QMP: add server mode to QEMUMonitorProtocol

QEMU supports socket chardevs that establish connections like a server
or a client.  The QEMUMonitorProtocol class only supports connecting as
a client.  It is not possible to connect race-free when launching QEMU
since trying to connect before QEMU has bound and is listening on the
socket results in failure.

Add the QEMUMonitorProtocol(server=True) argument to bind and listen on
the socket.  The QEMU process can then be launched and connects to the
already existing QMP socket without a race condition:

  qmp = qmp.QEMUMonitorProtocol(monitor_path, server=True)
  popen = subprocess.Popen(args)
  qmp.accept()

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

QMP/qmp.py

@@ -22,19 +22,24 @@ class QMPCapabilitiesError(QMPError):
     pass
 
 class QEMUMonitorProtocol:
-    def __init__(self, address):
+    def __init__(self, address, server=False):
         """
         Create a QEMUMonitorProtocol class.
 
         @param address: QEMU address, can be either a unix socket path (string)
                         or a tuple in the form ( address, port ) for a TCP
                         connection
-        @note No connection is established, this is done by the connect() method
+        @param server: server mode listens on the socket (bool)
+        @raise socket.error on socket connection errors
+        @note No connection is established, this is done by the connect() or
+              accept() methods
         """
         self.__events = []
         self.__address = address
         self.__sock = self.__get_sock()
-        self.__sockfile = self.__sock.makefile()
+        if server:
+            self.__sock.bind(self.__address)
+            self.__sock.listen(1)
 
     def __get_sock(self):
         if isinstance(self.__address, tuple):
@@ -43,6 +48,17 @@ def __get_sock(self):
             family = socket.AF_UNIX
         return socket.socket(family, socket.SOCK_STREAM)
 
+    def __negotiate_capabilities(self):
+        self.__sockfile = self.__sock.makefile()
+        greeting = self.__json_read()
+        if greeting is None or not greeting.has_key('QMP'):
+            raise QMPConnectError
+        # Greeting seems ok, negotiate capabilities
+        resp = self.cmd('qmp_capabilities')
+        if "return" in resp:
+            return greeting
+        raise QMPCapabilitiesError
+
     def __json_read(self, only_event=False):
         while True:
             data = self.__sockfile.readline()
@@ -67,14 +83,19 @@ def connect(self):
         @raise QMPCapabilitiesError if fails to negotiate capabilities
         """
         self.__sock.connect(self.__address)
-        greeting = self.__json_read()
-        if greeting is None or not greeting.has_key('QMP'):
-            raise QMPConnectError
-        # Greeting seems ok, negotiate capabilities
-        resp = self.cmd('qmp_capabilities')
-        if "return" in resp:
-            return greeting
-        raise QMPCapabilitiesError
+        return self.__negotiate_capabilities()
+
+    def accept(self):
+        """
+        Await connection from QMP Monitor and perform capabilities negotiation.
+
+        @return QMP greeting dict
+        @raise socket.error on socket connection errors
+        @raise QMPConnectError if the greeting is not received
+        @raise QMPCapabilitiesError if fails to negotiate capabilities
+        """
+        self.__sock, _ = self.__sock.accept()
+        return self.__negotiate_capabilities()
 
     def cmd_obj(self, qmp_cmd):
         """