Don't root FileSystemWatcher unnecessarily (#41872)

We already try to do this on Unix, though it seems we don't currently have tests for it, but we don't on Windows.

Author: stephentoub

src/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.Win32.cs

@@ -51,8 +51,19 @@ private void StartRaisingEvents()
                 // Store all state, including a preallocated overlapped, into the state object that'll be
                 // passed from iteration to iteration during the lifetime of the operation.  The buffer will be pinned
                 // from now until the end of the operation.
-                state = new AsyncReadState(session, buffer, _directoryHandle, ThreadPoolBoundHandle.BindHandle(_directoryHandle));
-                unsafe { state.PreAllocatedOverlapped = new PreAllocatedOverlapped(ReadDirectoryChangesCallback, state, buffer); }
+                state = new AsyncReadState(session, buffer, _directoryHandle, ThreadPoolBoundHandle.BindHandle(_directoryHandle), this);
+                unsafe
+                {
+                    state.PreAllocatedOverlapped = new PreAllocatedOverlapped((errorCode, numBytes, overlappedPointer) =>
+                    {
+                        AsyncReadState state = (AsyncReadState)ThreadPoolBoundHandle.GetNativeOverlappedState(overlappedPointer);
+                        state.ThreadPoolBinding.FreeNativeOverlapped(overlappedPointer);
+                        if (state.WeakWatcher.TryGetTarget(out FileSystemWatcher watcher))
+                        {
+                            watcher.ReadDirectoryChangesCallback(errorCode, numBytes, state);
+                        }
+                    }, state, buffer);
+                }
             }
             catch
             {
@@ -191,9 +202,8 @@ private unsafe void Monitor(AsyncReadState state)
         }
 
         /// <summary>Callback invoked when an asynchronous read on the directory handle completes.</summary>
-        private unsafe void ReadDirectoryChangesCallback(uint errorCode, uint numBytes, NativeOverlapped* overlappedPointer)
+        private void ReadDirectoryChangesCallback(uint errorCode, uint numBytes, AsyncReadState state)
         {
-            AsyncReadState state = (AsyncReadState)ThreadPoolBoundHandle.GetNativeOverlappedState(overlappedPointer);
             try
             {
                 if (IsHandleInvalid(state.DirectoryHandle))
@@ -232,11 +242,7 @@ private unsafe void ReadDirectoryChangesCallback(uint errorCode, uint numBytes,
             }
             finally
             {
-                // Clean up state associated with this one iteration
-                state.ThreadPoolBinding.FreeNativeOverlapped(overlappedPointer);
-
-                // Then call Monitor again to either start the next iteration or
-                // clean up the whole operation.
+                // Call Monitor again to either start the next iteration or clean up the whole operation.
                 Monitor(state);
             }
         }
@@ -338,7 +344,7 @@ private unsafe void ParseEventBufferAndNotifyForEach(byte[] buffer)
         /// </summary>
         private sealed class AsyncReadState
         {
-            internal AsyncReadState(int session, byte[] buffer, SafeFileHandle handle, ThreadPoolBoundHandle binding)
+            internal AsyncReadState(int session, byte[] buffer, SafeFileHandle handle, ThreadPoolBoundHandle binding, FileSystemWatcher parent)
             {
                 Debug.Assert(buffer != null);
                 Debug.Assert(buffer.Length > 0);
@@ -349,13 +355,15 @@ internal AsyncReadState(int session, byte[] buffer, SafeFileHandle handle, Threa
                 Buffer = buffer;
                 DirectoryHandle = handle;
                 ThreadPoolBinding = binding;
+                WeakWatcher = new WeakReference<FileSystemWatcher>(parent);
             }
 
-            internal int Session { get; private set; }
-            internal byte[] Buffer { get; private set; }
-            internal SafeFileHandle DirectoryHandle { get; private set; }
-            internal ThreadPoolBoundHandle ThreadPoolBinding { get; private set; }
-            internal PreAllocatedOverlapped PreAllocatedOverlapped { get; set; }
+            internal int Session { get; }
+            internal byte[] Buffer { get; }
+            internal SafeFileHandle DirectoryHandle { get; }
+            internal ThreadPoolBoundHandle ThreadPoolBinding { get; }
+            internal PreAllocatedOverlapped PreAllocatedOverlapped { get; set;  }
+            internal WeakReference<FileSystemWatcher> WeakWatcher { get; }
         }
     }
 }

src/System.IO.FileSystem.Watcher/tests/FileSystemWatcher.cs

@@ -2,12 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
-using System.Collections.Generic;
 using System.ComponentModel;
-using System.IO;
-using System.Linq;
-using System.Runtime.InteropServices;
+using System.Runtime.CompilerServices;
 using System.Threading;
 using Xunit;
 
@@ -265,5 +261,31 @@ public void EndInit_DoesNotEnableEventRaisedEvents()
                 Thread.Sleep(WaitForExpectedEventTimeout);
             }
         }
+
+        [Fact]
+        public void DroppedWatcher_Collectible()
+        {
+            WeakReference watcher = CreateEnabledWatcher(TestDirectory);
+            File.Create(GetTestFilePath()).Dispose();
+            Assert.True(SpinWait.SpinUntil(() =>
+            {
+                GC.Collect();
+                return !watcher.IsAlive;
+            }, LongWaitTimeout));
+
+        }
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private static WeakReference CreateEnabledWatcher(string path)
+        {
+            var fsw = new FileSystemWatcher(path);
+            fsw.Created += (s, e) => { };
+            fsw.Deleted += (s, e) => { };
+            fsw.Changed += (s, e) => { };
+            fsw.Renamed += (s, e) => { };
+            fsw.Error += (s, e) => { };
+            fsw.EnableRaisingEvents = true;
+            return new WeakReference(fsw);
+        }
     }
 }