Fix task host launch regressions from apphost support

src/Build/BackEnd/Components/Communications/NodeLauncher.cs

@@ -83,14 +83,17 @@ private string ResolveExecutableName(string msbuildLocation, out bool isNativeAp
             isNativeAppHost = false;
 
 #if RUNTIME_TYPE_NETCORE
-            // If msbuildLocation is a native app host (e.g., MSBuild.exe on Windows, MSBuild on Linux), run it directly.
-            // Otherwise, use dotnet.exe to run the managed assembly (e.g., MSBuild.dll).
             string fileName = Path.GetFileName(msbuildLocation);
-            isNativeAppHost = fileName.Equals(Constants.MSBuildExecutableName, StringComparison.OrdinalIgnoreCase);
-            if (!isNativeAppHost)
+
+            // Only managed assemblies (.dll) need dotnet.exe as a host.
+            // All native executables — MSBuild app host, MSBuildTaskHost.exe, etc. — run directly.
+            if (fileName.EndsWith(".dll", StringComparison.OrdinalIgnoreCase))
             {
                 return CurrentHost.GetCurrentHost();
             }
+
+            // Any .exe or extensionless binary (Linux app host) is a native executable.
+            isNativeAppHost = true;
 #endif
             return msbuildLocation;
         }

src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcTaskHost.cs

@@ -730,10 +730,19 @@ private NodeLaunchData ResolveAppHostOrFallback(
                         dotnetOverrides);
             }
 
-            CommunicationsUtilities.Trace("For a host context of {0}, app host not found at {1}, falling back to dotnet.exe from {2}.", hostContext, appHostPath, dotnetHostPath);
+            // Auto-discover dotnet host path when not explicitly provided.
+            string resolvedDotnetHostPath = dotnetHostPath;
+#if RUNTIME_TYPE_NETCORE
+            if (string.IsNullOrEmpty(resolvedDotnetHostPath))
+            {
+                resolvedDotnetHostPath = CurrentHost.GetCurrentHost();
+            }
+#endif
+
+            CommunicationsUtilities.Trace("For a host context of {0}, app host not found at {1}, falling back to dotnet.exe from {2}.", hostContext, appHostPath, resolvedDotnetHostPath);
 
             return new NodeLaunchData(
-                dotnetHostPath,
+                resolvedDotnetHostPath,
                 $"\"{Path.Combine(msbuildAssemblyPath, Constants.MSBuildAssemblyName)}\" {commandLineArgs}",
                 new Handshake(hostContext, toolsDirectory: msbuildAssemblyPath));
         }

src/Build/Instance/TaskFactories/TaskHostTask.cs

@@ -5,6 +5,7 @@
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.IO;
 using System.Reflection;
 using System.Threading;
 using Microsoft.Build.BackEnd.Logging;
@@ -13,6 +14,7 @@
 using Microsoft.Build.Framework;
 using Microsoft.Build.Internal;
 using Microsoft.Build.Shared;
+using Constants = Microsoft.Build.Framework.Constants;
 #if FEATURE_REPORTFILEACCESSES
 using Microsoft.Build.Experimental.FileAccess;
 using Microsoft.Build.FileAccesses;
@@ -700,13 +702,20 @@ private void HandleCoresRequest(TaskHostCoresRequest request)
         /// </summary>
         private void LogErrorUnableToCreateTaskHost(HandshakeOptions requiredContext, string runtime, string architecture, Exception e)
         {
-            string taskHostLocation = NodeProviderOutOfProcTaskHost.GetMSBuildExecutablePathForNonNETRuntimes(requiredContext);
-#if NETFRAMEWORK
+            string taskHostLocation;
+
             if (Handshake.IsHandshakeOptionEnabled(requiredContext, HandshakeOptions.NET))
             {
-                taskHostLocation = NodeProviderOutOfProcTaskHost.GetMSBuildLocationForNETRuntime(requiredContext, _taskHostParameters).MSBuildPath;
+                (_, string msbuildPath) = NodeProviderOutOfProcTaskHost.GetMSBuildLocationForNETRuntime(requiredContext, _taskHostParameters);
+                taskHostLocation = msbuildPath != null
+                    ? Path.Combine(msbuildPath, Constants.MSBuildExecutableName)
+                    : null;
             }
-#endif
+            else
+            {
+                taskHostLocation = NodeProviderOutOfProcTaskHost.GetMSBuildExecutablePathForNonNETRuntimes(requiredContext);
+            }
+
             string msbuildLocation = taskHostLocation ??
                 // We don't know the path -- probably we're trying to get a 64-bit assembly on a
                 // 32-bit machine.  At least give them the exe name to look for, though ...