Support symlinks on core backends

[pyramation]

I've been reading the docs and I'm under the impression that the core filesystems have symlinks?

I'm currently getting ENOTSUP not supported errors, so curious if there are any examples or pointers for getting symlinks to work, thanks!

[pyramation]

any one know how to create symlinks? Apparently they are readable, which is a good sign!

any help appreciated: https://stackoverflow.com/questions/55131565/how-to-create-a-symlink-within-browserfs

@jvilk 🙏🏻

[jvilk]

@pyramation I don't believe any backend currently supports symlinks. The basic abstraction is there should someone want to implement a backend that supports them, but I never got around to supporting them in the core backends.

[pyramation]

Thank you @jvilk!

Do you have an idea of how one could implement it for the Memory or LocalStorage backends? I'd be happy to dive in if there are some conceptual models for how to create the symlink.

I had one idea... I did look into potentially changing modes, and setting the contents manually. I found from this link that symlinks are mode 120000. I quickly realized I didn't know what the actual contents of a symlink look like, and couldn't find much info out there on the net or empirically in my shell (the OS wouldn't let me modify the mode of a sym link to turn it into a file and look at contents).

Any pointers on this thread are appreciated and I imagine would contribute to the creation of this feature ;)

[jvilk]

The contents of a symlink is the path to the file you're symlinking. So a symlink to ../foo will contain ../foo. It's symbolic, so if you delete ../foo and bring it back, your symlink will point to the new foo. If you rename ../foo, you'll have a broken symlink.

To implement symlinks, you'll need to somehow denote in the metadata of the file that it's a symlink (I think it's stashed in the mode as you suspect -- we stash file/dir in the mode as well), and then store it as a file with the contents of the symlink.

[pyramation]

Thanksd @jvilk ! I tried this out, and having issues editing mode. What's the best way to edit the mode or get into the metadata? I'm attempting to use chmodSync and getting ENOTSUP errors.

I'm trying to use chmod because If we try and write the mode on creation, we get issues because of umask, e.g.,

    fs.writeFileSync("value.txt", "hello world");
    fs.writeFileSync("alias", "./value.txt", {
      mode: 12000
    });
    console.log(fs.statSync("alias").mode.toString(8));

Which prints 127340 instead of 12000 because of UNIX permissions and umask, which makes sense. So in attempt to set mode after creation,

    fs.chmodSync("alias", 12000);

Then I get a nice not supported error: Error: ENOTSUP: Operation is not supported.

[abhishiv]

Hey @pyramation, did you manage to figure this out. I'm hitting the same problem.

Also, from the link that you posted should the mode change due to unix permissions for symlinks? It might make it easy to implement it If it doesn't.

In addition, on Linux systems you can't specify a symlink's permissions; all that matters is the target's permission (and effectively the full mode since that determines the symlink's behaviour too).

Also would readdir/readlink support symlinks automatically if we can figure this out? I think readdir might, but how would readlink know that it's a symlink if the mode changes? Does lstat help?

  /**
   * @return [Boolean] True if this item is a symbolic link (only valid through lstat)
   */
  public isSymbolicLink(): boolean {
    return (this.mode & 0xF000) === FileType.SYMLINK;
  }

[pyramation]

hey @abhishiv I don't think I ever got this working, nor did I see if there was work on it. Happy to see that more people want this feature, hopefully it can happen :)

[james-pre]

@pyramation

I'm attempting to use chmodSync and getting ENOTSUP errors.

chmod is supported with #334

If you do get it working a PR would definitely be welcome.