Re: [PATCH 0/2] Provide virtual /dev/fd and /dev/{stdin, stdout, stderr} symlinks

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Corinna,

On Mon, 28 Feb 2022, Corinna Vinschen wrote:

> On Feb 28 10:24, Corinna Vinschen wrote:
> > On Feb 25 16:46, Johannes Schindelin wrote:
> > > On Tue, 22 Feb 2022, Corinna Vinschen wrote:
> > > > On Feb 21 14:36, Johannes Schindelin wrote:
> > > > > If there is appetite for it, I wonder whether we should do something similar
> > > > > for `/dev/shm` and `/dev/mqueue`? Are these even still used in Cygwin?
> > > >
> > > > "still used"?  These are the dirs to store POSIX semaphors, message
> > > > queues and shared mem objects.
> > >
> > > Okay. I guess we do not really use them in Git for Windows ;-)
> >
> > Probably not.  I'm not aware that git uses POSIX IPC objects.
> >
> > > > These have to be real on-disk dirs.
> > >
> > > Could I ask you to help me understand why? Do they have to be writable? Or
> > > do the things that are written into them have to be persisted between
> > > Cygwin sessions?
> >
> > Cygwin uses ordinary on-disk files to emulate the objects, and
> > they have to persist over process exits.  Unfortunately I don't
> > see any other way to create persistent objects from user space.
>
> Btw., you don't have to create those dirs.  Only if you actually use
> POSIX IPC calls, the directories are required.
>
> In fact, the IPC objects are just mmaps (message queues, shared mem
> objects), or the file is just used to store the values after closing
> the object (semaphores).

Okay, I _think_ I understand the issue better now. Thank you for indulging
me!

> With "persistent" I mean, "DLL lifetime persistent".  It's not required
> that the objects are persistent until system shutdown, as it is now with
> file-based objects.
>
> It would be sufficient if the objects persist until the last Cygwin
> process of a Cygwin process tree exits.  I'm open to ideas, but they
> shouldn't further slow down the startup of a Cygwin process tree.

From my limited understanding, that _sounds_ as if a shared object might
be enough (similar to the shared parent directory that
`winsup/cygwin/shared.cc` is all about).

If this sounds like a viable approach, I'll put it into my ever-growing
backlog ;-)

Ciao,
Johannes