[WSL2] File changes made by Windows apps on Windows filesystem don't trigger notifications for Linux apps

WSL2 is really close to being a perfect runtime environment for server apps being developed in Windows. Great job! One missing feature however is breaking a core part of the developer flow.

For sources stored on the Windows filesystem, any changes made by Windows applications such as Visual Studio do not trigger any file change notifications as far as Linux apps are concerned. This means that all "live rebuild"-type tools don't work (examples: webpack --watch, jekyll --interactive, and Tilt.dev) when running under WSL2. This unfortunately renders many modern dev workflows unviable.

Notes:

• I strongly expect this is already a known issue as hinted here. However I was unable to find any open GitHub issue that specifically tracks this. #1956 is about a different scenario, #4064 isn't specifically about change notifications, #4224 is closed, #4293 doesn't fix it.
• I'm aware the WSL team suggests that with WSL2 it's good to store your application sources inside the Linux filesystem instead of the Windows one. However I don't think that's a viable solution since the whole point of WSL (for me) is to be able to use Windows-based editors such as full VS with a Linux dev environment. If it was sufficient to use Linux-based editors, I'd just use Mac/Linux as my actual OS and would have no need for WSL.
• This did work great on WSL1
• Changing fs.inotify.max_user_watches doesn't affect this, since the issue is about changes originating on the Windows side.

Bug report template

• Your Windows build number: 10.0.19033.1

• What you're doing and what's happening:

  This applies to all tools that listen for file change notifications, but as an example take webpack. Repro steps:

  • In your Windows filesystem, create an empty directory (example: c:\repro), and then add these three files to it
  • In a WSL2 Ubuntu 18.04 environment, install Node and NPM: sudo apt-get install nodejs npm
  • Still in WSL2, go into the directory from earlier: cd /mnt/c/repro
  • Restore NPM dependencies: npm i
  • Run Webpack in watch mode: npm run build:watch. Wait a few seconds until it completes the first build. It will now be waiting for further changes to your source files.
  • In a Windows application (e.g., Notepad or Visual Studio), open c:\repro\index.js and save some change to it. For example, change 'Hello, world' to 'Hello, world 2'.

• What's wrong / what should be happening instead:

  Expected behavior: Webpack should see the change and rebuild. That is, you'll see it log information about another build, and the output in dist/bundle.js will be updated.

  Actual behavior: Webpack doesn't respond at all, because there's no file change notification.

Finally I understand that the fix for this is likely to be "add file watch capabilities to the Plan9 server", and you may feel this is already being tracked by #4064. However #4064 describes a more obscure symptom of this missing feature and makes it sound like an intermittent issue. What I'm reporting here is not intermittent at all, and is a pretty mainstream scenario (using tools like webpack --watch). Thanks!

Run the listed WSL2 webpack watch reproduction under /mnt/c and confirm that a Windows-side save produces no Linux notification. Start by investigating the Plan9 server, which the issue identifies as the likely area for file-watch support. Done means Windows edits trigger the Linux watcher and webpack rebuilds, updating dist/bundle.js.