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

From: Dov Murik
Subject: Re: [PATCH] qmp-shell: Suppress banner and prompt when stdin is not a TTY
Date: Wed, 20 Jan 2021 10:25:25 +0200
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.1

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 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?




     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)
         $ echo 'print(19+23)' | python3

  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):
      def show_banner(self, msg='Welcome to the QMP low-level shell!'):
+        if not self._interactive:
+            return
          if not self._greeting:
@@ -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.
-            cmdline = input(prompt)
+            cmdline = self.read_command(prompt)
          except EOFError:
              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_interactive(sys.stdin.isatty())
+    qemu.show_banner()
      while qemu.read_exec_command(qemu.get_prompt()):

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