Enable -mt in official build pipeline (stacked on #13678)

.vsts-dotnet-ci.yml

@@ -98,7 +98,7 @@ jobs:
     displayName: cibuild_bootstrapped_msbuild.cmd
     inputs:
       filename: 'eng/cibuild_bootstrapped_msbuild.cmd'
-      arguments: -onlyDocChanged $(onlyDocChanged)
+      arguments: '-onlyDocChanged $(onlyDocChanged) -stage2Properties /mt'
     env:
       ForceUseXCopyMSBuild: 1
 # Task to collect code coverage on Windows. Disabled by default due to being unstable and sometimes it stucks forever
@@ -210,7 +210,7 @@ jobs:
     displayName: cibuild_bootstrapped_msbuild.cmd
     inputs:
       filename: 'eng/cibuild_bootstrapped_msbuild.cmd'
-      arguments: '-msbuildEngine dotnet -onlyDocChanged $(onlyDocChanged)'
+      arguments: '-msbuildEngine dotnet -onlyDocChanged $(onlyDocChanged) -stage2Properties /mt'
     env:
       MSBUILDUSESERVER: "1"
 # Task to collect code coverage on Windows. Disabled by default due to being unstable and sometimes it stucks forever
@@ -267,6 +267,9 @@ jobs:
     continueOnError: true
     condition: eq(variables.onlyDocChanged, 0)
 
+# Dedicated /mt validation that turns on the ThreadSafeTaskAnalyzer (BuildAnalyzer=true),
+# kept as a separate Linux Core job because the analyzer is more expensive and the regular
+# bootstrapped CI jobs (which all run /mt now too) do not need it.
 - job: BootstrapMSBuildWithMTMode
   displayName: "Linux Core Multithreaded Mode"
   pool:
@@ -406,7 +409,7 @@ jobs:
         Token: $(dn-bot-dnceng-artifact-feeds-rw)
   - bash: sudo apt-get update
   - bash: sudo apt-get install -y libxml2
-  - bash: . 'eng/cibuild_bootstrapped_msbuild.sh' --onlyDocChanged $(onlyDocChanged)
+  - bash: . 'eng/cibuild_bootstrapped_msbuild.sh' --onlyDocChanged $(onlyDocChanged) --stage2Properties '/mt'
     displayName: CI Build
     env:
         MSBUILDUSESERVER: "1"
@@ -494,7 +497,7 @@ jobs:
         arguments: $(Build.SourcesDirectory)/NuGet.config $Token
       env:
         Token: $(dn-bot-dnceng-artifact-feeds-rw)
-  - bash: . 'eng/cibuild_bootstrapped_msbuild.sh' --onlyDocChanged $(onlyDocChanged)
+  - bash: . 'eng/cibuild_bootstrapped_msbuild.sh' --onlyDocChanged $(onlyDocChanged) --stage2Properties '/mt'
     displayName: CI Build
     env:
         MSBUILDUSESERVER: "1"

.vsts-dotnet.yml

@@ -130,6 +130,7 @@ extends:
           enableComponentGovernance: true
           signTypeParameter: ${{ parameters.signTypeParameter }}
           enableOptProf: ${{ parameters.enableOptProf }}
+          additionalBuildArgs: '/mt'
 
     - template: /eng/common/templates-official/post-build/post-build.yml@self
       parameters:

eng/cibuild_bootstrapped_msbuild.ps1

@@ -5,6 +5,8 @@ Param(
   [switch] $prepareMachine,
   [bool] $buildStage1 = $True,
   [bool] $onlyDocChanged = 0,
+  [switch] $skipTests,
+  [string] $stage2Properties = "",
   [Parameter(ValueFromRemainingArguments=$true)][String[]]$properties
 )
 
@@ -117,14 +119,34 @@ try {
   # Opt into performance logging. https://github.com/dotnet/msbuild/issues/5900
   $env:DOTNET_PERFLOG_DIR=$PerfLogDir
 
+  # Mirrors cibuild_bootstrapped_msbuild.sh:96. Required so the apphost-based child task host
+  # (NodeProviderOutOfProcTaskHost.ResolveAppHostOrFallback) can locate the runtime when the
+  # parent MSBuild is launched as `dotnet exec MSBuild.dll` (which, unlike the SDK CLI
+  # `dotnet msbuild`, does not set DOTNET_HOST_PATH automatically). Trying to switch the
+  # invocation to `dotnet msbuild` here would be cleaner but the SDK CLI's argument
+  # translation mangles `/mt` into `/m` + `t`, so we keep the explicit `MSBuild.dll` path
+  # and set DOTNET_HOST_PATH ourselves.
+  $env:DOTNET_HOST_PATH=$dotnetExePath
+
   # When using bootstrapped MSBuild:
   # - Turn off node reuse (so that bootstrapped MSBuild processes don't stay running and lock files)
   # - Create bootstrap environment as it's required when also running tests
+  # - $stage2Properties are appended to the stage 2 build only (matching cibuild_bootstrapped_msbuild.sh).
+  #   Use this for switches like /mt that should not be passed to the stable MSBuild used in stage 1
+  #   until a stable version of MT is available in the images.
+  # Branches mirror cibuild_bootstrapped_msbuild.sh exactly:
+  #   onlyDocChanged=1 → bootstrap not created (artifacts not needed downstream)
+  #   skipTests        → bootstrap IS created (downstream MAY consume it), tests omitted
+  #   default          → bootstrap created, tests run
+  $stage2Args = if ($stage2Properties) { $stage2Properties -split '\s+' | Where-Object { $_ } } else { @() }
   if ($onlyDocChanged) {
-    & $PSScriptRoot\Common\Build.ps1 -restore -build -ci /p:CreateBootstrap=false /nr:false @properties
+    & $PSScriptRoot\Common\Build.ps1 -restore -build -ci /p:CreateBootstrap=false /nr:false @properties @stage2Args
+  }
+  elseif ($skipTests) {
+    & $PSScriptRoot\Common\Build.ps1 -restore -build -ci /nr:false @properties @stage2Args
   }
   else {
-    & $PSScriptRoot\Common\Build.ps1 -restore -build -test -ci /nr:false @properties
+    & $PSScriptRoot\Common\Build.ps1 -restore -build -test -ci /nr:false @properties @stage2Args
   }
 
   exit $lastExitCode