[release/8.0-staging] Fix absolute path check when loading hostfxr/hostpolicy/coreclr

[elinor-fung]

Backport of #116597 to release/8.0-staging

On Windows, we were incorrectly only checking for <letter>: and incorrectly determining that UNC and device paths were not absolute. This updates pal::is_path_rooted to match the managed Path.IsPathRooted and adds a pal::is_path_absolute with the logic of Path.IsPartiallyQualified

cc @dotnet/appmodel @AaronRobinsonMSFT

Customer Impact

• Customer reported
• Found internally

Customers are unable to run self-contained apps from or point framework-dependent apps to use a .NET runtime from a UNC share or device path.

Issue: #116521

Regression

• Yes
• No

Regressed in last servicing release.
b33d4e3

Testing

Manual testing for UNC share and device path. Automated test added for device paths.

Risk

Medium. The fix has to be in a path that every app will exercise. The updated logic is a mirror of existing logic in managed APIs.

[Copilot]

Pull Request Overview

This PR refines how hostfxr/hostpolicy/coreclr resolver checks for absolute paths on Windows by replacing is_path_rooted with a new is_path_fully_qualified, and adds tests for UNC and device path scenarios.

• Introduces pal::is_path_fully_qualified alongside updated is_path_rooted logic in pal.windows.cpp and a matching stub in pal.unix.cpp.
• Updates various resolver components (coreclr_resolver, deps_resolver, fxr_resolver, hostpolicy_resolver, apphost, corehost) to use the new qualification check.
• Adds Windows-specific tests for UNC (\\?\) and device (\\.\) paths, and an error trace on failure.

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/native/corehost/hostpolicy/standalone/coreclr_resolver.cpp	Use fully qualified path check instead of rooted check
src/native/corehost/hostpolicy/deps_resolver.cpp	Update assertion to use fully qualified path check
src/native/corehost/hostmisc/pal.windows.cpp	Implement is_path_fully_qualified and refine rooted logic
src/native/corehost/hostmisc/pal.unix.cpp	Stub is_path_fully_qualified to Unix-rooted logic
src/native/corehost/hostmisc/pal.h	Declare is_path_fully_qualified
src/native/corehost/fxr_resolver.h	Switch to fully qualified check for hostfxr path
src/native/corehost/fxr/standalone/hostpolicy_resolver.cpp	Switch to fully qualified check for hostpolicy path
src/native/corehost/corehost.cpp	Add error trace when fxr resolution fails
src/native/corehost/apphost/standalone/hostfxr_resolver.cpp	Switch to fully qualified check for apphost fxr path
src/installer/tests/HostActivation.Tests/StandaloneAppActivation.cs	Add tests for UNC and device EXE paths
src/installer/tests/HostActivation.Tests/PortableAppActivation.cs	Add tests for device paths in DotNetRoot setting

Comments suppressed due to low confidence (1)

src/native/corehost/hostmisc/pal.h:331

• Add a brief comment above is_path_fully_qualified explaining its purpose and how it differs from is_path_rooted, to aid future maintainers.

    bool is_path_fully_qualified(const string_t& path);

[jeffschwMSFT]

lgtm. we will take for consideration in 8.0.x

[dotnet-policy-service]

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