Use SpecialFolderOption.DoNotVerify in GetFolderPath to avoid empty path on Linux

[Copilot]

Environment.GetFolderPath(SpecialFolder.LocalApplicationData) returns an empty string on Linux when ~/.local/share doesn't exist (documented BCL behavior). Two locations in the SDK treat this as a fatal error, breaking functionality on minimal Linux environments (e.g., bare GitHub Ubuntu runners).

Changes

• DotnetupPaths.GetBaseDirectory() (primary fix): Switch to the two-argument overload with SpecialFolderOption.DoNotVerify, so the path is computed regardless of whether the directory exists yet — preventing dotnetup from throwing InvalidOperationException on bare Linux environments. Directory creation is handled later by the existing EnsureDataDirectoryExists call.

• VirtualProjectBuilder.GetTempSubdirectory(): Same fix applied — ensures dotnet run <file> does not fail on Linux when ~/.local/share is absent.

• VirtualProjectBuilderTests (new): Added a regression test (GetTempSubdirectory_ReturnsNonEmptyPath_WhenXdgDataHomeDoesNotExist) that sets XDG_DATA_HOME to a non-existent directory and verifies GetTempSubdirectory() returns a valid, non-empty path rooted under it. The test is gated to Linux/FreeBSD via [PlatformSpecificFact(TestPlatforms.Linux | TestPlatforms.FreeBSD)].

[nagilson]

@copilot is there a good way to add a test that checks this scenario (e.g. overriding environment variables based on the runtime implementation, launching the dotnetup e2e process) - since there is no file system mock? I'd like to be able to prevent regression here.

[Copilot]

@copilot is there a good way to add a test that checks this scenario (e.g. overriding environment variables based on the runtime implementation, launching the dotnetup e2e process) - since there is n...

Yes — since VirtualProjectBuilder internals are visible to dotnet.Tests, we can write a unit test that manipulates XDG_DATA_HOME directly in-process (the BCL's GetFolderPath reads it from the environment at call time).

I added VirtualProjectBuilderTests.GetTempSubdirectory_ReturnsNonEmptyPath_WhenXdgDataHomeDoesNotExist in test/dotnet.Tests/ProjectTools/VirtualProjectBuilderTests.cs (commit in this PR). The test:

1. Points XDG_DATA_HOME at a unique temp path that doesn't exist yet.
2. Calls VirtualProjectBuilder.GetTempSubdirectory().
3. Asserts the result is non-empty and is rooted under the new XDG_DATA_HOME (confirming SpecialFolderOption.Create created it).
4. Restores XDG_DATA_HOME and cleans up in a finally block.

It's gated to Linux/FreeBSD (where XDG_DATA_HOME / ~/.local/share is the fallback) via [PlatformSpecificFact(TestPlatforms.Linux | TestPlatforms.FreeBSD)].

[baronfel]

@copilot I think this is the wrong location - the linked issue is about the use of this method in dotnetup, not the dotnet CLI primarily.

[nagilson]

@copilot you need to use release/dnup as a base branch and make a PR into there, not main.

[lbussell]

Use SpecialFolderOption.DoNotVerify instead of SpecialFolderOption.Create, so that the path is computed regardless of whether it exists yet. dotnetup creates it later via EnsureDataDirectoryExists.

[Copilot]

Done — switched to SpecialFolderOption.DoNotVerify in both VirtualProjectBuilder.GetTempSubdirectory() and DotnetupPaths.GetBaseDirectory(). The directory creation is left to the existing EnsureDataDirectoryExists call. Commit: Use SpecialFolderOption.DoNotVerify instead of Create in GetFolderPath calls.

[Copilot]

@copilot I think this is the wrong location - the linked issue is about the use of this method in dotnetup, not the dotnet CLI primarily.

Fixed in the latest commit. DotnetupPaths.GetBaseDirectory() now uses SpecialFolderOption.Create so dotnetup no longer receives an empty path on bare Linux environments. The VirtualProjectBuilder fix is kept as well since it had the identical bug.

[Copilot]

@copilot you need to use release/dnup as a base branch and make a PR into there, not main.

Noted — I'm not able to retarget the PR's base branch directly. Please retarget this PR to release/dnup manually via the GitHub UI (Edit → Base), or close and re-open against release/dnup.