Fix flaky mount volume tests: refactor into separate test methods with typed Win32 exception handling

[Copilot]

finally cleanup blocks in both ReparsePoints_MountVolume.cs and Delete_MountVolume.cs could cause intermittent CI failures on Windows/ARM64 by either setting s_pass = false during cleanup or swallowing unexpected exceptions. Helix logs confirmed failures with Win32 errors 4390 ("not a reparse point") and 3 ("path not found") escaping cleanup and failing test scenarios.

The root cause is that MountHelper.Unmount throws whenever the directory exists but is not a valid mount point. This happens in two scenarios:

1. MountHelper.Mount fails after Directory.CreateDirectory has already created the directory — the directory exists but was never made a mount point.
2. The cleanup path is on a drive that is no longer accessible (e.g., the "other NTFS drive" used in Scenario 2 is gone by cleanup time), resulting in Win32 error 3.

Description

ReparsePointUtilities.cs

Changed MountHelper.Unmount to throw Win32Exception instead of raw Exception. This enables typed exception filtering by NativeErrorCode at all call sites, avoiding fragile message-string parsing.

ReparsePoints_MountVolume.cs and Delete_MountVolume.cs

The single monolithic RunTest() / runTest() methods have been broken into separate [ConditionalFact] test methods per scenario:

• ReparsePoints_MountVolume.cs: 4 separate methods (Scenario1_DifferentDriveMountedOnCurrentDrive, Scenario2_CurrentDriveMountedOnOtherDrive, Scenario31_CurrentDriveMountedOnCurrentDrive, Scenario32_CurrentDriveMountedOnCurrentDirectory), each with its own condition (IsNtfsWithOtherNtfsDrive, HasOtherNtfsDrive, or IsNtfs).
• Delete_MountVolume.cs: 7 separate methods (Scenario1 through Scenario35), same per-scenario conditional attributes.

The s_pass/Eval pattern has been removed and replaced with Assert.True/Assert.False directly in each test method, eliminating shared mutable state between scenarios.

All cleanup finally blocks now use a specific exception filter instead of bare catch { }:

// Before (swallows all exceptions)
try { MountHelper.Unmount(mountedDirName); } catch { }

// After (only suppresses expected cleanup errors)
try { MountHelper.Unmount(mountedDirName); }
catch (Win32Exception ex) when (ex.NativeErrorCode is 4390 or 3) { }

• 4390 = ERROR_NOT_A_REPARSE_POINT — directory exists but is not a mount point (Mount never succeeded or binding already removed)
• 3 = ERROR_PATH_NOT_FOUND — cleanup path is on a drive that is no longer accessible

Any other exception propagates normally so genuine issues are not silently ignored.

Delete.Windows.cs

The same specific exception filter applied to the existing Delete_VolumeMountPoint cleanup block.

Original prompt

Problem

The Directory_ReparsePoints_MountVolume.runTest() test in src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs fails (see issue #125624) because the finally blocks in Scenarios 1, 2, 3.1, and 3.2 unconditionally call MountHelper.Unmount(mountedDirName), but the mount point directory may have already been removed. When Unmount tries to call DeleteVolumeMountPoint on a path that no longer exists, it throws (Win32 error 4390 or 3), which gets caught by the scenario's catch block and sets s_pass = false, failing the test.

This is the exact same root cause that was fixed for the sibling test Directory_Delete_MountVolume in PR #125348, which was merged on 2026-03-11.

Fix

Change the finally blocks in Scenarios 1, 2, 3.1, and 3.2 to guard the Unmount and DeleteDir calls with Directory.Exists(mountedDirName), matching the pattern already applied in PR #125348 to Delete_MountVolume.cs.

There are exactly 4 finally blocks to change in src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs. Each one currently looks like:

finally
{
    MountHelper.Unmount(mountedDirName);
    DeleteDir(mountedDirName, true);
}

Each should be changed to:

finally
{
    if (Directory.Exists(mountedDirName))
    {
        MountHelper.Unmount(mountedDirName);
        DeleteDir(mountedDirName, true);
    }
}

The 4 locations in the file are:

1. Scenario 1 finally (lines 110–114)
2. Scenario 2 finally (lines 187–191)
3. Scenario 3.1 finally (lines 262–266)
4. Scenario 3.2 finally (lines 337–341)

Do not change anything else. No changes to s_pass, Eval, method signatures, test structure, or any other lines. Only add the Directory.Exists guard to these 4 finally blocks.

Fixes #125624

The following is the prior conversation context from the user's chat exploration (may be truncated):

User: Can you verify if there's a similar root cause to @dotnet/runtime/issues/125624 as @dotnet/runtime/pull/125348

Assistant:

Let me investigate both the issue and the pull request to understand their root causes.

Assistant: Now let me look at the actual test file that's failing in issue #125624 — the Directory_ReparsePoints_MountVolume test — to see if it has the same pattern:

Assistant: Yes, the root cause is essentially identical. Here's the analysis:

Same Root Cause

Both Directory_ReparsePoints_MountVolume.runTest() (issue #125624) and Directory_Delete_MountVolume.RunTest() (fixed by PR #125348) share the exact same bug pattern in the same test directory:

The pattern

The finally blocks in ReparsePoints_MountVolume.cs unconditionally call MountHelper.Unmount(mountedDirName) without first checking whether the mount point directory still exists. If the directory has already been removed (or is in a bad state), Unmount throws a Win32 exception, which gets caught by the scenario's catch block and sets s_pass = false, failing the test.

Affected finally blocks in ReparsePoints_MountVolume.cs

There are 4 finally blocks that need the same fix — at lines 110–114, 187–191, 262–266, and 337–341:

                        finally
                        {
                            MountHelper.Unmount(mountedDirName);
                            DeleteDir(mountedDirName, true);
                        }

What PR #125348 did for Delete_MountVolume.cs

PR #125348 wrapped the same unguarded Unmount/DeleteDir calls with a Directory.Exists check in Delete_MountVolume.cs:

// After fix (PR #125348)
finally
{
    if (Directory.Exists(mountedDirName))
    {
        MountHelper.Unmount(mountedDirName);
        DeleteDir(mountedDirName, true);
    }
}

The fix needed for #125624

The exact same Directory.Exists guard needs to be applied to the 4 finally blocks in ReparsePoints_MountVolume.cs (Scenarios 1, 2, 3.1, and 3.2). The two test files are structurally almost identical — they're sibling tests for mount volume operations — and the bug is the same: unguarded cleanup that throws when the mount point is already gone.

Would you like me to create a PR to apply this fix?

User: yes please

Assistant: I've started the process to create a pull request in dotnet/runtime. The coding agent is now working on applying the Directory.Exists guard to the 4 finally blocks in `ReparsePoints_MountVolum...

This pull request was created from Copilot chat.

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[jozkee]

@copilot can you double-check that the changes will fix the failed scenarios. Here's the console logs for both issues:

https://helixr18s23ayyeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-heads-main-c069cfeb95dd4cd0bb/System.IO.FileSystem.Tests/1/console.48d39a3e.log?helixlogtype=result

https://helixr18s23ayyeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-pull-125526-merge-ec064c4db40e4688b9/System.IO.FileSystem.Tests/1/console.d1be5f1e.log?skoid=8eda00af-b5ec-4be9-b69b-0919a2338892&sktid=72f988bf-86f1-41af-91ab-2d7cd011db47&skt=2026-03-16T18%3A03%3A40Z&ske=2026-03-16T19%3A03%3A40Z&sks=b&skv=2026-02-06&sv=2026-02-06&st=2026-03-16T18%3A03%3A40Z&se=2026-03-16T19%3A03%3A40Z&sr=c&sp=r&sig=rCVk68C1DmnrU8bz1t8WLIyZE2pA3lWyTU35hisxPiI%3D

[Copilot]

@copilot can you double-check that the changes will fix the failed scenarios. Here's the console logs for both issues:

https://helixr18s23ayyeko0k025g8.blob.core.windows.net/dotn...

The Helix log SAS tokens in your comment have expired (1-hour window) so I can't fetch them, but here's my code-level analysis:

Log 1 — ReparsePoints_MountVolume.runTest (main branch)

The 4 finally blocks had unconditional MountHelper.Unmount(mountedDirName). If MountHelper.Mount fails after CreateDirectory succeeds, the directory exists but is not a mount point → DeleteVolumeMountPoint fails with Win32 error 4390, caught by the scenario catch → s_pass = false. My Directory.Exists guard prevents calling Unmount when the directory doesn't exist (error 3 case). This matches the same pattern PR #125348 applied to Delete_MountVolume scenarios 1–3.2.

Log 2 — Delete_MountVolume.RunTest (PR #125526 build, still failing after #125348)

Scenarios 3.3–3.5 used if (!Eval(!Directory.Exists(mountedDirName), "Err_XXX!...")) { Unmount; DeleteDir; } in finally. Directory.Delete internally calls DeleteVolumeMountPoint first, then RemoveDirectory. If RemoveDirectory fails (e.g., antivirus holding a handle — exactly the case documented in EnsureDirectoryNotExist's comment), the mount binding is already removed but the directory still exists. The old Eval then fires → s_pass = false → cleanup tries Unmount → fails with Win32 error 4390 (no longer a reparse point) → exception caught → s_pass = false again.

My fix removes the Eval from the finally blocks entirely (no assertion in cleanup), using a plain if (Directory.Exists) guard instead. For scenario 3.3 specifically, the assertion was moved into the try block after Task.Delay(300). This prevents the spurious s_pass = false from cleanup code and matches the established pattern from #125348.

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[jozkee]

/azp run runtime-nativeaot-outerloop

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Azure Pipelines successfully started running 1 pipeline(s).

[jozkee]

@copilot the allowlist was updated, can you try #125625 (comment) again.

[Copilot]

@copilot the allowlist was updated, can you try #125625 (comment) again.

After fetching the first log, I found the root cause: the Directory.Exists guard from PR #125348 is insufficient. The log (from 2026-03-15, 4 days after PR #125348 merged on 2026-03-11) shows Scenario 1 still failing with Win32 error 4390:

Mounting volume E:\ at C:\LaksMountfpywadjz\   ← no uniqueName printed = GetVolumeNameForVolumeMountPoint failed
Unmounting the volume at C:\LaksMountfpywadjz\  ← Unmount IS called
Err_768lme! Win32 error: 4390

When GetVolumeNameForVolumeMountPoint fails, MountHelper.Mount throws after Directory.CreateDirectory has already created the directory. So Directory.Exists returns true, the guard passes, Unmount is called on a plain directory (not a mount point), and throws 4390.

Fix (commit c5e5852): wrap MountHelper.Unmount in try { } catch { } in all finally cleanup blocks in both files. This handles all cases — Mount failing, partial deletes, race conditions — without letting cleanup exceptions escape to the scenario's outer catch and set s_pass = false.

Changed 11 finally blocks total:

• Delete_MountVolume.cs: scenarios 1, 2, 3.1, 3.2, 3.3, 3.4, 3.5
• ReparsePoints_MountVolume.cs: scenarios 1, 2, 3.1, 3.2

[jozkee]

/azp run runtime-nativeaot-outerloop

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[Copilot]

Pull request overview

This PR reduces Windows/ARM64 test flakiness in mount-volume directory tests by preventing cleanup (finally) failures from incorrectly failing the test scenarios.

Changes:

• Wrap MountHelper.Unmount inside try/catch within finally blocks to prevent cleanup exceptions from failing scenarios.
• Keep Directory.Exists guards to skip cleanup when the mount directory is already removed.
• Restructure scenario 3.3 cleanup/validation in Delete_MountVolume.cs by moving the Eval assertion out of finally.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Makes cleanup robust by swallowing Unmount exceptions when the dir exists but isn’t a mount point.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Applies the same robust cleanup pattern across scenarios and moves an Eval check into the main scenario flow.

[jozkee]

@copilot address feedback and also take a look at this other log showing another error Err_231vwf! Exception caught in scenario: System.Exception: Win32 error: 3.
https://helixr18s23ayyeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-pull-125675-merge-4ba19134f92c454b8a/System.IO.FileSystem.Tests/1/console.c0e1f6cd.log?helixlogtype=result

Fix the code to don't eat up exceptions and also break down the scenarios in RunTest into separate test methods.

[Copilot]

@copilot address feedback and also take a look at this other log showing another error Err_231vwf! Exception caught in scenario: System.Exception: Win32 error: 3.
https://helixr18...

Done in commit c3c12b03:

• MountHelper.Unmount now throws Win32Exception instead of raw Exception, enabling typed filtering by NativeErrorCode at call sites.

• All 12 cleanup catch { } blocks (7 in Delete_MountVolume.cs, 4 in ReparsePoints_MountVolume.cs, 1 in Delete.Windows.cs) replaced with catch (Win32Exception ex) when (ex.NativeErrorCode is 4390 or 3) — only suppressing the two expected cleanup errors (4390 = not a reparse point, 3 = path not found when the other drive is gone). Any other exception propagates normally.

• RunTest broken into separate methods: Delete_MountVolume.cs → 7 [ConditionalFact] methods; ReparsePoints_MountVolume.cs → 4 [ConditionalFact] methods. Each has its own condition (IsNtfsWithOtherNtfsDrive, HasOtherNtfsDrive, or IsNtfs) matching the original skip logic.

• s_pass/Eval removed — replaced with Assert.True/Assert.False directly in each test method.

The Win32 error 3 case shown in the log (Scenario 2 cleanup failing because the E: drive path E:\LaksMountXxx\ was no longer accessible) is now handled by the when (ex.NativeErrorCode is ... or 3) filter.