = new List<Node> ();
+
+            var tasks = BootstrapRouters.Select (Dns.GetHostAddressesAsync).ToList ();
+            while (tasks.Count > 0) {
+                var completed = await Task.WhenAny (tasks);
+                tasks.Remove (completed);
+
+                try {
+                    var addresses = await completed;
+                    foreach (var v in addresses)
+                        results.Add (new Node (NodeId.Create (), new IPEndPoint (v, 6881)));
+                } catch {
+
+                }
+            }
+
+            return results.ToArray ();
         }
 
         async Task SendFindNode (IEnumerable<Node> newNodes)
@@ -139,7 +144,7 @@ async Task SendFindNode (IEnumerable<Node> newNodes)
                 }
             }
 
-            if (initialNodes.Count > 0 && engine.RoutingTable.NeedsBootstrap)
+            if (initialNodes.Count > 0 && engine.RoutingTable.NeedsBootstrap && BootstrapRouters.Length > 0)
                 await new InitialiseTask (engine).ExecuteAsync ();
         }
     }

src/MonoTorrent.Dht/MonoTorrent.Dht/DhtEngine.cs

@@ -63,6 +63,12 @@ class TransferMonitor : ITransferMonitor
 
     public class DhtEngine : IDisposable, IDhtEngine
     {
+        internal static readonly IList<string> DefaultBootstrapRouters = Array.AsReadOnly (new[] {
+            "router.bittorrent.com",
+            "router.utorrent.com",
+            "dht.transmissionbt.com"
+        });
+
         static readonly TimeSpan DefaultAnnounceInternal = TimeSpan.FromMinutes (10);
         static readonly TimeSpan DefaultMinimumAnnounceInterval = TimeSpan.FromMinutes (3);
 
@@ -272,7 +278,7 @@ public Task StartAsync ()
             => StartAsync (ReadOnlyMemory<byte>.Empty);
 
         public Task StartAsync (ReadOnlyMemory<byte> initialNodes)
-            => StartAsync (Node.FromCompactNode (BEncodedString.FromMemory (initialNodes)).Concat (PendingNodes), Array.Empty<string> ());
+            => StartAsync (Node.FromCompactNode (BEncodedString.FromMemory (initialNodes)).Concat (PendingNodes), DefaultBootstrapRouters.ToArray ());
 
         public Task StartAsync (params string[] bootstrapRouters)
             => StartAsync (Array.Empty<Node> (), bootstrapRouters);

src/Tests/Tests.MonoTorrent.Dht/MonoTorrent.Dht/TaskTests.cs

@@ -62,12 +62,10 @@ public async Task Setup ()
         [Repeat (10)]
         public async Task InitialiseFailure ()
         {
-            // FIXME: this flakily fails sometimes... why? DNS timeouts?
-            // Block until we've checked the status moved to Initializing
             var errorSource = new TaskCompletionSource<object> ();
             listener.MessageSent += (o, e) => errorSource.Task.GetAwaiter ().GetResult ();
 
-            await engine.StartAsync (new byte[26]);
+            await engine.StartAsync (new byte[26], Array.Empty<string> ());
             Assert.AreEqual (DhtState.Initialising, engine.State);
 
             // Then set an error and make sure the engine state moves to 'NotReady'