[Libguestfs] Progress bars

[Richard W.M. Jones]

On Tue, Jul 06, 2010 at 04:37:47PM +0100, Daniel P. Berrange wrote:
> In the wire protocol, when sending back the status messages, it is
> probably wise to make sure the 'serial' field of the status message
> always matches the 'serial' field of the initial RPC request.

Yup we do this already.

I'm really not attracted to the idea of making the library or the
daemon multithread safe right now.  Guestfs-browser is a multithreaded
program and it's using libguestfs quite happily by assigning a single
thread to manage the handle.

Also I don't see there's a lot of benefit to doing it.  Most
libguestfs operations are inherently serial (ie. create the partition,
*then* create the filesystem in the partition).  And those which could
be parallelized such as cloning several partitions at the same time in
virt-resize would be complex to implement and may be I/O bound in all
but a few real world cases.

[Speak up if you have a need for multithreaded access to the API]

> If you're not very careful adding a 'guestfs_get_progress' API
> could make it very hard to introduce multithread support. Since
> the main thread is blocked on the original API, I assume this
> get_progress will be called from a separate thread, so you can't
> just use thread locals to associate this info. 

Thinking about this more over the weekend, I came to reject the idea
of guestfs_get_progress also, but for different reasons.

Since guestfs_get_progress can be entirely implemented using the
callback, the user can implement thread safety if they need it.  By
dropping guestfs_get_progress and just implementing the callback we
don't need to consider threads at all now (covered by the existing
"thread == handle" rule).

And a callback (or closure) shouldn't prevent future thread safety ...
right?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines.  Supports shell scripting,
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