Add MSBUILDDEBUGONSTART=3 to skip debugging TaskHost child processes

src/Build.UnitTests/BackEnd/DebugUtils_tests.cs

@@ -5,6 +5,7 @@
 using System.IO;
 using System.Linq;
 using Microsoft.Build.Shared;
+using Microsoft.Build.Shared.Debugging;
 using Shouldly;
 using Xunit;
 
@@ -38,5 +39,13 @@ public void DumpExceptionToFileShouldWriteInDebugDumpPath()
                 File.Delete(exceptionFile);
             }
         }
+
+        [Fact]
+        public void IsInTaskHostNode_ReturnsFalseForCentralNode()
+        {
+            // When running in the main test process (no /nodemode argument),
+            // we should not be in a TaskHost node
+            DebugUtils.IsInTaskHostNode().ShouldBeFalse();
+        }
     }
 }

src/MSBuild/XMake.cs

@@ -597,6 +597,13 @@ private static void DebuggerLaunchCheck()
                 case "1":
                     Debugger.Launch();
                     break;
+                case "3":
+                    // Value "3" debugs the main MSBuild process but skips debugging child TaskHost processes
+                    if (!DebugUtils.IsInTaskHostNode())
+                    {
+                        Debugger.Launch();
+                    }
+                    break;
 #endif
                 case "2":
                     // Sometimes easier to attach rather than deal with JIT prompt

src/MSBuildTaskHost/OutOfProcTaskHost.cs

@@ -94,6 +94,10 @@ internal static ExitType Execute()
 
                     Console.ReadLine();
                     break;
+                case "3":
+                    // Value "3" skips debugging for TaskHost processes but debugs the main MSBuild process
+                    // This is useful when you want to debug MSBuild but not the child TaskHost processes
+                    break;
             }
 
             bool restart = false;

src/Shared/Debugging/DebugUtils.cs

@@ -105,6 +105,16 @@ private static bool CurrentProcessMatchesDebugName()
 
         public static string DebugPath { get; private set; }
 
+        /// <summary>
+        /// Returns true if the current process is an out-of-proc TaskHost node.
+        /// </summary>
+        /// <returns>
+        /// True if this process was launched with /nodemode:2 (indicating it's a TaskHost process),
+        /// false otherwise. This is useful for conditionally enabling debugging or other behaviors
+        /// based on whether the code is running in the main MSBuild process or a child TaskHost process.
+        /// </returns>
+        public static bool IsInTaskHostNode() => ProcessNodeMode.Value == NodeMode.OutOfProcTaskHostNode;
+
         public static string FindNextAvailableDebugFilePath(string fileName)
         {
             var extension = Path.GetExtension(fileName);