Related to https://github.com/Automattic/dotcom-forge/issues/8817

Proposed Changes

This PR aims to ensure that symlinks found inside of a site directory are working. We do this by first scanning and later watching the file system for symlinks and then creating mounts for the targets of each found symlink.

Since symlink targets inside of the NODEFS are not always predictable, we need to get the actual target of the link as seen by the FS inside the PHP runtime. When we determine the correct target, we create a mount point that points to the realpath of the symlink on the host system, essentially bypassing the need for chained links. (e.g.link -> link 2 -> target )

We also need to ensure that we keep a reference count of how many links point to a specific mounted target. It is possible that multiple symlinks point to the same target. As we are trying to unmount unnecessary targets, we need to be sure no other links point to it when we attempt to delete it.

Additionally, we need to ensure that we do not delete pre-existing files or directories at the target path. E.g. if a target already existed before we found a link pointing to it, we never want to remove it if the symlinks are removed.

Potential issue:
I found that deleting a symlink correctly removes files and links from the NODESF but when trying to load it via the browser, it throws an error of file not found. I think this might be related to how PHP or the underlying server is caching things or perhaps something related to the NODEFS. We might need to investigate that at some point.

Testing Instructions

• Create a Site
• Create a file somewhere on your filesystem, place something like <?php echo time(); in that file.
• Now, create a symlink from your Studio site directory to the file you just created by running ln -s ~/your-file.php test.php - this creates a symlink test.php that points to a file you created elsewhere.
• Try to load that file by requesting it in the browser http://localhost:<port>/test.php - you should see the current timestamp or the output of whatever you placed in the file.

Extra scenarios to test:

• Download a plugin somewhere and extract it, create a symlink inside wp-content/plugins to that plugin and see if it appears in wp-admin.
• Try adding a symlink before starting the site
• Try adding a symlink after the site has started
• Try to see if the symlink still works after 400 requests (this is when the PHP instance rotates)
• Try to delete a symlink while the site is running and verify that the file is no longer available.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?