dotnet.exe prints error messages to console when launched with empty DOTNET_MULTILEVEL_LOOKUP

[pedrobsaila]

Fixes #54073
Fixes #82260

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Issue Details

---

Fixes #54073

Author:	pedrobsaila
Assignees:	-
Labels:	area-Host
Milestone:	-

[vitek-karas]

We should add a test to validate that nothing goes to stderr.

[pedrobsaila]

Don't print to stderr on error getting env. variable, just trace a warning (which by default doesn't go anywhere)

since warnings do not go anywhere, checking stderr would not be useful.

[vitek-karas]

since warnings do not go anywhere, checking stderr would not be useful.

We try to add regression tests for pretty much every fix. So in this case the test should fail before the change and pass after. The tests also often enable tracing which will make the warnings visible and verifiable.
And also just checking that stderr is indeed empty in this case is a good test.

[elinor-fung]

Since we don't always read DOTNET_MULTILEVEL_LOOKUP (since it is disabled in latest versions) and the root issue concerns any environment variable we read, it probably makes sense to target a another specific variable.

Incorrectly writing to stderr in the STARTUP_HOOK case was reported in #82260. We have an existing test that sets an empty value for the STARTUP_HOOK environment variable - that should be updated to check that nothing is written to stderr.

runtime/src/installer/tests/HostActivation.Tests/StartupHooks.cs

Line 84 in 47aab33

public void Muxer_activation_of_Empty_StartupHook_Variable_Succeeds()

We also have tests setting an empty DOTNET_ROOT:

runtime/src/installer/tests/HostActivation.Tests/PortableAppActivation.cs

Line 172 in 47aab33

public void AppHost_FrameworkDependent_GlobalLocation_Succeeds(bool useRegisteredLocation)

[vitek-karas]

This can be replaced with .EnableTracingAndCaptureOutputs, it does not set verbosity (so it defaults to everything), but that should work for this test as well.

Similarly in the other places.

[elinor-fung]

Thank you!

[agocke]

/backport to release/7.0-staging

[github-actions]

Started backporting to release/7.0-staging: https://github.com/dotnet/runtime/actions/runs/5299837208

[elinor-fung]

/backport to release/6.0-staging

[github-actions]

Started backporting to release/6.0-staging: https://github.com/dotnet/runtime/actions/runs/5522137951