Re: Making QEMU easier for management tools and applications

[Stefan Hajnoczi]

On Thu, Jan 16, 2020 at 12:03:14PM +0100, Kashyap Chamarthy wrote:
> On Thu, Jan 02, 2020 at 02:47:22PM +0000, Stefan Hajnoczi wrote:
> > On Sat, Dec 21, 2019 at 10:02:23AM +0100, Markus Armbruster wrote:
> > > Stefan Hajnoczi <address@hidden> writes:
> 
> [...]
> 
> > > > 2. scripts/qmp/ contains command-line tools for QMP communication.
> > > > They could use some polish and then be shipped.
> > > 
> > > MAINTAINERS blames them on me, but they're effectively unmaintained.
> > > Prerequisite for shipping: having a maintainer who actually gives a
> > > damn.
> > ...
> > > * scripts/qmp/qmp-shell
> > > 
> > >   Half-hearted attempt at a human-friendly wrapper around the JSON
> > >   syntax.  I have no use for this myself.
> > 
> > I think this one is used by people.  John Snow comes to mind.
> 
> FWIW I too frequently use 'qmp-shell'.  And some of the examples in this
> document[1] are demonstrated with it.
> 
> I'm reasonably happy with it (particularly the persistent history
> captured in ~/.qmp-shell_history), and it has some "known issues" that
> can trip up a new user.  The one that immediately jumps to mind:
> asynchronous events won't be printed without a prompt from the user --
> e.g. after a `blockdev-commit`, you won't see BLOCK_JOB_{READY,
> COMPLETED} events printed unless you manually hit enter from the
> 'qmp-shell'.
> 
> (Not complaining; I have a long-standing TODO to make time to
> investigate this.)
> 
> [1] https://qemu.readthedocs.io/en/latest/interop/live-block-operations.html

John and I discussed async events in the past.  qmp-shell currently uses
the input() built-in function.  If we modify it with a
select(2)/poll(2)-style function that monitors both stdin and the QMP
socket then it could print QMP events as soon as they are received.

There might be a nicer way of doing it, but pseudo-code for the idea is:

  def input_with_events(prompt):
      while True:
          print(prompt, end='', flush=True)
          readable_files = select([sys.stdin, qmp_socket])
          if qmp_socket in readable_files:
              print_qmp_events()

      # stdin is ready, read a line
      return input()

Stefan

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