MRTCore using MICROSOFT_WINDOWSAPPRUNTIME_BASE_DIRECTORY breaks launching one WinAppSDK app from another

[DHowett]

Describe the bug

WinAppSDK applications which use C# registration free WinRT set the environment variable MICROSOFT_WINDOWSAPPRUNTIME_BASE_DIRECTORY.

WindowsAppSDK/dev/UndockedRegFreeWinRT/UndockedRegFreeWinRT-AutoInitializer.cs

Lines 24 to 27 in bd8e2c1

	internal static void AccessWindowsAppSDK()
	{
	// Set base directory env var for PublishSingleFile support (referenced by SxS redirection)
	Environment.SetEnvironmentVariable("MICROSOFT_WINDOWSAPPRUNTIME_BASE_DIRECTORY", AppContext.BaseDirectory);

This environment variable is used by MRTCore to locate resources.

WindowsAppSDK/dev/MRTCore/mrt/Core/src/MRM.cpp

Line 999 in bd8e2c1

if (SUCCEEDED(wil::TryGetEnvironmentVariableW(L"MICROSOFT_WINDOWSAPPRUNTIME_BASE_DIRECTORY", baseDir)) && baseDir)

When that env var is present, MRTCore skips all local resource PRI locations including ModuleName.pri and just loads resources.pri out of the specified directory.

WindowsAppSDK/dev/MRTCore/mrt/Core/src/MRM.cpp

Line 1001 in bd8e2c1

searchStart = SearchPass::BaseDirForResourcesPri;

This explodes any application that relies on resources when it is spawned from another WinAppSDK application using CreateProcess.

---

A word of advice: don't use Environment Variables to communicate information across your own processes. They get inherited in places you would not expect.

[DHowett]

Oh, and it allows a user environment variable to control where an elevated process reads a data file from. I hope that MRTCore is fuzzed. :)

[Scottj1s]

@DHowett MICROSOFT_WINDOWSAPPRUNTIME_BASE_DIRECTORY is used in the implementation of PublishSingleFile, not reg-free WinRT activation. Is that your scenario? Do you see the same thing without PublishSingleFile?

(The env var was a requirement imposed upon the PublishSingleFile by fusion manifest redirection.)

[DHowett]

@Scottj1s honestly, I looked at the filename and path ("WindowsAppSDK/dev/UndockedRegFreeWinRT/UndockedRegFreeWinRT-AutoInitializer.cs") which is all about "reg" "free" and "winrt" before concluding that it had something to do with undocked reg free winrt. Perhaps my confusion is understandable. 🙂

However, I am happy to confirm that there are 300 instances of PublishSingleFile in the project where we saw this and they are all set to FALSE...

[Scottj1s]

Fix will be serviced to 1.8 and 2.0 shortly