Fix Connect call on Linux.

Socket.Connect overloads that might attempt to create connections to multiple IP addresses are not supported: https://github.com/dotnet/corefx/issues/5829.

As per dotnet/corefx@30bd4b7, the workaround is to resolve the name to a collection of IP addresses and try them sequentially.

Also implemented support for comma-delimited hostnames (even though this appears to have been dropped from the official connector in mysql/mysql-connector-net@03f3a23.)

src/MySql.Data/MySqlClient/MySqlConnection.cs

@@ -103,7 +103,13 @@ public override async Task OpenAsync(CancellationToken cancellationToken)
 				if (m_session == null)
 				{
 					m_session = new MySqlSession(pool);
-					await m_session.ConnectAsync(m_connectionStringBuilder.Server, (int) m_connectionStringBuilder.Port).ConfigureAwait(false);
+					var connected = await m_session.ConnectAsync(m_connectionStringBuilder.Server.Split(','), (int) m_connectionStringBuilder.Port).ConfigureAwait(false);
+					if (!connected)
+					{
+						SetState(ConnectionState.Closed);
+						throw new MySqlException("Unable to connect to any of the specified MySQL hosts.");
+					}
+
 					var payload = await m_session.ReceiveAsync(cancellationToken).ConfigureAwait(false);
 					var reader = new ByteArrayReader(payload.ArraySegment.Array, payload.ArraySegment.Offset, payload.ArraySegment.Count);
 					var initialHandshake = new InitialHandshakePacket(reader);

src/MySql.Data/Serialization/MySqlSession.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Collections.Generic;
+using System.Net;
 using System.Net.Sockets;
 using System.Threading;
 using System.Threading.Tasks;
@@ -34,16 +36,48 @@ public void Dispose()
 			m_state = State.Closed;
 		}
 
-		public async Task ConnectAsync(string hostname, int port)
+		public async Task<bool> ConnectAsync(IEnumerable<string> hostnames, int port)
 		{
-			m_socket = new Socket(SocketType.Stream, ProtocolType.Tcp);
+			foreach (var hostname in hostnames)
+			{
+				IPAddress[] ipAddresses;
+				try
+				{
+					ipAddresses = await Dns.GetHostAddressesAsync(hostname).ConfigureAwait(false);
+				}
+				catch (SocketException)
+				{
+					// name couldn't be resolved
+					continue;
+				}
+
+				// need to try IP Addresses one at a time: https://github.com/dotnet/corefx/issues/5829
+				foreach (var ipAddress in ipAddresses)
+				{
+					Socket socket = null;
+					try
+					{
+						socket = new Socket(SocketType.Stream, ProtocolType.Tcp);
 #if NETSTANDARD1_3
-			await m_socket.ConnectAsync(hostname, port).ConfigureAwait(false);
+						await socket.ConnectAsync(ipAddress, port).ConfigureAwait(false);
 #else
-			await Task.Factory.FromAsync(m_socket.BeginConnect, m_socket.EndConnect, hostname, port, null).ConfigureAwait(false);
+						await Task.Factory.FromAsync(socket.BeginConnect, socket.EndConnect, hostname, port, null).ConfigureAwait(false);
 #endif
-			m_transmitter = new PacketTransmitter(m_socket);
-			m_state = State.Connected;
+					}
+					catch (SocketException)
+					{
+						Utility.Dispose(ref socket);
+						continue;
+					}
+
+					m_socket = socket;
+					m_transmitter = new PacketTransmitter(m_socket);
+					m_state = State.Connected;
+					return true;
+				}
+			}
+
+			return false;
 		}
 
 		// Starts a new conversation with the server by sending the first packet.

src/MySql.Data/project.json

@@ -30,6 +30,7 @@
 				"System.Data.Common": "4.0.1-*",
 				"System.Globalization": "4.0.11-*",
 				"System.IO": "4.1.0-*",
+				"System.Net.NameResolution": "4.0.0-*",
 				"System.Net.Sockets": "4.1.0-*",
 				"System.Runtime": "4.1.0-*",
 				"System.Runtime.Extensions": "4.1.0-*",

tests/SideBySide.New/ConnectAsync.cs

@@ -54,6 +54,28 @@ public async Task State()
 			}
 		}
 
+#if BASELINE
+		[Fact(Skip = "Doesn't implement \"Multiple hosts can be specified separated by commas.\" behavior as documented.")]
+#else
+		[Fact]
+#endif
+		public async Task ConnectMultipleHostNames()
+		{
+			var csb = new MySqlConnectionStringBuilder
+			{
+				Server = "www.mysql.com,invalid.example.net,localhost",
+				Port = 3306,
+				UserID = Constants.UserName,
+				Password = Constants.Password,
+			};
+			using (var connection = new MySqlConnection(csb.ConnectionString))
+			{
+				Assert.Equal(ConnectionState.Closed, connection.State);
+				await connection.OpenAsync();
+				Assert.Equal(ConnectionState.Open, connection.State);
+			}
+		}
+
 		[Fact]
 		public async Task ServerVersion()
 		{

tests/SideBySide.New/ConnectSync.cs

@@ -66,6 +66,30 @@ public void PersistSecurityInfo(bool persistSecurityInfo)
 		public void State()
 		{
 			using (var connection = new MySqlConnection(m_database.Connection.ConnectionString))
+			{
+				Assert.Equal(ConnectionState.Closed, connection.State);
+				connection.Open();
+				Assert.Equal(ConnectionState.Open, connection.State);
+				connection.Close();
+				Assert.Equal(ConnectionState.Closed, connection.State);
+			}
+		}
+
+#if BASELINE
+		[Fact(Skip = "Doesn't implement \"Multiple hosts can be specified separated by commas.\" behavior as documented.")]
+#else
+		[Fact]
+#endif
+		public void ConnectMultipleHostNames()
+		{
+			var csb = new MySqlConnectionStringBuilder
+			{
+				Server = "www.mysql.com,invalid.example.net,localhost",
+				Port = 3306,
+				UserID = Constants.UserName,
+				Password = Constants.Password,
+			};
+			using (var connection = new MySqlConnection(csb.ConnectionString))
 			{
 				Assert.Equal(ConnectionState.Closed, connection.State);
 				connection.Open();