socket: fixing possible mixed up response with events

[rouming]

Patch drains possible events on qmp socket until true "qmp_capabilities" response is received.

This targets a nasty and rare problem in "Connect(), Run()" sequence, when asynchronous QEMU "event" just after the "qmp_capabilities" request is accepted as response and further any QMP request gets completed with response from the preceding "qmp_capabilities" response.

The following QMP protocol sequence is possible:

1 read {"QMP": {"version": {"qemu": {"micro": 0, "minor": 1, "major": 5}, ...}"
2 write {"execute":"qmp_capabilities"}
3 read {"timestamp": {"seconds": xxx, "microseconds": xxx}, "event": "any"}
4 read {"return": {}}

the 3. is unexpected by the current Connect() implementation and "event" is considered as a proper response on "qmp_capabilities", in other turn 4. is read in the go.mos.listen() and immediately pushed to the stream channel, so any further QMP command (Run() call) will be immediately completed by an empty response from line 4.

The described problem of unexpected empty response line was observed on this code qmp.SocketMonitor sequence:

Connect()
Run('{"execute":"query-status"}') <<< Returns empty response
Disconnect()

The problem is very rare and was observed ~5 times on different machines over a fairly long period of time (several months), which corresponds to nature of the described rare protocol race.

The current patch was tested on modified QEMU, where an aritifical sleep() was introduced in the qmp_marshal_qmp_capabilities() call just right after the qmp_qmp_capabilities() was invoked, so all further events can be accepted by the QMP socket:

 --- qapi/qapi-commands-control.c        2023-11-08 08:55:16.209007741 +0100
 +++ qapi/qapi-commands-control.c.orig   2023-11-08 08:55:13.929005997 +0100
 @@ -42,10 +42,6 @@

      qmp_qmp_capabilities(arg.has_enable, arg.enable, &err);

 +    printf(">>> BEFORE SLEEP 10\n");
 +    sleep(10);
 +    printf(">>> AFTER SLEEP 10\n");
 +
      error_propagate(errp, err);

  out:

Any QMP event can be freely received by the QMP client, while the execution flow of the qmp_marshal_qmp_capabilities() was interrupted or scheduled out.

The fix of the described is simple: read from the QMP socket until response is received and drop all possible events.