python/qemu: Add set_qmp_monitor() to QEMUMachine

The QEMUMachine VM has a monitor setup on which an QMP
connection is always attempted on _post_launch() (executed
by launch()). In case the QEMU process immediatly exits
then the qmp.accept() (used to establish the connection) stalls
until it reaches timeout and consequently an exception raises.

That behavior is undesirable when, for instance, it needs to
gather information from the QEMU binary ($ qemu -cpu list) or a
test which launches the VM expecting its failure.

This patch adds the set_qmp_monitor() method to QEMUMachine that
allows turn off the creation of the monitor machinery on VM launch.

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20191211185536.16962-2-wainersm@redhat.com>
[Cleber: trivial indentation fix]
Signed-off-by: Cleber Rosa <crosa@redhat.com>

python/qemu/machine.py

@@ -104,6 +104,7 @@ class QEMUMachine(object):
         self._events = []
         self._iolog = None
         self._socket_scm_helper = socket_scm_helper
+        self._qmp_set = True   # Enable QMP monitor by default.
         self._qmp = None
         self._qemu_full_args = None
         self._test_dir = test_dir
@@ -228,15 +229,16 @@ class QEMUMachine(object):
                 self._iolog = iolog.read()
 
     def _base_args(self):
-        if isinstance(self._monitor_address, tuple):
-            moncdev = "socket,id=mon,host=%s,port=%s" % (
-                self._monitor_address[0],
-                self._monitor_address[1])
-        else:
-            moncdev = 'socket,id=mon,path=%s' % self._vm_monitor
-        args = ['-chardev', moncdev,
-                '-mon', 'chardev=mon,mode=control',
-                '-display', 'none', '-vga', 'none']
+        args = ['-display', 'none', '-vga', 'none']
+        if self._qmp_set:
+            if isinstance(self._monitor_address, tuple):
+                moncdev = "socket,id=mon,host=%s,port=%s" % (
+                    self._monitor_address[0],
+                    self._monitor_address[1])
+            else:
+                moncdev = 'socket,id=mon,path=%s' % self._vm_monitor
+            args.extend(['-chardev', moncdev, '-mon',
+                         'chardev=mon,mode=control'])
         if self._machine is not None:
             args.extend(['-machine', self._machine])
         if self._console_set:
@@ -255,20 +257,21 @@ class QEMUMachine(object):
 
     def _pre_launch(self):
         self._temp_dir = tempfile.mkdtemp(dir=self._test_dir)
-        if self._monitor_address is not None:
-            self._vm_monitor = self._monitor_address
-        else:
-            self._vm_monitor = os.path.join(self._sock_dir,
-                                            self._name + "-monitor.sock")
-            self._remove_files.append(self._vm_monitor)
         self._qemu_log_path = os.path.join(self._temp_dir, self._name + ".log")
         self._qemu_log_file = open(self._qemu_log_path, 'wb')
 
-        self._qmp = qmp.QEMUMonitorProtocol(self._vm_monitor,
-                                            server=True)
+        if self._qmp_set:
+            if self._monitor_address is not None:
+                self._vm_monitor = self._monitor_address
+            else:
+                self._vm_monitor = os.path.join(self._sock_dir,
+                                                self._name + "-monitor.sock")
+                self._remove_files.append(self._vm_monitor)
+            self._qmp = qmp.QEMUMonitorProtocol(self._vm_monitor, server=True)
 
     def _post_launch(self):
-        self._qmp.accept()
+        if self._qmp:
+            self._qmp.accept()
 
     def _post_shutdown(self):
         if self._qemu_log_file is not None:
@@ -330,7 +333,8 @@ class QEMUMachine(object):
         Wait for the VM to power off
         """
         self._popen.wait()
-        self._qmp.close()
+        if self._qmp:
+            self._qmp.close()
         self._load_io_log()
         self._post_shutdown()
 
@@ -346,12 +350,13 @@ class QEMUMachine(object):
             self._console_socket = None
 
         if self.is_running():
-            try:
-                if not has_quit:
-                    self._qmp.cmd('quit')
-                self._qmp.close()
-            except:
-                self._popen.kill()
+            if self._qmp:
+                try:
+                    if not has_quit:
+                        self._qmp.cmd('quit')
+                    self._qmp.close()
+                except:
+                    self._popen.kill()
             self._popen.wait()
 
         self._load_io_log()
@@ -368,6 +373,21 @@ class QEMUMachine(object):
 
         self._launched = False
 
+    def set_qmp_monitor(self, enabled=True):
+        """
+        Set the QMP monitor.
+
+        @param enabled: if False, qmp monitor options will be removed from
+                        the base arguments of the resulting QEMU command
+                        line. Default is True.
+        @note: call this function before launch().
+        """
+        if enabled:
+            self._qmp_set = True
+        else:
+            self._qmp_set = False
+            self._qmp = None
+
     def qmp(self, cmd, conv_keys=True, **args):
         """
         Invoke a QMP command and return the response dict