Re: [Xen-devel] [RFC PATCH v2 00/17] Add support for qemu-xen runnning in a Linux-based stubdomain.

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Thu, Nov 01, 2018 at 05:48:37PM +0000, Ian Jackson wrote:
> libxl has code for waiting for xs watches and domain death and so on
> already.
> 
> How about this: provide a new variant of libxenvchan_client_init which
> can give a return indication of the form `this server does not appear
> to be set up; please watch on the following xenstore key and then call
> me again when the watch fires'.  That would be simple, and not involve
> further event loop entanglement, and would fit nicely into libxl.

Doing it in this order would be susceptible to a race condition -
server appearing after libxenvchan_client_init check, but before libxl
register the watch. Also, right now libxenvchan_client_init have only
one possible error code: NULL (instead of struct libxenvchan *). Adding
more elaborate error reporting would require API change.

As the xs path is provided by libxenvchan_client_init caller anyway,
libxl can register watch before calling libxenvchan_client_init and wait
on it if libxenvchan_client_init fails. In _this particular case_ other
conditions don't need to be handled specifically, as in the worst case it
will hit startup/qmp timeout. 

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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