Improve error messages when ToolTask overrides exit code 0 to -1 due to logged errors

src/Samples/TestExitCodeOverride/TestExitCodeOverride.proj

@@ -0,0 +1,119 @@
+<Project>
+  <!--
+    Manual test project for ExitCodeOverriddenToIndicateErrors feature.
+
+    Both targets run a command that exits with code 0 but produces output
+    in canonical error format, which MSBuild parses as an error. This causes
+    ExitCode to be overridden from 0 to -1 and triggers the new error messages.
+
+    Usage (from repo root, after building with .\build.cmd):
+      artifacts\bin\bootstrap\core\dotnet.exe build src\Samples\TestExitCodeOverride\TestExitCodeOverride.proj /t:TestExec -v:diag
+      artifacts\bin\bootstrap\core\dotnet.exe build src\Samples\TestExitCodeOverride\TestExitCodeOverride.proj /t:TestToolTask -v:diag
+      artifacts\bin\bootstrap\core\dotnet.exe build src\Samples\TestExitCodeOverride\TestExitCodeOverride.proj /t:TestXamlToolTask -v:diag  (Windows only)
+      artifacts\bin\bootstrap\core\dotnet.exe build src\Samples\TestExitCodeOverride\TestExitCodeOverride.proj /t:TestAll -v:diag
+
+    Expected results:
+      TestExec         -> MSB3077: The command "..." exited with return value 0, but errors were detected ...
+      TestToolTask     -> (low-importance) "The command exited with return value 0, but errors were detected ..."
+                          plus MSB6006 error from the base ToolTask.HandleTaskExecutionErrors
+      TestXamlToolTask -> MSB3725: The command "..." exited with return value 0, but errors were detected ...
+                          (Windows only, uses XamlTaskFactory with cmd.exe)
+  -->
+
+  <!-- ============================================================ -->
+  <!-- Test 1: Exec task                                            -->
+  <!--   The echo command outputs a canonical error and exits 0.    -->
+  <!--   Exec.HandleTaskExecutionErrors logs MSB3077.               -->
+  <!-- ============================================================ -->
+  <Target Name="TestExec">
+    <Message Text="=== Test 1: Exec task — canonical error on stdout, exit code 0 ===" Importance="high" />
+    <Exec Command="echo file.cs(1,1): error ERR001: This is a fake error from a tool that exited successfully"
+          ContinueOnError="true" />
+    <Message Text="=== Test 1 complete. Look for MSB3077 above. ===" Importance="high" />
+  </Target>
+
+  <!-- ============================================================ -->
+  <!-- Test 2: Basic ToolTask (inline task, no HandleTaskExecutionErrors override) -->
+  <!--   Uses RoslynCodeTaskFactory to define a ToolTask derivative  -->
+  <!--   that does NOT override HandleTaskExecutionErrors.           -->
+  <!--   Base class logs low-importance message + MSB6006 error.     -->
+  <!-- ============================================================ -->
+  <UsingTask TaskName="BasicToolTask"
+             TaskFactory="RoslynCodeTaskFactory"
+             AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.Core.dll">
+    <Task>
+      <Reference Include="$(MSBuildToolsPath)\Microsoft.Build.Utilities.Core.dll" />
+      <Code Type="Class" Language="cs">
+        <![CDATA[
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+
+public class BasicToolTask : ToolTask
+{
+    protected override string ToolName => 
+        System.Runtime.InteropServices.RuntimeInformation.IsOSPlatform(
+            System.Runtime.InteropServices.OSPlatform.Windows) ? "cmd.exe" : "sh";
+
+    protected override string GenerateFullPathToTool() =>
+        System.Runtime.InteropServices.RuntimeInformation.IsOSPlatform(
+            System.Runtime.InteropServices.OSPlatform.Windows)
+            ? System.IO.Path.Combine(
+                System.Environment.GetFolderPath(System.Environment.SpecialFolder.System), "cmd.exe")
+            : "/bin/sh";
+
+    protected override string GenerateCommandLineCommands() =>
+        System.Runtime.InteropServices.RuntimeInformation.IsOSPlatform(
+            System.Runtime.InteropServices.OSPlatform.Windows)
+            ? "/C echo file.cs(1,1): error ERR002: Fake error from basic ToolTask"
+            : "-c \"echo 'file.cs(1,1): error ERR002: Fake error from basic ToolTask'\"";
+}
+        ]]>
+      </Code>
+    </Task>
+  </UsingTask>
+
+  <Target Name="TestToolTask">
+    <Message Text="=== Test 2: Basic ToolTask — canonical error on stdout, exit code 0 ===" Importance="high" />
+    <BasicToolTask ContinueOnError="true" />
+    <Message Text="=== Test 2 complete. Look for MSB6006 above (and 'exited with return value 0' at -v:diag). ===" Importance="high" />
+  </Target>
+
+  <!-- ============================================================ -->
+  <!-- Test 3: XamlDataDrivenToolTask (via XamlTaskFactory)         -->
+  <!--   Uses XamlTaskFactory to define a task backed by cmd.exe.   -->
+  <!--   The command template echoes a canonical error and exits 0. -->
+  <!--   XamlDataDrivenToolTask.HandleTaskExecutionErrors logs      -->
+  <!--   MSB3725.                                                   -->
+  <!-- ============================================================ -->
+  <UsingTask TaskName="XamlToolTask"
+             TaskFactory="XamlTaskFactory"
+             AssemblyName="Microsoft.Build.Tasks.Core">
+    <Task>
+      <![CDATA[
+        <ProjectSchemaDefinitions xmlns="clr-namespace:Microsoft.Build.Framework.XamlTypes;assembly=Microsoft.Build.Framework"
+                                  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                                  xmlns:sys="clr-namespace:System;assembly=mscorlib">
+          <Rule Name="XamlToolTask" ToolName="cmd.exe">
+            <StringProperty Name="CommandLineTemplate" />
+          </Rule>
+        </ProjectSchemaDefinitions>
+      ]]>
+    </Task>
+  </UsingTask>
+
+  <Target Name="TestXamlToolTask" Condition="'$(OS)' == 'Windows_NT'">
+    <Message Text="=== Test 3: XamlDataDrivenToolTask — canonical error on stdout, exit code 0 ===" Importance="high" />
+    <XamlToolTask CommandLineTemplate="echo file.cs(1,1): error ERR003: Fake error from XamlDataDrivenToolTask"
+                  EchoOff="true"
+                  ContinueOnError="true" />
+    <Message Text="=== Test 3 complete. Look for MSB3725 above. ===" Importance="high" />
+  </Target>
+
+  <!-- ============================================================ -->
+  <!-- Run all tests                                                -->
+  <!-- ============================================================ -->
+  <Target Name="TestAll" DependsOnTargets="TestExec;TestToolTask;TestXamlToolTask">
+    <Message Text="=== All tests complete ===" Importance="high" />
+  </Target>
+
+</Project>

src/Tasks.UnitTests/Exec_Tests.cs

@@ -246,7 +246,18 @@ public void LoggedErrorsCauseFailureDespiteExitCode0()
             Assert.False(result);
             // Exitcode is set to -1
             Assert.Equal(-1, exec.ExitCode);
-            ((MockEngine)exec.BuildEngine).AssertLogContains("MSB3073");
+
+            MockEngine engine = (MockEngine)exec.BuildEngine;
+
+            // Should log the special "exited zero with errors" message
+            engine.AssertLogContains("MSB3077");
+
+            // Should not log other failure-related error codes
+            engine.AssertLogDoesntContain("MSB3073");
+            engine.AssertLogDoesntContain("MSB6006");
+
+            // Errors: canonical error from tool output + MSB3077 from HandleTaskExecutionErrors
+            engine.Errors.ShouldBe(2);
         }
 
         [Fact]

src/Tasks.UnitTests/XamlDataDrivenToolTask_Tests.cs

@@ -5,8 +5,14 @@
 using System.IO;
 using System.Reflection;
 using Microsoft.Build.Evaluation;
+using Microsoft.Build.Framework;
 using Microsoft.Build.Shared;
+using Microsoft.Build.Tasks;
+using Microsoft.Build.Tasks.Xaml;
+using Microsoft.Build.Utilities;
+using Shouldly;
 using Xunit;
+using Xunit.Abstractions;
 
 #nullable disable
 
@@ -382,4 +388,65 @@ public void SquareBracketEscaping()
             logger.AssertLogContains("echo 14) [[notaproperty]] end");
         }
     }
+
+    /// <summary>
+    /// Tests for XamlDataDrivenToolTask error handling using direct task instantiation.
+    /// </summary>
+    public class ErrorHandlingTests
+    {
+        private readonly ITestOutputHelper _output;
+
+        public ErrorHandlingTests(ITestOutputHelper output)
+        {
+            _output = output;
+        }
+
+        /// <summary>
+        /// Tests that when a XamlDataDrivenTask's tool exits with code 0 but canonical errors
+        /// were logged during execution, the special "exited zero with errors" message (MSB3725)
+        /// is used instead of the normal command failure message (MSB3721).
+        /// </summary>
+        [Fact]
+        public void ExitCodeZeroWithLoggedErrors_LogsMSB3725()
+        {
+            var cmdLine = NativeMethodsShared.IsWindows
+                ? "echo myfile(88,37): error AB1234: thisisacanonicalerror"
+                : "echo \"myfile(88,37): error AB1234: thisisacanonicalerror\"";
+
+            var task = new TestableXamlDataDrivenToolTask();
+            task.BuildEngine = new MockEngine(_output);
+            task.CommandLineTemplate = cmdLine;
+            task.EchoOff = true;
+
+            bool result = task.Execute();
+
+            result.ShouldBeFalse();
+
+            MockEngine engine = (MockEngine)task.BuildEngine;
+
+            // Should log the special "exited zero with errors" message, not the normal command failure
+            engine.AssertLogContains("MSB3725");
+
+            // Should not log other error messages related to XamlDataDrivenToolTask command failure 
+            engine.AssertLogDoesntContain("MSB3721");
+            engine.AssertLogDoesntContain("MSB3722");
+
+            // Errors: canonical error from tool output + MSB3725 from HandleTaskExecutionErrors
+            engine.Errors.ShouldBe(2);
+        }
+    }
+
+    /// <summary>
+    /// A concrete subclass of XamlDataDrivenToolTask for direct unit testing
+    /// without requiring a project file or XamlTaskFactory.
+    /// </summary>
+    internal sealed class TestableXamlDataDrivenToolTask : XamlDataDrivenToolTask
+    {
+        public TestableXamlDataDrivenToolTask()
+            : base(Array.Empty<string>(), null)
+        {
+        }
+
+        protected override string ToolName => "unused";
+    }
 }

src/Tasks/Exec.cs

@@ -325,10 +325,15 @@ protected override bool HandleTaskExecutionErrors()
             {
                 Log.LogErrorWithCodeFromResources("Exec.CommandFailedAccessDenied", commandForLog, ExitCode);
             }
+            else if (ExitCodeOverriddenToIndicateErrors)
+            {
+                Log.LogErrorWithCodeFromResources("Exec.CommandExitedZeroWithErrors", commandForLog);
+            }
             else
             {
                 Log.LogErrorWithCodeFromResources("Exec.CommandFailed", commandForLog, ExitCode);
             }
+
             return false;
         }
 

src/Tasks/Resources/Strings.resx

@@ -397,6 +397,10 @@
     <value>MSB3071: All drive letters from A: through Z: are currently in use. Since the working directory "{0}" is a UNC path, the "Exec" task needs a free drive letter to map the UNC path to. Disconnect from one or more shared resources to free up drive letters, or specify a local working directory before attempting this command again.</value>
     <comment>{StrBegin="MSB3071: "}LOCALIZATION: "Exec", "A:", and "Z:" should not be localized.</comment>
   </data>
+  <data name="Exec.MissingCommandError">
+    <value>MSB3072: The "Exec" task needs a command to execute.</value>
+    <comment>{StrBegin="MSB3072: "}LOCALIZATION: "Exec" should not be localized.</comment>
+  </data>
   <data name="Exec.CommandFailed">
     <value>MSB3073: The command "{0}" exited with code {1}.</value>
     <comment>{StrBegin="MSB3073: "}</comment>
@@ -412,9 +416,9 @@
     <value>MSB3076: The regular expression "{0}" that was supplied is invalid. {1}</value>
     <comment>{StrBegin="MSB3076: "}</comment>
   </data>
-  <data name="Exec.MissingCommandError">
-    <value>MSB3072: The "Exec" task needs a command to execute.</value>
-    <comment>{StrBegin="MSB3072: "}LOCALIZATION: "Exec" should not be localized.</comment>
+  <data name="Exec.CommandExitedZeroWithErrors">
+    <value>MSB3077: The command "{0}" exited with return value 0, but errors were detected during execution. ExitCode was set to -1.</value>
+    <comment>{StrBegin="MSB3077: "}</comment>
   </data>
   <data name="Exec.InvalidWorkingDirectory">
     <value>The working directory "{0}" does not exist.</value>
@@ -2319,6 +2323,10 @@
       <value>MSB3724: Unable to create Xaml task.  Duplicate property name '{0}'.</value>
     <comment>{StrBegin="MSB3724: "}</comment>
   </data>
+  <data name="Xaml.CommandExitedZeroWithErrors">
+    <value>MSB3725: The command "{0}" exited with return value 0, but errors were detected during execution. ExitCode was set to -1.</value>
+    <comment>{StrBegin="MSB3725: "}</comment>
+  </data>
 
   <!--
         The XslTransformation message bucket is: MSB3701-3710.