python/machine: use socketpair() for qtest connection

Like the QMP and console sockets, begin using socketpairs for the qtest
connection, too. After this patch, we'll be able to remove the vestigial
sock_dir argument, but that cleanup is best done in its own patch.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20230928044943.849073-6-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/machine/qtest.py

@@ -24,6 +24,7 @@
     Optional,
     Sequence,
     TextIO,
+    Tuple,
 )
 
 from qemu.qmp import SocketAddrT
@@ -38,23 +39,41 @@ class QEMUQtestProtocol:
     :param address: QEMU address, can be either a unix socket path (string)
                     or a tuple in the form ( address, port ) for a TCP
                     connection
-    :param server: server mode, listens on the socket (bool)
+    :param sock: An existing socket can be provided as an alternative to
+                 an address. One of address or sock must be provided.
+    :param server: server mode, listens on the socket. Only meaningful
+                   in conjunction with an address and not an existing
+                   socket.
+
     :raise socket.error: on socket connection errors
 
     .. note::
        No connection is established by __init__(), this is done
        by the connect() or accept() methods.
     """
-    def __init__(self, address: SocketAddrT,
+    def __init__(self,
+                 address: Optional[SocketAddrT] = None,
+                 sock: Optional[socket.socket] = None,
                  server: bool = False):
+        if address is None and sock is None:
+            raise ValueError("Either 'address' or 'sock' must be specified")
+        if address is not None and sock is not None:
+            raise ValueError(
+                "Either 'address' or 'sock' must be specified, but not both")
+        if sock is not None and server:
+            raise ValueError("server=True is meaningless when passing socket")
+
         self._address = address
-        self._sock = self._get_sock()
+        self._sock = sock or self._get_sock()
         self._sockfile: Optional[TextIO] = None
+
         if server:
+            assert self._address is not None
             self._sock.bind(self._address)
             self._sock.listen(1)
 
     def _get_sock(self) -> socket.socket:
+        assert self._address is not None
         if isinstance(self._address, tuple):
             family = socket.AF_INET
         else:
@@ -67,7 +86,8 @@ def connect(self) -> None:
 
         @raise socket.error on socket connection errors
         """
-        self._sock.connect(self._address)
+        if self._address is not None:
+            self._sock.connect(self._address)
         self._sockfile = self._sock.makefile(mode='r')
 
     def accept(self) -> None:
@@ -127,29 +147,40 @@ def __init__(self,
                          base_temp_dir=base_temp_dir,
                          sock_dir=sock_dir, qmp_timer=qmp_timer)
         self._qtest: Optional[QEMUQtestProtocol] = None
-        self._qtest_path = os.path.join(sock_dir, name + "-qtest.sock")
+        self._qtest_sock_pair: Optional[
+            Tuple[socket.socket, socket.socket]] = None
 
     @property
     def _base_args(self) -> List[str]:
         args = super()._base_args
+        assert self._qtest_sock_pair is not None
+        fd = self._qtest_sock_pair[0].fileno()
         args.extend([
-            '-qtest', f"unix:path={self._qtest_path}",
+            '-chardev', f"socket,id=qtest,fd={fd}",
+            '-qtest', 'chardev:qtest',
             '-accel', 'qtest'
         ])
         return args
 
     def _pre_launch(self) -> None:
+        self._qtest_sock_pair = socket.socketpair()
+        os.set_inheritable(self._qtest_sock_pair[0].fileno(), True)
         super()._pre_launch()
-        self._qtest = QEMUQtestProtocol(self._qtest_path, server=True)
+        self._qtest = QEMUQtestProtocol(sock=self._qtest_sock_pair[1])
 
     def _post_launch(self) -> None:
         assert self._qtest is not None
         super()._post_launch()
-        self._qtest.accept()
+        if self._qtest_sock_pair:
+            self._qtest_sock_pair[0].close()
+        self._qtest.connect()
 
     def _post_shutdown(self) -> None:
+        if self._qtest_sock_pair:
+            self._qtest_sock_pair[0].close()
+            self._qtest_sock_pair[1].close()
+            self._qtest_sock_pair = None
         super()._post_shutdown()
-        self._remove_if_exists(self._qtest_path)
 
     def qtest(self, cmd: str) -> str:
         """