Why didn't `TestOldNuget()` fail?

[rainersigwald]

The regressions fixed by #11488 would ideally have been caught by this test

msbuild/src/Build.OM.UnitTests/NugetRestoreTests.cs

Lines 24 to 52 in 36b265d

	public void TestOldNuget()
	{
	string msbuildExePath = Path.GetDirectoryName(RunnerUtilities.PathToCurrentlyRunningMsBuildExe)!;
	using TestEnvironment testEnvironment = TestEnvironment.Create();
	TransientTestFolder folder = testEnvironment.CreateFolder(createFolder: true);
	// The content of the solution isn't known to matter, but having a custom solution makes it easier to add requirements should they become evident.
	TransientTestFile sln = testEnvironment.CreateFile(folder, "test.sln",
	@"
	Microsoft Visual Studio Solution File, Format Version 12.00
	\# Visual Studio 15
	VisualStudioVersion = 15.0.26124.0
	MinimumVisualStudioVersion = 15.0.26124.0
	Global
	GlobalSection(SolutionConfigurationPlatforms) = preSolution
	Debug|Any CPU = Debug|Any CPU
	Debug|x64 = Debug|x64
	Debug|x86 = Debug|x86
	Release|Any CPU = Release|Any CPU
	Release|x64 = Release|x64
	Release|x86 = Release|x86
	EndGlobalSection
	GlobalSection(SolutionProperties) = preSolution
	HideSolutionNode = FALSE
	EndGlobalSection
	EndGlobal
	");
	RunnerUtilities.RunProcessAndGetOutput(Path.Combine(msbuildExePath, "nuget", "NuGet.exe"), "restore " + sln.Path + " -MSBuildPath \"" + msbuildExePath + "\"", out bool success, outputHelper: _output);
	success.ShouldBeTrue();
	}

But weren't. Why?

[JanProvaznik]

I debugged this for several hours and could not come up with an answer, the scenario just works and I could not break into what is happening in this old NuGet that is different from the scenario where it fails.
My suspicion was that nuget somehow sanitized the project file before using msbuild but that was not confirmed.

[GangWang01]

With VS 17.13.1 (msbuild 17.13.9.7704), the regression could be reproduced when old nuget.exe restores if the path to 64-bit MSBuild is specified explicitly. Note that under the path to 64-bit MSBuild there is not the assembly Microsoft.VisualStudio.SolutionPersistence.dll and then the issue occurs.

Regarding the test that couldn't catch the regression, it's because the msbuildExe path used in the test is the build output folder of the test project and this folder contains the assembly Microsoft.VisualStudio.SolutionPersistence.dll. The package reference Microsoft.VisualStudio.SolutionPersistence in ./src/Build/Microsoft.Build.csproj is transitively copied to the build output of the test project.

We need to change the test to use msbuildExe in the boostrap and make sure 64-bit msbuild there doesn't contain the assembly Microsoft.VisualStudio.SolutionPersistence.dll.