Re: Xenstore.h clarifications

[Ian Campbell <Ian.Campbell@citrix.com>]

On Thu, 2014-11-27 at 12:51 +0200, Razvan Cojocaru wrote:

It's a good idea to CC the relevant maintainers if you want their input.

> Hello,
> 
> I know that xc_interface_open() can be safely called several times from
> the same process, and that several processes can each have a bunch of
> xc_interface handles open, and that I shouldn't use an xc_interface
> inherited from the parent in a child process, because xenctrl.h says so.
> 
> Is it safe to assume that the same restrictions / conventions apply to
> xs_handles obtained via xs_open()? Xenstore.h is not explicit. Looking
> at the code, it would seem safe to assume that it can be used in a
> similar manner, but it would be nice to have this confirmed if possible.

I think there's a pretty good chance that the same applies to xenstore
connections made over the device/shared-ring interface.

I'm not really sure about the semantics of a Unix domain socket after a
fork, but I don't expect both parent and child could sanely make use of
it.

So I think the answer is:

      * Connections made with xs_open(0) (which might be shared page or
        socket based) are only guaranteed to work in the parent after
        fork.
      * Connections made with xs_open(XS_OPEN_SOCKETONLY) will be usable
        in either the parent or the child after fork, but not both.
      * xs_daemon_open*() and xs_domain_open() are deprecated synonyms
        for xs_open(0)
      * XS_OPEN_READONLY has not bearing on any of this.

Ian, does that seem right?

Razvan, assuming Ian concurs with the above (or corrects it) then could
you knock up a patch to document the result please.

Ian.