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

[Dov Murik]

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.

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

-Dov


> --js
>
> > ---
> >
> > Notes:
> >      Note that this might be considered a breaking change; if users have
> >      automated scripts which assume that qmp-shell prints 3 lines of 
> > banner,
> >      this change will break their scripts.  If there are special
> >      considerations/procedures for breaking changes, please let me know.
> >      The rationale behaind the TTY check is to imitate python's 
> > behaviour:
> >          $ python3
> >          Python 3.7.5 (default, Apr 19 2020, 20:18:17)
> >          [GCC 9.2.1 20191008] on linux
> >          Type "help", "copyright", "credits" or "license" for more 
> > information.
> >          >>> print(19+23)
> >          42
> >          >>>
> >          $ echo 'print(19+23)' | python3
> >          42
> >
> >   scripts/qmp/qmp-shell | 19 +++++++++++++++++--
> >   1 file changed, 17 insertions(+), 2 deletions(-)
> >
> > diff --git a/scripts/qmp/qmp-shell b/scripts/qmp/qmp-shell
> > index b4d06096ab..9336066fa8 100755
> > --- a/scripts/qmp/qmp-shell
> > +++ b/scripts/qmp/qmp-shell
> > @@ -288,6 +288,8 @@ class QMPShell(qmp.QEMUMonitorProtocol):
> >           self.__completer_setup()
> >       def show_banner(self, msg='Welcome to the QMP low-level shell!'):
> > +        if not self._interactive:
> > +            return
> >           print(msg)
> >           if not self._greeting:
> >               print('Connected')
> > @@ -300,6 +302,15 @@ class QMPShell(qmp.QEMUMonitorProtocol):
> >               return "TRANS> "
> >           return "(QEMU) "
> > +    def read_command(self, prompt):
> > +        if self._interactive:
> > +            return input(prompt)
> > +        else:
> > +            line = sys.stdin.readline()
> > +            if not line:
> > +                raise EOFError
> > +            return line
> > +
> >       def read_exec_command(self, prompt):
> >           """
> >           Read and execute a command.
> > @@ -307,7 +318,7 @@ class QMPShell(qmp.QEMUMonitorProtocol):
> >           @return True if execution was ok, return False if disconnected.
> >           """
> >           try:
> > -            cmdline = input(prompt)
> > +            cmdline = self.read_command(prompt)
> >           except EOFError:
> >               print()
> >               return False
> > @@ -322,6 +333,9 @@ class QMPShell(qmp.QEMUMonitorProtocol):
> >       def set_verbosity(self, verbose):
> >           self._verbose = verbose
> > +    def set_interactive(self, interactive):
> > +        self._interactive = interactive
> > +
> >   class HMPShell(QMPShell):
> >       def __init__(self, address):
> >           QMPShell.__init__(self, address)
> > @@ -449,8 +463,9 @@ def main():
> >       except qemu.error:
> >           die('Could not connect to %s' % addr)
> > -    qemu.show_banner()
> >       qemu.set_verbosity(verbose)
> > +    qemu.set_interactive(sys.stdin.isatty())
> > +    qemu.show_banner()
> >       while qemu.read_exec_command(qemu.get_prompt()):
> >           pass
> >       qemu.close()