Merge pull request #2786 from stephentoub/fix_fsw_regression

Fix regression in FileSystemWatcher

Author: stephentoub

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

@@ -75,8 +75,13 @@ private void StopRaisingEvents()
             // This operation doesn't need to be atomic because the API will deal with a closed
             // handle appropriately. If we get here while asynchronously waiting on a change notification, 
             // closing the directory handle should cause ReadDirectoryChangesCallback be be called,
-            // cleaning up the operation.
+            // cleaning up the operation.  Note that it's critical to also null out the handle.  If the
+            // handle is currently in use in a P/Invoke, it will have its reference count temporarily
+            // increased, such that the disposal operation won't take effect and close the handle
+            // until that P/Invoke returns; if during that time the FSW is restarted, the IsHandleInvalid
+            // check will see a valid handle, unless we also null it out.
             _directoryHandle.Dispose();
+            _directoryHandle = null;
 
             // Set enabled to false
             _enabled = false;

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

@@ -146,7 +146,6 @@ public static void FileSystemWatcher_Changed_FileDataChange()
         }
     }
 
-    [ActiveIssue(2740)]
     [Theory]
     [InlineData(true)]
     [InlineData(false)]