Re: How to fix |mkfifo()| failure if |pathname| is on NFS ? / was: Re: [EXTERNAL] Re: mkfifo: cannot set permissions of 'x.fifo': Not a directory

[Cedric Blancher <cedric.blancher@gmail.com>]

On Fri, 25 Aug 2023 at 17:15, Roland Mainz via Cygwin <cygwin@cygwin.com> wrote:
>
> On Fri, Aug 25, 2023 at 2:18 PM Corinna Vinschen via Cygwin
> <cygwin@cygwin.com> wrote:
> >
> > On Aug 23 01:05, Roland Mainz via Cygwin wrote:
> > > Note that Cygwin does not interpret the file |myfifo.fifo| as FIFO,
> > > instead it comes back as a symlink "myfifo.fifo -> ':\0:c4:1000'".
> > >
> > > AFAIK there are (at least) these two options to fix the problems:
> > > 1. Check whether the filesystem for the fifos path is NFS
> > > (cgywin.dll's |fs.fs_is_nfs()|), and if it is a symlink check if it
> > > starts with ':\0:c4:' (assuming "c4" is the prefix for inodes created
> > > with |mkfifo()|). If this condition is |true|, then cygwin |stat()|,
> > > |open()| etc. should treat this inode as FIFO.
> >
> > The downside is that it is not possible to diffentiate between Cygwin
> > FIFOs and real FIFOs created from the remote side in `ls -l'
> > output.  Note that Cygwin returns the NFS stat info verbatim, so
> > a real FIFO is returned as a real FIFO:
> >
> >   linux$ mkfifo bar
> >   cygwin$ ls -l bar
> >   prw-r--r-- 1 corinna vinschen  0 Aug 25 13:58 bar
>
> I know.
>
> > The idea was always to use NFS as exchange medium, but not as
> > installation medium for the entire distro or to keep Cygwin home
> > dirs on NFS.  There were times where NFS was pretty unstable.
> > I used NFS for quite some time to build Cygwin packages, but at one
> > point I got trouble (performance problems with multiple concurrent
> > processes accessing an NFS share, build errors out of the blue),
> > so I switched to Samba shares, albeit grudgingly.  I'm not yet
> > sure if the problems are fixed.  At least a recent OpenSSH build
> > ran through without problems...
>
> I think most issues have been fixed for the Microsoft NFSv3 client,
> and for the CITI NFSv4.1 client (technically it's called
> "ms-nfsv41-client") the situation is even better since it's
> opensource, and we can fix problems even faster there.
> From what I see the ms-nfsv41-client is stable enough for daily
> routine usage, and I know that other institutions like DFG and CERN
> use it for daily work too. The only nasty part is the damn lack of
> documentation, and that there are no signed binaries, so any kernel
> with UEFI/SecureBook cannot use them.
>
> > Anyway.  How would you like to make sure that a Cygwin application
> > can differ between real FIFOs and Cygwin FIFOs?
>
> For now I can provide a migration script, and in the medium term we
> should get Microsoft to provide some kind of |mknod()| API. see below.
>
> > > 2. Check whether the filesystem for the fifos path is NFS
> > > (cgywin.dll's |fs.fs_is_nfs()|), and then just refuse |mkfifo()| with
> > > |ENOSYS| (not implemented)
> >
> > I like the idea.
>
> See below.
>
> > > Better algorithm for [1] might be to check whether the inode is a
> > > symlink, and then do a |fs.fs_is_nfs()| on the symlinks |pathname|,
> > > assuming this is more performant.
> >
> > Even better would be if we could just create and use real FIFOs
> > on NFS(*).  But while NtQueryEaFile can be used to fetch real
> > NFS file info, and while NtCreateFile can be used to create real
> > synmlinks via NFS, I don't see an interface resembling mknod/mkfifo.
>
> Looking at the ms-nfs41-client source code, there is no API for that *YET*.
>
> So my plan would be like this:
> 1. Cygwin: implements the proposed devnodes-as-symlink emulation code,
> if the env var CYGWIN has "nfs_emulate_dev_special_files_as_symlink"
> set
> 2. Cygwin: By default |mkfifo()| will fail with |ENOSYS| (not |EPERM|,
> as we intend to fix the issue!) on NFS filesystems, unless
> CYGWIN=nfs_emulate_dev_special_files_as_symlink

I think the emulation should be on by default, or you break things.

Ced
-- 
Cedric Blancher <cedric.blancher@gmail.com>
[https://plus.google.com/u/0/+CedricBlancher/]
Institute Pasteur