FileSystemWatcher does not detect watched directory deletion on Linux

[svick]

Description

On Windows, when a directory watched by FileSystemWatcher is deleted, an error is raised that notifies the user of this. But on Linux, the user has no way of learning about it. From cursory reading of inotify docs, FileSystemWatcher could listen to the IN_DELETE_SELF event to make this work.

Reproduction Steps

Run the following code:

string dir = Path.Combine(Path.GetTempPath(), "fsw-demo");
if (Directory.Exists(dir)) Directory.Delete(dir, true);
Directory.CreateDirectory(dir);

using var watcher = new FileSystemWatcher(dir);
watcher.Created += (_, e) => Console.WriteLine($"Created: {e.Name}");
watcher.Deleted += (_, e) => Console.WriteLine($"Deleted: {e.Name}");
watcher.Error   += (_, e) => Console.WriteLine($"Error:   {e.GetException()}");
watcher.EnableRaisingEvents = true;

File.WriteAllText(Path.Combine(dir, "file.txt"), "text");

Directory.Delete(dir, true);

await Task.Delay(1000);

try { Directory.Delete(dir, true); } catch { }

Expected behavior

On Windows, it prints:

Created: file.txt
Error:   System.ComponentModel.Win32Exception (5): Access is denied.

Actual behavior

On Linux (WSL), it prints instead:

Created: file.txt
Deleted: file.txt

Regression?

No.

Configuration

• .Net 11 Preview 2 on both OSes
• Ubuntu 22.04.1 LTS on WSL

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[adamsitnik]

cc @tmds who is the expert in this field

[svick]

Forgot to mention, this is somewhat similar to the closed FileSystemWatcher doesn't trigger Deleted event for file when watched directory is deleted on OSX, even though OSX uses a different implementation than Linux.

[tmds]

There's a comment in FSW.Linux about why it isn't using the IN_DELETE_SELF:

runtime/src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.Linux.cs

Lines 1251 to 1254 in c3727d9

	// of the supplied filters values. We explicitly don't include IN_DELETE_SELF.
	// The Windows implementation doesn't include notifications for the root directory,
	// and having this for subdirectories results in duplicate notifications, one from
	// the parent and one from self.

It is assumed that there was no event generated.

We can generate one to match the Windows behavior.

[tmds]

Similarly, when the directory being watched gets moved we should probably do something that Windows is handling already.

It would be nice if we have some tests that capture the Windows behavior on delete and move. Then we can see what other platforms fail those tests and tackle them.