[libvirt] [PATCH 25/27] fdstream: Suppress use of IO helper for sparse streams

[Eric Blake]

On 05/05/2016 09:32 AM, Daniel P. Berrange wrote:
> On Thu, Apr 28, 2016 at 12:05:12PM +0200, Michal Privoznik wrote:
>> This is kind of a hacky approach to the following problem, but so
>> far I am unable to come up with anything better. On some
>> occasions (esp. when dealing with regular files) libvirt_iohelper
>> is spawned to prefetch data for us. We will then have a pipe then
>> for reading the data from it. This does not fit in our sparse
>> stream implementation as one simply doesn't lseek() over a pipe.
>> Until this is resolved, let's suppress use of the IO helper and
>> read data from FD directly.
> 
> This doesn't really fly - the problem is that with regular files,
> poll() on the FD will always return ready, even if the read or
> write will block in I/O. So by nomt using the iohelper this is
> going to cause our main loop to block on I/O for streams.

The only real solution is to teach libvirt_iohelper to do structured
reads when requested.  That is, you'll have to add a command-line flag
to libvirt_iohelper, which if present, says all of the output from
libvirt_iohelper will be structured as tuples of either
<type=data,length,bytes> or of <type=hole,length>.  When used in this
mode, the client HAS to parse the tuples, rather than assuming that the
pipe can be read literally.  So that means we also have to teach the
consumer of libvirt_iohelper how to read tuples off the pipe, at which
point it then knows whether to send a regular VIR_NET_STREAM or the
compact VIR_NET_STREAM_SKIP.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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