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

[Dov Murik <dovmurik@linux.vnet.ibm.com>]

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>
---

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()
-- 
2.20.1