Allow ignoring files based on an xattr value

[DragoonAethis]

This feature request is partially inspired by this PR on rye, a new Python project manager. It introduces a feature where it can automatically mark a directory excluded from sync for Dropbox and iCloud clients by setting a special extended attribute on the directory (com.dropbox.ignored or com.apple.fileprovider.ignore#P). It would be useful if Syncthing supported a similar mechanism.

I sync my workspaces which generally includes quite a few Python projects, most of which come with large virtualenvs. tl;dr: You don't want to sync virtualenvs. Virtualenvs contain installed Python libraries and helper scripts - they can differ a lot between OSes, so you can't cleanly reuse the same virtualenv if syncing files between Windows, Linux or macOS. They also contain thousands of almost exclusively automatically-managed files with unpleasant side effects in case of automatic sync conflict resolutions picking the wrong files, or mixing and matching files synced between different OSes. The project manager should take care of all this automatically on demand.

In general I think it would be useful for people syncing their code between multiple computers. The alternative is to put a special .stignore into project/workspace directories, but that requires either loose files per project, or having a top-level .stignore with /*/.venv and !/exception-project/.venv rules that is then managed separately on each device.

On the negative side, this feature can be very "opaque" - if a directory is ignored via xattrs, there's no clear feedback on why Syncthing just ignores a specific directory (nothing in .stignore, nothing wrong in logs) and can confuse users if enabled unconditionally.

[calmh]

This is probably simple and clear on the source side of things, but less so on the destination, and ignore patterns apply to both. On the destination, architecture wise in Syncthing, it’s a bit late to discover a file is supposed to be ignored when we are at the point where it’s going to get written to disk. We want to know that when we process the incoming index data…

[calmh]

There’s also a fairly straightforward workaround available in that you can write a small script that walks your folder and writes these directories to the ignore file if they have a specific attribute set.