[RFC PATCH] discover/pxe-parser: Retrieve configs asynchronously

[Samuel Mendoza-Jonas]

On Mon, Jun 13, 2016 at 06:47:01PM +1000, Jeremy Kerr wrote:
> Hi Sam,
> 
> > +	/*
> > +	 * Retrieving PXE configs over the network can take some time depending
> > +	 * on factors such as slow network, malformed paths, bad DNS, and
> > +	 * overzealous firewalls. Instead of blocking the discover server while
> > +	 * we wait for these, spawn an asynchronous job for each URL we can
> > +	 * parse and process the resulting files in a callback.
> > +	 * Since the caller will free the discover_context once
> > +	 * iterate_parsers() completes, we need to copy the discover_context
> > +	 * under a different talloc context so that it survives until the
> > +	 * callback it finished with it.
> > +	 */
> 
> This sounds a little dangerous - there may be other allocations made
> under the original discover_context that get freed after
> iterate_parsers() returns there.

Yeah it certainly blew up a few times before it was ready :) I've sanity
checked the lifetimes of what happens with each discover context to the
point that I'm fairly happy with it - but it still leaves it a bit
vulnerable to future changes.

> 
> Can we either:
> 
>  - have this code create a second reference to the talloc-ed
>    discover_context, so that it doesn't get freed until both references
>    have finished using the context (which may require checking that
>    talloc_ref and friends work the way we assume), or

I like this, sounds it should be fairly straightforward with
talloc_reference and talloc_free/talloc_unlink, I'll give it a shot.

> 
>  - rethink the lifetime of the contexts, so that we only free once we
>    know that the context is unused.
> 
> Cheers,
> 
> 
> Jeremy