Port in use error message

src/Tgstation.Server.Api/Models/ErrorCode.cs

@@ -477,5 +477,11 @@ namespace Tgstation.Server.Api.Models
 		/// </summary>
 		[Description("Cannot start DreamDaemon headless with the BYOND pager running!")]
 		DeploymentPagerRunning,
+
+		/// <summary>
+		/// Could not bind to port we wanted to launch DreamDaemon on.
+		/// </summary>
+		[Description("Could not bind to requested DreamDaemon port! Is there another service running on that port?")]
+		DreamDaemonPortInUse,
 	}
 }

src/Tgstation.Server.Host/Components/Session/SessionControllerFactory.cs

@@ -3,6 +3,8 @@ using Microsoft.Extensions.Logging;
 using System;
 using System.Globalization;
 using System.Linq;
+using System.Net;
+using System.Net.Sockets;
 using System.Threading;
 using System.Threading.Tasks;
 using Tgstation.Server.Api;
@@ -201,6 +203,7 @@ namespace Tgstation.Server.Host.Components.Session
 					if (launchParameters.SecurityLevel == DreamDaemonSecurity.Trusted)
 						await byondLock.TrustDmbPath(ioManager.ConcatPath(basePath, dmbProvider.DmbName), cancellationToken).ConfigureAwait(false);
 
+					PortBindTest(portToUse.Value);
 					await CheckPagerIsNotRunning(cancellationToken).ConfigureAwait(false);
 
 					var accessIdentifier = cryptographySuite.GetSecureString();
@@ -214,7 +217,7 @@ namespace Tgstation.Server.Host.Components.Session
 					// important to run on all ports to allow port changing
 					var arguments = String.Format(CultureInfo.InvariantCulture, "{0} -port {1} -ports 1-65535 {2}-close -{3} -{4} -public -params \"{5}\"",
 						dmbProvider.DmbName,
-						primaryPort ? launchParameters.PrimaryPort : launchParameters.SecondaryPort,
+						portToUse,
 						launchParameters.AllowWebClient.Value ? "-webclient " : String.Empty,
 						SecurityWord(launchParameters.SecurityLevel.Value),
 						visibility,
@@ -422,5 +425,23 @@ namespace Tgstation.Server.Host.Components.Session
 			if(otherUserName.Equals(ourUserName, StringComparison.Ordinal))
 				throw new JobException(ErrorCode.DeploymentPagerRunning);
 		}
+
+		/// <summary>
+		/// Check if a given <paramref name="port"/> can be bound to.
+		/// </summary>
+		/// <param name="port">The port number to test.</param>
+		void PortBindTest(ushort port)
+		{
+			using var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+
+			try
+			{
+				socket.Bind(new IPEndPoint(IPAddress.Loopback, port));
+			}
+			catch (Exception ex)
+			{
+				throw new JobException(ErrorCode.DreamDaemonPortInUse, ex);
+			}
+		}
 	}
 }