Re: ztcp should not pick fd 0

[Peter Stephenson <Peter.Stephenson@csr.com>]

On Wed, 11 Aug 2010 12:21:27 -0400
Greg Klanderman <gak@klanderman.net> wrote:
> I'm using ztcp to connect to a server; when the connection is lost and
> needs to be re-opened from within the completion system (compctl),
> calling 'ztcp <host> <port>' will actually choose fd 0 for the socket
> file descriptor which is hosing other parts of my program (calling out
> to a python process specifically).  I can work around this problem by
> requesting the previous fd with the '-d' option (I suppose that's not
> 100% safe because after closing the original socket that fd might get
> reallocated in the intervening time), but it would be really nice if
> ztcp could be made to never choose fds 0 through 2.  This is also a
> problem because if you try to close fd 0 with ztcp you'll get the
> error '0 is an invalid argument to -c'.

Moving it above 9 is probably OK.  I'm trying to work out whether there's
any use at present where that might cause problems, but it would have to be
somewhere that assumed it was less than 10, which would be an unsafe
assumption.  It's more consistent to have it over 9, other "magic" file
descriptors work like that.

> The obvious patch below does not work because movefd() marks the fd as
> FDT_INTERNAL, and that causes the fd to get closed when external
> programs are exec'd.  This calls into question the other use of movefd
> in tcp.c as well.

There's nothing very special about fdtable; adding "fdtable[sess->fd] =
FDT_EXTERNAL" to the patch on success should be OK, or invent another FDT_
type (fdtable is already exported to modules).  The other use in the file
may need this too --- it's quite likely the CLOEXEC behaviour of
zsh/net/tcp hasn't been thought through, but the default should obviously
be the system default of not closing fd's.

-- 
Peter Stephenson <pws@csr.com>            Software Engineer
Tel: +44 (0)1223 692070                   Cambridge Silicon Radio Limited
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