Re: [PATCH 2/6] tools/libxl: Update datacopier to support sending data only

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

On Fri, 2015-02-20 at 11:10 +0000, Andrew Cooper wrote:
> On 20/02/15 10:27, Ian Campbell wrote:
> > On Wed, 2015-02-18 at 16:34 +0000, Andrew Cooper wrote:
> >> From: Wen Congyang <wency@cn.fujitsu.com>
> >>
> >> datacopier is to read some data and write it out. If we
> >> have some data to send it over network, we cannot use
> >> datacopier. Update it to support this case.
> > Please can you clarify this commit message. Questions I'm left with
> > after reading it:
> >
> >       * What is the relevance of "send it over network" here, why does
> >         it matter what the output fd is? Or is this something to do with
> >         the lack of an input fd (in which case where does the incoming
> >         data come from?)
> >       * Why can datacopier not currently be used in this case, what
> >         actually goes wrong?
> >       * What is the nature of the update which makes it work? (possibly
> >         becomes obvious after the previous answers)
> >
> > Thanks.
> 
> This is one of several patches which didn't get included as part of the
> libxl remus series in the end (i.e. git rebase didn't drop this hunk
> out), but is still needed for migration v2.
> 
> Currently, the datacopier only functions when copying from readfd to
> writefd until EOF is hit on readfd.
> 
> The datacopier infrastructure already has prefixdata which will be
> inserted into writefd ahead of the content of readfd, but lacks the
> ability to simply copy from a local buffer to writefd.
> 
> This patch makes the lack of readfd non-fatal, which allows the rest of
> the existing infrastructure function as a proper copy from local buffer.

Where "copy from local buffer" is using the prefixdata support?

Anyway, please expand the commit message with all that info.

> 
> ~Andrew
>