python/qmp: clear events on get_events() call

All callers in the tree *already* clear the events after a call to
get_events(). Do it automatically instead and update callsites to remove
the manual clear call.

These semantics are quite a bit easier to emulate with async QMP, and
nobody appears to be abusing some emergent properties of what happens if
you decide not to clear them, so let's dial down to the dumber, simpler
thing.

Specifically: callers of clear() right after a call to get_events() are
more likely expressing their desire to not see any events they just
retrieved, whereas callers of clear_events() not in relation to a recent
call to pull_event/get_events are likely expressing their desire to
simply drop *all* pending events straight onto the floor. In the sync
world, this is safe enough; in the async world it's nearly impossible to
promise that nothing happens between getting and clearing the
events.

Making the retrieval also clear the queue is vastly simpler.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20210923004938.3999963-9-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/machine/machine.py

@@ -631,7 +631,6 @@ def get_qmp_events(self, wait: bool = False) -> List[QMPMessage]:
         events = self._qmp.get_events(wait=wait)
         events.extend(self._events)
         del self._events[:]
-        self._qmp.clear_events()
         return events
 
     @staticmethod

python/qemu/qmp/__init__.py

@@ -361,7 +361,7 @@ def pull_event(self,
 
     def get_events(self, wait: bool = False) -> List[QMPMessage]:
         """
-        Get a list of available QMP events.
+        Get a list of available QMP events and clear all pending events.
 
         @param wait (bool): block until an event is available.
         @param wait (float): If wait is a float, treat it as a timeout value.
@@ -374,7 +374,9 @@ def get_events(self, wait: bool = False) -> List[QMPMessage]:
         @return The list of available QMP events.
         """
         self.__get_events(wait)
-        return self.__events
+        events = self.__events
+        self.__events = []
+        return events
 
     def clear_events(self) -> None:
         """

python/qemu/qmp/qmp_shell.py

@@ -381,7 +381,6 @@ def read_exec_command(self) -> bool:
         if cmdline == '':
             for event in self.get_events():
                 print(event)
-            self.clear_events()
             return True
 
         return self._execute_cmd(cmdline)