Fixed issue where SolutionName variable could go missing. Added comments

.gitignore

@@ -352,4 +352,5 @@ MigrationBackup/
 .ionide/
 
 # Ignored as part of the Labs builds process
-Common/Labs.TargetFrameworks.props*
+Common/Labs.TargetFrameworks.props*
+Common/Scripts/UpdateTargetFrameworks.CurrentVariant.*

Common/ConditionalTargetFramework.targets

@@ -5,6 +5,10 @@
         <DisableFastUpToDateCheck>True</DisableFastUpToDateCheck>
     </PropertyGroup>
 
+    <Target Name="CleanupConditionalTargetFrameworkCache" BeforeTargets="Clean">
+        <Delete Files="$(RepositoryDirectory)Common/Scripts/UpdateTargetFrameworks.CurrentVariant.txt" />
+    </Target>
+
     <Target Name="PruneExtraTargetFrameworks" BeforeTargets="PreBuildEvent">
         <PropertyGroup>
             <PowerShellCommand Condition=" '$(OS)' == 'Windows_NT' ">powershell</PowerShellCommand>
@@ -19,4 +23,4 @@
 
         <Exec Command="$(PowerShellCommand) -ExecutionPolicy Unrestricted -NoProfile -File $(RepositoryDirectory)Common/Scripts/UpdateTargetFrameworks.ps1 $(LabsTargetPlatformsVariant) --Verbose" WorkingDirectory="$(RepositoryDirectory)Common/Scripts/" />
     </Target>
-</Project>
+</Project>

Common/Scripts/UpdateTargetFrameworks.ps1

@@ -1,3 +1,5 @@
+# This file can execute in parallel, causing potential file access issues.
+# This lock prevent us from reading or writing concurrently and getting access exceptions.
 if (Test-Path ".\..\Labs.TargetFrameworks.props.lock") {
     return;
 }
@@ -9,6 +11,17 @@ if ($args.Length -eq 0 -or ![bool]$args[0]) {
 
 $variant = $args[0];
 
+# MSBuild uses the SolutionName to determine the variant, but SolutionName occassionally goes missing during
+# the build process, which causes the default/fallback variant to be used when it shouldn't.
+# This check ensures we're always using the same variant, until the file is deleted by MSBuild during a clean.
+if ((Get-Content -Path .\UpdateTargetFrameworks.CurrentVariant.txt).Trim() -notlike $variant) {
+    # for debug only
+    Set-Content -Path .\UpdateTargetFrameworks.CircumventedVariant.$variant.txt -Value '';
+    return;
+}
+
+Set-Content -Path .\UpdateTargetFrameworks.CurrentVariant.txt -Value $variant;
+
 Set-Content -Path .\..\Labs.TargetFrameworks.props.lock -Value '';
 $fileContents = Get-Content -Path .\..\Labs.TargetFrameworks.default.props
 
@@ -20,20 +33,19 @@ if ($variant -eq "All") {
 if ($variant -eq "Wasm") {
     # Remove all non-wasm TFMs
     $newFileContents = $fileContents -replace '<(UwpTargetFramework|WinAppSdkTargetFramework|WpfLibTargetFramework|LinuxLibTargetFramework|AndroidLibTargetFramework|MacOSLibTargetFramework|iOSLibTargetFramework)>.+?>', '';
-    Set-Content -Path .\..\Labs.TargetFrameworks.props.wasmonly -Value '';
 }
 
 if ($variant -eq "Windows") {
     # Minimal Windows dependencies.
     $newFileContents = $fileContents -replace '<(LinuxLibTargetFramework|AndroidLibTargetFramework|MacOSLibTargetFramework|iOSLibTargetFramework)>.+?>', '';
-    Set-Content -Path .\..\Labs.TargetFrameworks.props.windowsonly -Value '';
 }
 
+# MSBuild uses last modified date instead of file hashes to determine when the content has changed.
+# Only update the props file if the new content is different.
 if ($newFileContents -eq $fileContents) {
     Remove-Item -Path .\..\Labs.TargetFrameworks.props.lock;
     return;
 }
 
 Set-Content -Force -Path .\..\Labs.TargetFrameworks.props -Value $newFileContents;
-
-Remove-Item -Path .\..\Labs.TargetFrameworks.props.lock;
+Remove-Item -Path .\..\Labs.TargetFrameworks.props.lock;