[Gluster-devel] GlusterFS API to manipulate open file descriptor - glfs_fcntl?

[Anoop C S]

On Tue, 2019-10-15 at 12:40 +0530, Amar Tumballi wrote:
> It would be great if you can point at the requirements, or new
> additions you are talking about.

Explained below..

> On Tue, Oct 15, 2019 at 12:26 PM Anoop C S <anoopcs at cryptolab.net>
> wrote:
> > Hi all,
> > 
> > This is to check and confirm whether we have an API(or an internal
> > implementation which can be exposed as API) to perform operations
> > on an
> > open file descriptor as a wrapper around existing fcntl() system
> > call.
> > We do have specific APIs for locks(glfs_posix_lock) and file
> > descriptor
> > duplication(glfs_dup) which are important among those operations
> > listed
> > as per man fcntl(2). 
> > At present we have a requirement(very recent) from Samba to set
> > file
> > descriptor flags through its VFS layer which would need a
> > corresponding
> > mechanism inside GlusterFS. Due to its absence, VFS module for
> > GlusterFS inside Samba will have to workaround with the hack of
> > creating fake local file descriptors outside GlusterFS.
> > 
> > Thoughts and suggestions are welcome.
> > 
> 
> If there is a need have a feature, it makes sense to extend fd_t
> structure and provide it inside. If my memory serve right, we didn't
> support fcntl() behavior in gluster as there was no fcntl() through
> fuse when we started.. 
> 
> Would be good to understand what is needed,

During open/create code path Samba tries to set blocking
flag(O_NONBLOCK on Linux) on open file descriptor through its VFS
layer. Comparing to fcntl() call command would be F_SETFL with required
flag(O_NONBLOCK). It is an expectation that backend file system(in this
case GlusterFS) executes this operation and return the result. VFS
module for GlusterFS inside Samba can extract required file descriptor
but would require a way to set the flag on it.

> and then start working on design discussions. (if it makes sense).
> 
> -Amar
>  
> > Anoop C S.
> > 
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