Escape hatch: share stdout with node children (#3130)

The VSTest runner emits log messages to its stdout. So far, in .NET
Core, that has resulted in them getting piped all the way up through
the worker MSBuild that launched the task, the entry-point MSBuild
coordinating the build, and the `dotnet test` invocation that started
it all.

The introduction of node reuse for .NET Core makes this untenable:
worker nodes should not share stdout with the process that happened to
launch them, because they will be long-lived and may do entirely
unrelated builds.

But for now, this breaks the `dotnet test` scenario. This commit creates
an escape hatch environment variable
`MSBUILDENSURESTDOUTFORTASKPROCESSES` that can be used in combination
with `/nodereuse:false` to create a cone of MSBuild processes that all
share std handles.

Enables workaround for microsoft/vstest#1503.

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

@@ -23,6 +23,7 @@
 
 using BackendNativeMethods = Microsoft.Build.BackEnd.NativeMethods;
 using System.Threading.Tasks;
+using Microsoft.Build.Utilities;
 
 namespace Microsoft.Build.BackEnd
 {
@@ -470,14 +471,21 @@ private int LaunchNode(string msbuildLocation, string commandLineArgs)
             // Null out the process handles so that the parent process does not wait for the child process
             // to exit before it can exit.
             uint creationFlags = 0;
-            startInfo.dwFlags = BackendNativeMethods.STARTFUSESTDHANDLES;
+            if (Traits.Instance.EscapeHatches.EnsureStdOutForChildNodesIsPrimaryStdout)
+            {
+                creationFlags = BackendNativeMethods.NORMALPRIORITYCLASS;
+            }
 
             if (String.IsNullOrEmpty(Environment.GetEnvironmentVariable("MSBUILDNODEWINDOW")))
             {
-                startInfo.hStdError = BackendNativeMethods.InvalidHandle;
-                startInfo.hStdInput = BackendNativeMethods.InvalidHandle;
-                startInfo.hStdOutput = BackendNativeMethods.InvalidHandle;
-                creationFlags = creationFlags | BackendNativeMethods.CREATENOWINDOW;
+                if (!Traits.Instance.EscapeHatches.EnsureStdOutForChildNodesIsPrimaryStdout)
+                {
+                    startInfo.hStdError = BackendNativeMethods.InvalidHandle;
+                    startInfo.hStdInput = BackendNativeMethods.InvalidHandle;
+                    startInfo.hStdOutput = BackendNativeMethods.InvalidHandle;
+                    startInfo.dwFlags = BackendNativeMethods.STARTFUSESTDHANDLES;
+                    creationFlags = creationFlags | BackendNativeMethods.CREATENOWINDOW;
+                }
             }
             else
             {
@@ -497,12 +505,6 @@ private int LaunchNode(string msbuildLocation, string commandLineArgs)
             // Run the child process with the same host as the currently-running process.
             exeName = GetCurrentHost();
             commandLineArgs = "\"" + msbuildLocation + "\" " + commandLineArgs;
-
-            if (NativeMethodsShared.IsWindows)
-            {
-                // Repeat the executable name _again_ because Core MSBuild expects it
-                commandLineArgs = exeName + " " + commandLineArgs;
-            }
 #endif
 
             if (!NativeMethodsShared.IsWindows)
@@ -511,7 +513,10 @@ private int LaunchNode(string msbuildLocation, string commandLineArgs)
                 ProcessStartInfo processStartInfo = new ProcessStartInfo();
                 processStartInfo.FileName = exeName;
                 processStartInfo.Arguments = commandLineArgs;
-                processStartInfo.CreateNoWindow = (creationFlags | BackendNativeMethods.CREATENOWINDOW) == BackendNativeMethods.CREATENOWINDOW;
+                if (!Traits.Instance.EscapeHatches.EnsureStdOutForChildNodesIsPrimaryStdout)
+                {
+                    processStartInfo.CreateNoWindow = (creationFlags | BackendNativeMethods.CREATENOWINDOW) == BackendNativeMethods.CREATENOWINDOW;
+                }
                 processStartInfo.UseShellExecute = false;
 
                 Process process;
@@ -537,6 +542,14 @@ private int LaunchNode(string msbuildLocation, string commandLineArgs)
             }
             else
             {
+#if RUNTIME_TYPE_NETCORE
+                if (NativeMethodsShared.IsWindows)
+                {
+                    // Repeat the executable name in the args to suit CreateProcess
+                    commandLineArgs = exeName + " " + commandLineArgs;
+                }
+#endif
+
                 BackendNativeMethods.PROCESS_INFORMATION processInfo = new BackendNativeMethods.PROCESS_INFORMATION();
 
                 bool result = BackendNativeMethods.CreateProcess

src/Shared/Traits.cs

@@ -127,6 +127,12 @@ internal class EscapeHatches
         /// </summary>
         public readonly bool UseAutoRunWhenLaunchingProcessUnderCmd = Environment.GetEnvironmentVariable("MSBUILDUSERAUTORUNINCMD") == "1";
 
+        /// <summary>
+        /// Workaround for https://github.com/Microsoft/vstest/issues/1503.
+        /// </summary>
+        public readonly bool EnsureStdOutForChildNodesIsPrimaryStdout = Environment.GetEnvironmentVariable("MSBUILDENSURESTDOUTFORTASKPROCESSES") == "1";
+
+
         private static bool? ParseNullableBoolFromEnvironmentVariable(string environmentVariable)
         {
             var value = Environment.GetEnvironmentVariable(environmentVariable);