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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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.

~Andrew