[2/2] 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>
---
 QMP/qmp.py |   43 ++++++++++++++++++++++++++++++++-----------
 1 files changed, 32 insertions(+), 11 deletions(-)

On Wed, 25 May 2011 19:48:01 +0100
Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> wrote:

> 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()

Would be cool to get the demo shell doing this (not a required for the
initial merge, of course).

> 
> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
> ---
>  QMP/qmp.py |   43 ++++++++++++++++++++++++++++++++-----------
>  1 files changed, 32 insertions(+), 11 deletions(-)
> 
> diff --git a/QMP/qmp.py b/QMP/qmp.py
> index 2565508..c7dbea0 100644
> --- a/QMP/qmp.py
> +++ b/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 @@ class QEMUMonitorProtocol:
>              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 @@ class QEMUMonitorProtocol:
>          @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):
>          """