MacOS File Change Events Not Triggering? Here's Why!

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MacOS File Change Events Not Triggering? Here's Why!

Hey everyone, have you ever run into a situation where your file watcher on macOS just stops working after you save a file using your favorite editor? It's super frustrating, right? Well, you're not alone! This is a pretty common issue, and it's all down to how macOS handles file saving, specifically with atomic saves. Let's dive into the nitty-gritty and figure out what's going on, and of course, how you might be able to handle it.

The Core Problem: Atomic Saves and File Watchers

So, what exactly are atomic saves, and why are they causing us headaches? Let's break it down, guys.

Understanding Atomic Saves

Most modern editors, like Neovim, VS Code, and MacVim, use a clever trick called atomic saves to ensure that your files are never corrupted during the saving process. Here's how it works:

  1. Write to a Temporary File: When you hit save, the editor doesn't directly modify your original file. Instead, it writes all the changes to a brand new, temporary file.
  2. Rename the Temporary File: Once the changes are safely written, the editor renames the temporary file to the same name as your original file. This is an incredibly quick operation and ensures that you always have a complete, valid version of your file, even if something goes wrong during the save process.

This method is super safe and reliable, which is why it's so widely used. But, here's where the trouble starts.

The Role of kqueue and uv_fs_event

Our file watchers, the ones that are supposed to detect changes, often use a mechanism called kqueue under the hood. On macOS, kqueue is a powerful system call that's used to monitor file system events. It works by keeping track of specific things, like the inode of a file.

  • Inode? What's That? Think of an inode as a unique identifier for a file. It's like the file's fingerprint within the file system. When the editor renames the temporary file to overwrite the original file, it also changes the inode. Now, the file watcher is still watching the old inode, which no longer points to the actual file on your system.

The core of the problem lies in the fact that kqueue (and therefore uv_fs_event) is tracking the old inode. Since a new inode has been created, any subsequent changes to the new file are no longer being tracked. This is why our file watcher suddenly goes silent after that first save, unable to track those changes. No more "mtime change" or any file update logs will appear after that initial write.

Reproducing the Issue: A Step-by-Step Guide

Want to see this problem in action? Here's how you can reproduce the issue and witness the problem firsthand:

Step-by-Step Instructions

  1. Run the Extension on macOS: Start by running the file watcher, application, or extension that you are using. This can be any application that has a file monitoring feature, such as a file synchronizer, code editor extension, or any tool that reacts to file changes.
  2. Open an Editor: Choose an editor like Neovim or MacVim (with :set backupcopy=auto). This is key, as most editors utilize this atomic save behavior.
  3. Save Changes: Make some changes to the temporary file opened by the extension and save them using your editor.
  4. Observe the Behavior: After the first save, you'll likely notice that your file watcher stops responding to changes. Subsequent saves won't trigger any updates in the output or behavior of the extension. It will act as though nothing is happening. You're left hanging! This is our core problem.

What You'll See

After you've reproduced the issue by following these steps, you'll see that file change events (via uv_fs_event) should trigger after each save, and the editor's output should update dynamically, similar to what you'd experience on Linux or Windows. However, the sad reality is that nothing happens. No further change events are received after that first save. Your file watcher silently gives up on you, completely oblivious to your ongoing editing endeavors.

Expected vs. Actual Behavior: A Comparison

The Ideal World: Expected Behavior

In a perfect world, when working with file watchers and editors that use atomic saves, here's what we'd expect:

  • Real-time Updates: As you save your changes in the editor, the file watcher should immediately recognize the changes and update accordingly. This could manifest as refreshing content in a live preview, automatically running tests, or synchronizing files across devices.
  • Seamless Integration: The process should be invisible. You'd be able to edit and save files without a second thought, and your file watcher would always be there, diligently keeping things in sync.

The Reality: Actual Behavior

Unfortunately, because of the limitations of kqueue and uv_fs_event, here's what you'll encounter on macOS:

  • Initial Update Only: After the first save, your file watcher registers the initial write, and then it goes silent.
  • Delayed Updates (Sometimes): The new contents of the file might eventually appear in the application or output, but with a noticeable delay, after you close the editor. This is because macOS eventually registers the change at the file system level, but there is no real-time update.
  • Frustration: You're left waiting for the file watcher to catch up, which disrupts your workflow and forces you to manually refresh the content or restart the process.

Possible Solutions and Workarounds

So, what can we do to tackle this issue? Here are some possible solutions and workarounds you can consider:

1. Polling for Changes

One potential workaround is to use polling instead of relying solely on file system events. Polling involves periodically checking the file's modification time (mtime) to see if it has changed. This approach is less efficient but can detect changes even when the file's inode changes.

2. Monitoring the Parent Directory

Instead of watching the individual file, you could watch the directory that contains the file. This way, you might be able to detect the renaming of the file, which would trigger a new event. However, this approach can be less precise and may require additional logic to determine which file has changed.

3. Using FSEvents (macOS-Specific)

macOS has a more advanced file system event notification mechanism called FSEvents. This framework is specifically designed to handle file system events on macOS, including situations where files are renamed or replaced. You can try to switch your file watcher to the use of FSEvents. However, there are some extra steps involved in integrating with FSEvents.

4. Editor Configuration

Some editors offer options that might influence how they save files. For instance, in some editors, you might be able to configure it to create backups of files before saving instead of using atomic saves. That could make the file watcher trigger the right event, but it would come with the downsides of having to deal with backup copies of your files.

Conclusion: Navigating File Change Events on macOS

Dealing with the limitation of the uv_fs_event on macOS can be a bit of a pain, especially if you're a heavy user of file watchers and editors with atomic save capabilities. Understanding how atomic saves work, and how they interact with file system event monitors like kqueue, is the first step in solving this issue. By using the right solution, such as polling, directory monitoring, or macOS-specific solutions, you can achieve your desired behavior and stay productive!

So, if you are hitting this issue, don't worry, you are not alone! It's a well-known problem, and thankfully, there are some great options for working around it. Keep experimenting with the solutions above, and I am sure you can get your file watchers working perfectly!