[release/9.0] Disable ResolveLinkTarget_Succeeds returnFinalTarget=true for UNC paths

[Copilot]

ResolveLinkTarget_Succeeds fails with IOException: The specified network name is no longer available on Windows versions that have UNC access disabled, because SymbolicLink_ResolveLinkTarget_PathToTarget_Data was yielding UNC paths with returnFinalTarget=true, causing Windows to actually attempt to resolve \\LOCALHOST\share\path.

main PR

Description

• In BaseSymbolicLinks.cs, split SymbolicLink_ResolveLinkTarget_PathToTarget_Data to iterate PathToTargetData with both returnFinalTarget=false/true, but restrict PathToTargetUncData to returnFinalTarget=false only
• Non-UNC path behavior is unchanged; UNC paths still get basic symlink target verification without the network resolution step that fails on restricted machines

foreach (string path in PathToTargetData)
{
    yield return new object[] { path, false };
    yield return new object[] { path, true };
}
// UNC paths are excluded from the returnFinalTarget=true case since
// they throw "The specified network name is no longer available" in
// various Windows versions. https://github.com/dotnet/runtime/issues/120380
foreach (string path in PathToTargetUncData)
{
    yield return new object[] { path, false };
}

Customer Impact

Tests in FileInfo_SymbolicLinks, File_SymbolicLinks, DirectoryInfo_SymbolicLinks, and Directory_SymbolicLinks are failing on affected Windows machines, blocking CI.

Regression

Not a product regression — test infrastructure issue. Triggered by Windows policy changes disabling UNC paths on certain machines.

Testing

Minimal test-only change. The fix narrows the test matrix for UNC paths to avoid exercising the network resolution path that is unavailable on restricted machines.

Risk

Very low. Single-line logic change scoped entirely to test data generation. No production code touched. Reduces test coverage only for a scenario (returnFinalTarget=true on UNC paths) that cannot reliably run in affected environments.

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

Original prompt

Problem

As reported in #120380, the test ResolveLinkTarget_Succeeds (used by FileInfo_SymbolicLinks, File_SymbolicLinks, DirectoryInfo_SymbolicLinks, Directory_SymbolicLinks) is failing on various Windows versions that have recently disabled UNC paths with:

System.IO.IOException : The specified network name is no longer available

The failure occurs because SymbolicLink_ResolveLinkTarget_PathToTarget_Data in BaseSymbolicLinks.cs yields UNC paths (from PathToTargetUncData) with returnFinalTarget=true. When resolving a symlink target with returnFinalTarget=true, Windows tries to actually resolve the UNC path \\LOCALHOST\share\path, which fails on machines where UNC is disabled.

Fix

In the file src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Base/SymbolicLinks/BaseSymbolicLinks.cs, modify the SymbolicLink_ResolveLinkTarget_PathToTarget_Data property (lines 56-66) so that UNC paths only yield test cases with returnFinalTarget=false, not true.

Current code (lines 56-66):

public static IEnumerable<object[]> SymbolicLink_ResolveLinkTarget_PathToTarget_Data
{
    get
    {
        foreach (string path in PathToTargetData.Union(PathToTargetUncData))
        {
            yield return new object[] { path, false };
            yield return new object[] { path, true };
        }
    }
}

The fix should change this to iterate PathToTargetData with both false and true, but iterate PathToTargetUncData with only false:

public static IEnumerable<object[]> SymbolicLink_ResolveLinkTarget_PathToTarget_Data
{
    get
    {
        foreach (string path in PathToTargetData)
        {
            yield return new object[] { path, false };
            yield return new object[] { path, true };
        }
        // UNC paths are excluded from the returnFinalTarget=true case since
        // they throw "The specified network name is no longer available" in
        // various Windows versions. https://github.com/dotnet/runtime/issues/120380
        foreach (string path in PathToTargetUncData)
        {
            yield return new object[] { path, false };
        }
    }
}

Only this one property needs to change. The other data properties (SymbolicLink_LinkTarget_PathToTarget_Data, Junction_*) should remain unchanged.

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[Copilot]

Pull request overview

Updates symbolic link test data generation to avoid UNC network resolution failures on Windows configurations where UNC access is disabled, stabilizing CI for ResolveLinkTarget_Succeeds.

Changes:

• Split SymbolicLink_ResolveLinkTarget_PathToTarget_Data generation to cover returnFinalTarget=false/true only for non-UNC paths.
• Restrict UNC path cases to returnFinalTarget=false to prevent Windows from attempting UNC resolution.

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