Re: [PATCH] qmp-shell: Suppress banner and prompt when stdin is not a TTY

From: "Daniel P. Berrangé" <berrange@redhat.com>
To: Dov Murik <dovmurik@linux.vnet.ibm.com>
Cc: John Snow <jsnow@redhat.com>,
	qemu-devel@nongnu.org, Markus Armbruster <armbru@redhat.com>
Subject: Re: [PATCH] qmp-shell: Suppress banner and prompt when stdin is not a TTY
Date: Wed, 20 Jan 2021 09:45:39 +0000	[thread overview]
Message-ID: <20210120094539.GC3015589@redhat.com> (raw)
In-Reply-To: <1d63a097-27d5-5b3a-2ce6-f6a2402e135e@linux.vnet.ibm.com>

On Wed, Jan 20, 2021 at 10:25:25AM +0200, Dov Murik wrote:
> Hi John,
> 
> On 19/01/2021 22:02, John Snow wrote:
> > On 1/17/21 2:27 AM, Dov Murik wrote:
> > > Detect whether qmp-shell's standard input is not a TTY; in such case,
> > > assume a non-interactive mode, which suppresses the welcome banner and
> > > the "(QEMU)" prompt.  This allows for easier consumption of qmp-shell's
> > > output in scripts.
> > > 
> > > Example usage before this change:
> > > 
> > >      $ printf "query-status\nquery-kvm\n" | sudo
> > > scripts/qmp/qmp-shell qmp-unix-sock
> > >      Welcome to the QMP low-level shell!
> > >      Connected to QEMU 5.1.50
> > > 
> > >      (QEMU) {"return": {"status": "running", "singlestep": false,
> > > "running": true}}
> > >      (QEMU) {"return": {"enabled": true, "present": true}}
> > >      (QEMU)
> > > 
> > > Example usage after this change:
> > > 
> > >      $ printf "query-status\nquery-kvm\n" | sudo
> > > scripts/qmp/qmp-shell qmp-unix-sock
> > >      {"return": {"status": "running", "singlestep": false,
> > > "running": true}}
> > >      {"return": {"enabled": true, "present": true}}
> > > 
> > > Signed-off-by: Dov Murik <dovmurik@linux.vnet.ibm.com>
> > 
> > Hiya! I've been taking lead on modernizing a lot of our python
> > infrastructure, including our QMP library and qmp-shell.
> > 
> > (Sorry, not in MAINTAINERS yet; but I am in the process of moving these
> > scripts and tools over to ./python/qemu/qmp.)
> 
> Thanks for this effort.
> 
> > 
> > This change makes me nervous, because qmp-shell is not traditionally a
> > component we've thought of as needing to preserve backwards-compatible
> > behavior. Using it as a script meant to be consumed in a headless
> > fashion runs a bit counter to that assumption.
> > 
> > I'd be less nervous if the syntax of qmp-shell was something that was
> > well thought-out and rigorously tested, but it's a hodge-podge of
> > whatever people needed at the moment. I am *very* reluctant to cement
> > it.
> 
> Yes, I understand your choice.
> 
> 
> > 
> > Are you trying to leverage the qmp.py library from bash?
> 
> Yes, I want to send a few QMP commands and record their output.  If I use
> socat to the unix-socket I need to serialize the JSON request myself, so
> using qmp-shell saves me that; also not sure if there's any negotiation done
> at the beginning by qmp-shell.

There is a handshake, but it is just a single json message.

See docs/interop/qmp-intro.txt and qmp-spec.txt for guidance.

> Is there an easier way to script qmp commands, short of writing my own
> python program which uses the qmp.py library?

Yes, writing your own python program is probably best. Doing anything
complex is shell is almost always a mistake, as it is a very crude
and poor language compared to something like managing QEMU/QMP.

Note that I don't believe that we've declared qmp.py to be a long
term stable interface for users outside of QEMU either. An alternative
is to just use the python sockets APIs directly to speak to QEMU/QMP

Regards,
Daniel
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