[libvirt] creating stream

[Michal Privoznik]

On 04/05/2017 12:35 AM, Vasiliy Tolstov wrote:
> Hi. I know that libvirt devs does not like devs that used internal rpc
> of libvirt... 

Not entirely true. We like all devs, but we just discourage people to
use our RPC as it's internal. It doesn't offer the full set of features
provided by libvirt APIs: for instance some drivers (e.g. ESX) don't use
it and use curl instead to communicate with remote hypervisor. The
second reason is that we might change the RPC suddenly without prior
notice and thus your application would just stop working.

> I need to create stream to use with upload/download and
> as i see it does not created via rpc call, can you suggest me what
> files i need to check to understand how virStreamNew works and how to
> read write works internally?
> Thanks!
> 

The libvirt streams are a bit kind of magic. Esp. on RPC level.
Basically, when client and server wants to transfer some data over a
stream, both of them call virStreamNew() which creates a local object
which so far is not tied to the other stream in any way. The point where
these two streams start to be connected is the public API that involves
the stream, for instance virStorageVolDownload().

The streams can be viewed as an UNIX socket or some other type of
socket. virStreamSend() corresponds to write() and virStreamRecv() then
corresponds to read(). So if one side sends some data to the stream, the
other side can receive the data.

On the code level, there's again a driver that implements the methods as
there are several implementations available. Firstly, there's network
implementation in our RPC code (this is responsible for transferring
stream data to the other side), then there's local implementation for
working with files (virFDStream), and finally ESX has its own
implementation (it uses curl for fetching/uploading data onto ESX
hypervisor).

I'm currently working on sparse streams (so that files transferred
through our streams stay sparse), and let me tell you - the code is
pretty complicated.

http://libvirt.org/internals/rpc.html

Michal