|IO_REPARSE_TAG_MOUNTPOINT| (Junctions) not working for remote filesystems in Cygwin ?

Thu Feb 6 09:47:24 GMT 2025

Greetings, Corinna Vinschen via Cygwin!
> On Feb 4 14:47, Jeremy Drake via Cygwin wrote:
>> On Tue, 4 Feb 2025, Roland Mainz via Cygwin wrote:
>>>> > it seems that Cygwin does not support |IO_REPARSE_TAG_MOUNTPOINT| for
>> > "remote" filesystems:
>> > ---- snip ----
>> > 2582 /* Don't handle junctions on remote filesystems as
>> > symlinks. This type
>> > 2583 of reparse point is handled transparently by the OS so that the
>> > 2584 target of the junction is the remote directory it is
>> > supposed to
>> > 2585 point to. If we handle it as symlink, it will be mistreated as
>> > 2586 pointing to a dir on the local system. */
>> >
>> > The matching code in our filesystems seems to work in PowerShell and
>> > cmd.exe - so what context am I missing ?
>>>> The comment seemed to explain it pretty well. Junctions are always
>> absolute. If it is absolute to a local path, that path is local to the
>> server, not the client. If Cygwin treated it as a symlink, it would see
>> the target as /cygdrive/c/whatever and would try to follow that to the
>> client-local directory. By *not* treating those as symlinks, it will
>> instead treat them as ordinary directories to be traversed, which will
>> allow the OS to handle them as normal.

> Well explained.

>> Perhaps it could be relaxed to allow remote junctions to be treated as
>> symlinks if their targets are UNC rather than local? Is that the case
>> your filesystems are exposing?

> Just to be clear, there are two types.

> The official volume mount points using the GUID-style volume names as
> introduced with the Vista volume manager shouldn't be touched at all for
> the reason stated above.

> The junctions points are usually pointing to some local directory
> in the form \??\X:\... We can't use them for the same reason.

> But if your NFS client would be so kind to convert them to the UNC
> type of path, i. e., \??\UNC\server\path, then we could test it in
> Cygwin and actually expose them as symlinks.

> However, is it really worth the effort?

> Right now, those remote reparse points of type
> IO_REPARSE_TAG_MOUNT_POINT are transparently handled by the OS, that's
> why there's no problem using them in PS or cmd. They are just passed
> through.

> In Cygwin, symlinks of any type are handled as symlinks. That means,
> evaluating a path with a symlink requires to open the symlink and read
> the target path from it, then replace parts or all of the current path
> with the symlink content, to create a final POSIX/Win32 path pair from
> it.

> So you have a (small) performance hit, for the not so obvious gain to
> see a remote junction as symlink in Cygwin.

> I'm not judging here, I'm really asking for your opinion.

To add another stone to the pile,
$ fsutil behavior set symlinkEvaluation
…
symlinkEvaluation {L2L|L2R|R2R|R2L}:{0|1} [...]
…
Sample SymlinkEvaluation command:
 "fsutil behavior set symlinkEvaluation L2L:1 L2R:0"
 - Will enable local to local symbolic links and disable local to
 remote symbolic links. It will not change the state of remote to
 remote links or remote to local links.
 - This operation takes effect immediately (no reboot required)
The platform behavior could be different from user's expectations. And it is
not library's job to second-guess the OS behavior.
-- 
With best regards,
Andrey Repin
Thursday, February 6, 2025 12:44:40
Sorry for my terrible english...