Fix flaky File_Move_Multiple_From_Watched_To_Unwatched_Mac test caused by duplicate FSEvents

[Copilot]

Description

macOS FSEvents can deliver duplicate Deleted events when moving files/directories from a watched to an unwatched directory. The existing isFilteredOut filter (Mac path, skipOldEvents=true) only discarded stale Created/Changed events—not duplicate Deleted events—causing ExpectEvents to signal completion prematurely and collect an extra event:

Expected: [Deleted file0, Deleted file1]
Actual:   [Deleted file0, Deleted file0, Deleted file1]

Changes:

• FileSystemWatcher.File.Move.cs / FileSystemWatcher.Directory.Move.cs: When skipOldEvents=true, replace the single-line lambda isFilteredOut with a ConcurrentDictionary<FiredEvent, bool>-backed deduplicator. TryAdd is atomic: first occurrence passes through, subsequent duplicates of the same (EventType, Path) are filtered out.

Changes

• Modified FileMove_Multiple_FromWatchedToUnwatched and DirectoryMove_Multiple_FromWatchedToUnwatched to deduplicate events via ConcurrentDictionary.TryAdd in addition to the existing stale-event filtering.

Testing

Existing tests cover the changed paths. The fix directly addresses the observed failure mode (duplicate Deleted events inflating the collected event list).

Original prompt

---

This section details on the original issue you should resolve

<issue_title>System.IO.Tests.File_Move_Tests.File_Move_Multiple_From_Watched_To_Unwatched_Mac test failure</issue_title>
<issue_description>```
Discovered: System.IO.FileSystem.Watcher.Tests (found 166 of 210 test cases)
Starting: System.IO.FileSystem.Watcher.Tests (parallel test collections = on [6 threads], stop on fail = off)
System.IO.Tests.File_Move_Tests.File_Move_Multiple_From_Watched_To_Unwatched_Mac(filesCount: 2) [FAIL]
Assert.Equal() Failure: Collections differ
↓ (pos 1)
Expected: ArraySelectIterator<<>f__AnonymousType1<string, string>, FiredEvent> [Deleted /tmp/helix/working/9FEF0885/t/#File_Move_Tests_r5rmsjek.mw5/dir_watched/file0 , Deleted /tmp/helix/working/9FEF0885/t/#File_Move_Tests_r5rmsjek.mw5/dir_watched/file1 ]
Actual: List [Deleted /tmp/helix/working/9FEF0885/t/#File_Move_Tests_r5rmsjek.mw5/dir_watched/file0 , Deleted /tmp/helix/working/9FEF0885/t/#File_Move_Tests_r5rmsjek.mw5/dir_watched/file0 , Deleted /tmp/helix/working/9FEF0885/t/#File_Move_Tests_r5rmsjek.mw5/dir_watched/file1 ]
↑ (pos 1)
Stack Trace:
//src/libraries/System.IO.FileSystem.Watcher/tests/FileSystemWatcher.File.Move.cs(234,0): at System.IO.Tests.File_Move_Tests.FileMove_Multiple_FromWatchedToUnwatched(Int32 filesCount, Boolean skipOldEvents)
//src/libraries/System.IO.FileSystem.Watcher/tests/FileSystemWatcher.File.Move.cs(48,0): at System.IO.Tests.File_Move_Tests.File_Move_Multiple_From_Watched_To_Unwatched_Mac(Int32 filesCount)
at System.Object.InvokeStub_File_Move_Tests.File_Move_Multiple_From_Watched_To_Unwatched_Mac(Object, Span`1)
/_/src/libraries/System.Private.CoreLib/src/System/Reflection/MethodBaseInvoker.cs(95,0): at System.Reflection.MethodBaseInvoker.InvokeWithOneArg(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
System.IO.Tests.FileSystemWatcherTests_netstandard17.DroppedWatcher_Collectible [SKIP]
Condition(s) not met: "IsPreciseGcSupported"
Finished: System.IO.FileSystem.Watcher.Tests

## Build Information
Build: https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1314668
Build error leg or test failing: System.IO.Tests.File_Move_Tests.File_Move_Multiple_From_Watched_To_Unwatched_Mac
Pull request: https://github.com/dotnet/runtime/pull/125014
<!-- Error message template  -->
## Error Message

Fill the error message using [step by step known issues guidance](https://github.com/dotnet/arcade/blob/main/Documentation/Projects/Build%20Analysis/KnownIssueJsonStepByStep.md).

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```json
{
  "ErrorMessage": "",
  "ErrorPattern": "System.IO.Tests.File_Move_Tests.File_Move_Multiple_From_Watched_To_Unwatched_Mac.*FAIL",
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

Known issue validation

Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=1314668
Error message validated: [System.IO.Tests.File_Move_Tests.File_Move_Multiple_From_Watched_To_Unwatched_Mac.*FAIL]
Result validation: ✅ Known issue matched with the provided build.
Validation performed at: 3/1/2026 12:32:32 AM UTC

Report

Build	Definition	Test	Pull Request
1338073	dotnet/runtime	System.IO.Tests.File_Move_Tests.File_Move_Multiple_From_Watched_To_Unwatched_Mac	#125634
1330792	dotnet/runtime	System.IO.Tests.File_Move_Tests.File_Move_Multiple_From_Watched_To_Unwatched_Mac	#125283
1324517	dotnet/runtime	[System.IO.Tests.File_Move_Tests.File_Move_Multiple_From_Watched_To_Unwatc...

• Fixes System.IO.Tests.File_Move_Tests.File_Move_Multiple_From_Watched_To_Unwatched_Mac test failure #125018

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[Copilot]

Pull request overview

This PR fixes a macOS-specific flakiness in System.IO.FileSystem.Watcher move tests by preventing duplicate FSEvents Deleted notifications from causing ExpectEvents to complete early and record extra events.

Changes:

• Add a ConcurrentDictionary<FiredEvent, bool>-backed deduplication filter (via TryAdd) when skipOldEvents=true.
• Apply the same deduplication approach to both file-move and directory-move “watched → unwatched” multi-item tests on macOS.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.IO.FileSystem.Watcher/tests/FileSystemWatcher.File.Move.cs	When skipOldEvents is enabled (macOS path), filters duplicate fired events (same (EventType, Path[, OldPath])) to avoid premature completion.
src/libraries/System.IO.FileSystem.Watcher/tests/FileSystemWatcher.Directory.Move.cs	Same deduplication strategy applied to directory move tests under the macOS skipOldEvents configuration.