Add locking around subdirectoryWatchers changes

github.com//pull/124716#discussion_r2838494096

Author: gwr

src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.SunOS.cs

@@ -390,7 +390,10 @@ private void CreateSubdirectoryWatchers(DirectorySnapshot snapshot)
                             // Share parent's cancellation token so entire tree can be cancelled together
                             var childWatcher = new RunningInstance(watcher, subdirPath, subdirRelativePath, true, _notifyFilters, _cancellationToken);
                             childWatcher.Start();
-                            _subdirectoryWatchers.Add(childWatcher);
+                            lock (_subdirectoryWatchers)
+                            {
+                                _subdirectoryWatchers.Add(childWatcher);
+                            }
                             subdirCount++;
                         }
                     }
@@ -756,10 +759,17 @@ private void ProcessDeletion(FileSystemWatcher watcher, string name, FileEntry o
                 if (isDir && _includeSubdirectories)
                 {
                     string subdirPath = System.IO.Path.Combine(_directoryPath, name);
-                    var childToRemove = _subdirectoryWatchers.Find(c => c._directoryPath == subdirPath);
+                    RunningInstance? childToRemove;
+                    lock (_subdirectoryWatchers)
+                    {
+                        childToRemove = _subdirectoryWatchers.Find(c => c._directoryPath == subdirPath);
+                        if (childToRemove is not null)
+                        {
+                            _subdirectoryWatchers.Remove(childToRemove);
+                        }
+                    }
                     if (childToRemove is not null)
                     {
-                        _subdirectoryWatchers.Remove(childToRemove);
                         childToRemove.Cancel();
                     }
                 }
@@ -787,10 +797,17 @@ private void ProcessRename(FileSystemWatcher watcher, string oldName, FileEntry
                 if (isDir && _includeSubdirectories)
                 {
                     string oldSubdirPath = System.IO.Path.Combine(_directoryPath, oldName);
-                    var childToUpdate = _subdirectoryWatchers.Find(c => c._directoryPath == oldSubdirPath);
+                    RunningInstance? childToUpdate;
+                    lock (_subdirectoryWatchers)
+                    {
+                        childToUpdate = _subdirectoryWatchers.Find(c => c._directoryPath == oldSubdirPath);
+                        if (childToUpdate is not null)
+                        {
+                            _subdirectoryWatchers.Remove(childToUpdate);
+                        }
+                    }
                     if (childToUpdate is not null)
                     {
-                        _subdirectoryWatchers.Remove(childToUpdate);
                         childToUpdate.Cancel();
                         CreateSingleSubdirectoryWatcher(newName);
                     }
@@ -833,7 +850,10 @@ private void CreateSingleSubdirectoryWatcher(string name)
                     {
                         var childWatcher = new RunningInstance(watcher, subdirPath, subdirRelativePath, true, _notifyFilters, _cancellationToken);
                         childWatcher.Start();
-                        _subdirectoryWatchers.Add(childWatcher);
+                        lock (_subdirectoryWatchers)
+                        {
+                            _subdirectoryWatchers.Add(childWatcher);
+                        }
                     }
                 }
                 catch (Exception ex)
@@ -980,9 +1000,13 @@ internal void Cancel()
                 _isCancelling = true;
 
                 // First, cancel all subdirectory watchers (depth-first)
-                // ToArray() creates a snapshot to avoid race conditions with ProcessEvents thread
-                // modifying the list during iteration
-                foreach (var child in _subdirectoryWatchers.ToArray())
+                // Take a lock when creating the snapshot to avoid races with concurrent modifications
+                RunningInstance[] children;
+                lock (_subdirectoryWatchers)
+                {
+                    children = _subdirectoryWatchers.ToArray();
+                }
+                foreach (RunningInstance child in children)
                 {
                     child.Cancel();
                 }