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

[Martin Wege <martin.l.wege@gmail.com>]

On Wed, Aug 23, 2023 at 1:06 AM Roland Mainz via Cygwin
<cygwin@cygwin.com> wrote:
>
> On Tue, Aug 22, 2023 at 4:52 PM Lavrentiev, Anton (NIH/NLM/NCBI) [C]
> via Cygwin <cygwin@cygwin.com> wrote:
> > > FIFOs which don't make *any* sense
> > > ... FWIW, a remote NFS fileystem.
> >
> > I got an impression that the OP is trying to deploy something (maybe the entire Cygwin) onto an
> > NFS share.  So the named FIFO "file" is also created in there.
>
> I agree with that impression. This is basically what large sites
> (universities etc) do with UNIX and Linux: The machines mount an
> user's ${HOMR} directory via automounter over NFS, and users are
> discouraged (e.g. grumpy admin visiting you in person, blocking all
> escape routes... =:-) ) from using the machine's local filesystems (in
> Cygwin's case that includes "C:"!).
>
> In that case people want to use |mkfifo()|/|mkfifoat()| and/or
> /usr/bin/mkfifo in their home directory, and don't expect that it does
> not work.
>
> But that is what happens on Cygwin 3.4.8 right now, if someone tries
> to do a |mkfifo()| on a NFS home directory (tested with MS NFSv3 and
> CITI NFSv4 clients):
> |mkfifo()| succeeds, but the resulting inode is *NOT* a FIFO as requested
>
> Example (/cygdrive/h/ is my home directory shared from my Linux machine):
> ---- snip ----
> roland_mainz@winkrakra1 /cygdrive/h/work/cygwin_mkfifo_on_nfs
> $ uname -a
> CYGWIN_NT-10.0-19045 winkrakra1 3.4.8-1.x86_64 2023-08-17 17:02 UTC
> x86_64 Cygwin
>
> roland_mainz@winkrakra1 /cygdrive/h/work/cygwin_mkfifo_on_nfs
> $ mount
> C:/cygwin64/bin on /usr/bin type ntfs (binary,auto)
> C:/cygwin64/lib on /usr/lib type ntfs (binary,auto)
> C:/cygwin64 on / type ntfs (binary,auto)
> C: on /cygdrive/c type ntfs (binary,posix=0,user,noumount,auto)
> H: on /cygdrive/h type nfs (binary,posix=0,user,noumount,auto)
>
> roland_mainz@winkrakra1 /cygdrive/h/work/cygwin_mkfifo_on_nfs
> $ ls -l
> total 1
> -rw-rw-rw- 1 Unix_User+0 Unix_Group+0 330 Aug 22 23:58 cygwin_mkfifo_on_nfs.c
>
> roland_mainz@winkrakra1 /cygdrive/h/work/cygwin_mkfifo_on_nfs
> $ cat -n cygwin_mkfifo_on_nfs.c
>      1  #include <stdlib.h>
>      2  #include <stdio.h>
>      3  #include <stdio.h>
>      4  #include <errno.h>
>      5  #include <sys/types.h>
>      6  #include <sys/stat.h>
>      7
>      8  int main(int ac, char *av[])
>      9  {
>     10          (void)puts("# start");
>     11
>     12          if
> (mkfifo("/cygdrive/h/work/cygwin_mkfifo_on_nfs/myfifo.fifo", 0) != 0)
>     13                  perror("mkfifo failed");
>     14          (void)puts("# done.");
>     15          return EXIT_SUCCESS;
>     16  }
>     17
>
> roland_mainz@winkrakra1 /cygdrive/h/work/cygwin_mkfifo_on_nfs
> $ gcc -g cygwin_mkfifo_on_nfs.c
>
> roland_mainz@winkrakra1 /cygdrive/h/work/cygwin_mkfifo_on_nfs
> $ ./a
> # start
> # done.
>
> roland_mainz@winkrakra1 /cygdrive/h/work/cygwin_mkfifo_on_nfs
> $ ls -l
> total 68
> -rwxr-xr-x 1 Unix_User+0 Unix_Group+0 66951 Aug 23 00:12 a.exe
> -rw-rw-rw- 1 Unix_User+0 Unix_Group+0   330 Aug 22 23:58 cygwin_mkfifo_on_nfs.c
> lrwxrwxrwx 1 Unix_User+0 Unix_Group+0    11 Aug 23 00:12 myfifo.fifo
> -> ':\0:c4:1000'
>
> roland_mainz@winkrakra1 /cygdrive/h/work/cygwin_mkfifo_on_nfs
> $ cat <myfifo.fifo
> -bash: myfifo.fifo: No such file or directory
> ---- snip ----
>
> 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.
> 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)
>
> 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.

We would like to see option 1 implemented, maybe as an option in the
CYGWIN environment variable
(CYGWIN=nfs_emulate_dev_special_files_as_symlink ?)

Thanks,
Martin