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

From: John Snow
Subject: Re: [PATCH] qmp-shell: Suppress banner and prompt when stdin is not a TTY
Date: Wed, 20 Jan 2021 10:46:27 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0

On 1/20/21 4:45 AM, Daniel P. Berrangé wrote:
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}}

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

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

Right. qmp.py is technically not stable either, but it at least doesn't use an invented syntax we don't have a spec for ... and it is used by quite a few other things in the tree, so I trust it /slightly/ more. I cannot promise compatibility for scripts that aren't in the tree at this time, though.

(I am working on an asyncio variant of the QMP library that I do hope to promise stability for, but that's probably not something you can hope to see in the short term. It will likely have an API that is at least somewhat similar to the existing library, but will use asyncio coroutines instead of blocking calls.)

You can look at ./tests/qemu-iotests/ for some examples of using the QMP library that we have today; grep for '.qmp(' to find examples.

The connection for these tests is established in python/qemu/machine.py, look at the 'self._qmp_connection' field. This connection is exposed via the qmp(...) method, which the tests use. The library handles the (very small) handshake.

There are also bash tests in ./tests/qemu-iotests/ that handle some QMP by themselves, and might be up your alley for very simple cases. Test 060 sets up its own QMP connection and just echoes JSON into the pipe.


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