Add env var to control host priority

src/Microsoft.TestPlatform.CoreUtilities/Helpers/EnvironmentVariableHelper.cs

@@ -13,8 +13,15 @@ namespace Microsoft.VisualStudio.TestPlatform.CoreUtilities.Helpers;
 
 internal class EnvironmentVariableHelper : IEnvironmentVariableHelper
 {
+    /// <inheritdoc />
     public string GetEnvironmentVariable(string variable)
         => Environment.GetEnvironmentVariable(variable);
+
+    /// <inheritdoc />
+    public TEnum GetEnvironmentVariableAsEnum<TEnum>(string variable, TEnum defaultValue = default) where TEnum : struct, Enum
+        => Environment.GetEnvironmentVariable(variable) is string value && !string.IsNullOrEmpty(value)
+            ? Enum.TryParse<TEnum>(value, out var enumValue) ? enumValue : defaultValue
+            : defaultValue;
 }
 
 #endif

src/Microsoft.TestPlatform.CoreUtilities/Helpers/Interfaces/IEnvironmentVariableHelper.cs

@@ -3,9 +3,25 @@
 
 #nullable disable
 
+using System;
+
 namespace Microsoft.VisualStudio.TestPlatform.Utilities.Helpers.Interfaces;
 
 internal interface IEnvironmentVariableHelper
 {
+    /// <summary>
+    /// Retrieves the value of an environment variable from the current process.
+    /// </summary>
+    /// <param name="variable">The name of the environment variable.</param>
+    /// <returns>The value of the environment variable specified by variable, or null if the environment variable is not found.</returns>
     string GetEnvironmentVariable(string variable);
+
+    /// <summary>
+    /// Retrieves the value of an environment variable from the current process and converts it to the given type.
+    /// </summary>
+    /// <typeparam name="TEnum">The type used for conversion.</typeparam>
+    /// <param name="variable">The name of the environment variable.</param>
+    /// <param name="defaultValue">The default value to return if the environment variable is not found.</param>
+    /// <returns></returns>
+    TEnum GetEnvironmentVariableAsEnum<TEnum>(string variable, TEnum defaultValue = default) where TEnum : struct, Enum;
 }

src/Microsoft.TestPlatform.TestHostProvider/Hosting/DefaultTestHostManager.cs

@@ -17,6 +17,7 @@
 using Microsoft.TestPlatform.TestHostProvider.Hosting;
 using Microsoft.TestPlatform.TestHostProvider.Resources;
 using Microsoft.VisualStudio.TestPlatform.CoreUtilities.Extensions;
+using Microsoft.VisualStudio.TestPlatform.CoreUtilities.Helpers;
 using Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Helpers;
 using Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Helpers.Interfaces;
 using Microsoft.VisualStudio.TestPlatform.DesktopTestHostRuntimeProvider;
@@ -57,6 +58,7 @@ public class DefaultTestHostManager : ITestRuntimeProvider2
     private readonly IFileHelper _fileHelper;
     private readonly IEnvironment _environment;
     private readonly IDotnetHostHelper _dotnetHostHelper;
+    private readonly IEnvironmentVariableHelper _environmentVariableHelper;
 
     private ITestHostLauncher _customTestHostLauncher;
     private Process _testHostProcess;
@@ -68,7 +70,12 @@ public class DefaultTestHostManager : ITestRuntimeProvider2
     /// Initializes a new instance of the <see cref="DefaultTestHostManager"/> class.
     /// </summary>
     public DefaultTestHostManager()
-        : this(new ProcessHelper(), new FileHelper(), new PlatformEnvironment(), new DotnetHostHelper())
+        : this(
+            new ProcessHelper(),
+            new FileHelper(),
+            new DotnetHostHelper(),
+            new PlatformEnvironment(),
+            new EnvironmentVariableHelper())
     {
     }
 
@@ -79,12 +86,18 @@ public DefaultTestHostManager()
     /// <param name="fileHelper">File helper instance.</param>
     /// <param name="environment">Instance of platform environment.</param>
     /// <param name="dotnetHostHelper">Instance of dotnet host helper.</param>
-    internal DefaultTestHostManager(IProcessHelper processHelper, IFileHelper fileHelper, IEnvironment environment, IDotnetHostHelper dotnetHostHelper)
+    internal DefaultTestHostManager(
+        IProcessHelper processHelper,
+        IFileHelper fileHelper,
+        IDotnetHostHelper dotnetHostHelper,
+        IEnvironment environment,
+        IEnvironmentVariableHelper environmentVariableHelper)
     {
         _processHelper = processHelper;
         _fileHelper = fileHelper;
-        _environment = environment;
         _dotnetHostHelper = dotnetHostHelper;
+        _environment = environment;
+        _environmentVariableHelper = environmentVariableHelper;
     }
 
     /// <inheritdoc/>
@@ -469,8 +482,31 @@ private bool LaunchHost(TestProcessStartInfo testHostStartInfo, CancellationToke
             _processHelper.SetExitCallback(processId, ExitCallBack);
         }
 
+        if (_testHostProcess is null)
+        {
+            return false;
+        }
+
+        SetProcessPriority(_testHostProcess, _environmentVariableHelper);
         OnHostLaunched(new HostProviderEventArgs("Test Runtime launched", 0, _testHostProcess.Id));
-        return _testHostProcess != null;
+
+        return true;
+    }
+
+    internal static void SetProcessPriority(Process testHostProcess, IEnvironmentVariableHelper environmentVariableHelper)
+    {
+        ProcessPriorityClass testHostPriority = ProcessPriorityClass.BelowNormal;
+        try
+        {
+            testHostPriority = environmentVariableHelper.GetEnvironmentVariableAsEnum("VSTEST_HOST_INTERNAL_PRIORITY", testHostPriority);
+            testHostProcess.PriorityClass = testHostPriority;
+            EqtTrace.Verbose("Setting test host process priority to {0}", testHostProcess.PriorityClass);
+        }
+        // Setting the process Priority can fail with Win32Exception, NotSupportedException or InvalidOperationException.
+        catch (Exception ex)
+        {
+            EqtTrace.Error("Failed to set test host process priority to {0}. Exception: {1}", testHostPriority, ex);
+        }
     }
 
     private string GetUwpSources(string uwpSource)

src/Microsoft.TestPlatform.TestHostProvider/Hosting/DotnetTestHostManager.cs

@@ -688,9 +688,14 @@ private bool LaunchHost(TestProcessStartInfo testHostStartInfo, CancellationToke
             _processHelper.SetExitCallback(processId, ExitCallBack);
         }
 
-        OnHostLaunched(new HostProviderEventArgs("Test Runtime launched", 0, _testHostProcess.Id));
+        if (_testHostProcess is null)
+        {
+            return false;
+        }
 
-        return _testHostProcess != null;
+        DefaultTestHostManager.SetProcessPriority(_testHostProcess, _environmentVariableHelper);
+        OnHostLaunched(new HostProviderEventArgs("Test Runtime launched", 0, _testHostProcess.Id));
+        return true;
     }
 
     private string GetTestHostPath(string runtimeConfigDevPath, string depsFilePath, string sourceDirectory)

test/Microsoft.TestPlatform.TestHostProvider.UnitTests/Hosting/DefaultTestHostManagerTests.cs

@@ -12,6 +12,7 @@
 using System.Threading.Tasks;
 
 using Microsoft.VisualStudio.TestPlatform.CoreUtilities.Extensions;
+using Microsoft.VisualStudio.TestPlatform.CoreUtilities.Helpers;
 using Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Helpers;
 using Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Helpers.Interfaces;
 using Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Hosting;
@@ -39,6 +40,7 @@ public class DefaultTestHostManagerTests
     private readonly Mock<IFileHelper> _mockFileHelper;
     private readonly Mock<IDotnetHostHelper> _mockDotnetHostHelper;
     private readonly Mock<IEnvironment> _mockEnvironment;
+    private readonly Mock<IEnvironmentVariableHelper> _mockEnvironmentVariable;
     private readonly DefaultTestHostManager _testHostManager;
 
     private TestableTestHostManager? _testableTestHostManager;
@@ -53,10 +55,11 @@ public DefaultTestHostManagerTests()
         _mockProcessHelper.Setup(ph => ph.GetCurrentProcessFileName()).Returns("vstest.console.exe");
         _mockDotnetHostHelper = new Mock<IDotnetHostHelper>();
         _mockEnvironment = new Mock<IEnvironment>();
+        _mockEnvironmentVariable = new Mock<IEnvironmentVariableHelper>();
 
         _mockMessageLogger = new Mock<IMessageLogger>();
 
-        _testHostManager = new DefaultTestHostManager(_mockProcessHelper.Object, _mockFileHelper.Object, _mockEnvironment.Object, _mockDotnetHostHelper.Object);
+        _testHostManager = new DefaultTestHostManager(_mockProcessHelper.Object, _mockFileHelper.Object, _mockDotnetHostHelper.Object, _mockEnvironment.Object, _mockEnvironmentVariable.Object);
         _testHostManager.Initialize(_mockMessageLogger.Object, $"<?xml version=\"1.0\" encoding=\"utf-8\"?><RunSettings> <RunConfiguration> <TargetPlatform>{Architecture.X64}</TargetPlatform> <TargetFrameworkVersion>{Framework.DefaultFramework}</TargetFrameworkVersion> <DisableAppDomain>{false}</DisableAppDomain> </RunConfiguration> </RunSettings>");
         _startInfo = _testHostManager.GetTestHostProcessStartInfo(Enumerable.Empty<string>(), null, default);
     }
@@ -592,7 +595,7 @@ public TestableTestHostManager(
             IProcessHelper processHelper,
             bool shared,
             IMessageLogger logger)
-            : base(processHelper, new FileHelper(), new PlatformEnvironment(), new DotnetHostHelper())
+            : base(processHelper, new FileHelper(), new DotnetHostHelper(), new PlatformEnvironment(), new EnvironmentVariableHelper())
         {
             Initialize(logger, $"<?xml version=\"1.0\" encoding=\"utf-8\"?><RunSettings> <RunConfiguration> <TargetPlatform>{architecture}</TargetPlatform> <TargetFrameworkVersion>{framework}</TargetFrameworkVersion> <DisableAppDomain>{!shared}</DisableAppDomain> </RunConfiguration> </RunSettings>");
         }