chore: Add fail-fast logics when --serve option enabled & port is already used

src/docfx/Models/BuildCommand.cs

@@ -14,6 +14,12 @@ public override int Execute(CommandContext context, BuildCommandOptions settings
     {
         return CommandHelper.Run(settings, () =>
         {
+            if (settings.Serve && CommandHelper.IsTcpPortAlreadyUsed(settings.Host, settings.Port))
+            {
+                Logger.LogError($"Serve option specified. But TCP port {settings.Port ?? 8080} is already being in use.");
+                return;
+            }
+
             var (config, baseDirectory) = Docset.GetConfig(settings.ConfigFile);
             MergeOptionsToConfig(settings, config.build, baseDirectory);
             var serveDirectory = RunBuild.Exec(config.build, new(), baseDirectory, settings.OutputFolder);

src/docfx/Models/CommandHelper.cs

@@ -1,8 +1,12 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Net;
+using System.Net.NetworkInformation;
 using Docfx.Common;
 
+#nullable enable
+
 namespace Docfx;
 
 internal class CommandHelper
@@ -49,4 +53,33 @@ public static int Run(LogOptions options, Action run)
 
         return Logger.HasError ? -1 : 0;
     }
+
+    public static bool IsTcpPortAlreadyUsed(string? host, int? port)
+    {
+        port ??= 8080;
+
+        var ipGlobalProperties = IPGlobalProperties.GetIPGlobalProperties();
+        var ipEndpoints = ipGlobalProperties.GetActiveTcpListeners()
+                                            .Where(x => x.Port == port);
+
+        if (!ipEndpoints.Any())
+            return false; // Specified port is not used by any endpoint.
+
+        switch (host)
+        {
+            case null:
+            case "localhost":
+                return ipEndpoints.Any(x => IPAddress.IsLoopback(x.Address)); // Check both IPv4/IPv6 loopback address.
+            default:
+                if (IPAddress.TryParse(host, out var address))
+                {
+                    return ipEndpoints.Any(x => x.Address == address);
+                }
+                else
+                {
+                    // Anything not recognized as a valid IP address (e.g. `*`) binds to all IPv4 and IPv6 IPAddresses.
+                    return true;
+                }
+        }
+    }
 }

src/docfx/Models/DefaultCommand.cs

@@ -3,6 +3,7 @@
 
 using System.ComponentModel;
 using System.Reflection;
+using Docfx.Common;
 using Docfx.Dotnet;
 using Docfx.Pdf;
 using Spectre.Console.Cli;
@@ -29,6 +30,12 @@ public override int Execute(CommandContext context, Options options)
 
         return CommandHelper.Run(options, () =>
         {
+            if (options.Serve && CommandHelper.IsTcpPortAlreadyUsed(options.Host, options.Port))
+            {
+                Logger.LogError($"Serve option specified. But TCP port {options.Port ?? 8080} is already being in use.");
+                return;
+            }
+
             var (config, configDirectory) = Docset.GetConfig(options.ConfigFile);
             var outputFolder = options.OutputFolder;
             string serveDirectory = null;