[PATCH v2 4/6] python/machine: use socketpair() for console connections

[John Snow <jsnow@redhat.com>]

Create a socketpair for the console output. This should help eliminate
race conditions around console text early in the boot process that might
otherwise have been dropped on the floor before being able to connect to
QEMU under "server,nowait".

Signed-off-by: John Snow <jsnow@redhat.com>
---
 python/qemu/machine/machine.py | 30 +++++++++++++++++++++++++++---
 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/python/qemu/machine/machine.py b/python/qemu/machine/machine.py
index 26f0fb8a81..09f214c95c 100644
--- a/python/qemu/machine/machine.py
+++ b/python/qemu/machine/machine.py
@@ -159,6 +159,8 @@ def __init__(self,
 
         self._name = name or f"{id(self):x}"
         self._sock_pair: Optional[Tuple[socket.socket, socket.socket]] = None
+        self._cons_sock_pair: Optional[
+            Tuple[socket.socket, socket.socket]] = None
         self._temp_dir: Optional[str] = None
         self._base_temp_dir = base_temp_dir
         self._sock_dir = sock_dir
@@ -315,8 +317,9 @@ def _base_args(self) -> List[str]:
         for _ in range(self._console_index):
             args.extend(['-serial', 'null'])
         if self._console_set:
-            chardev = ('socket,id=console,path=%s,server=on,wait=off' %
-                       self._console_address)
+            assert self._cons_sock_pair is not None
+            fd = self._cons_sock_pair[0].fileno()
+            chardev = f"socket,id=console,fd={fd}"
             args.extend(['-chardev', chardev])
             if self._console_device_type is None:
                 args.extend(['-serial', 'chardev:console'])
@@ -351,6 +354,10 @@ def _pre_launch(self) -> None:
                 nickname=self._name
             )
 
+        if self._console_set:
+            self._cons_sock_pair = socket.socketpair()
+            os.set_inheritable(self._cons_sock_pair[0].fileno(), True)
+
         # NOTE: Make sure any opened resources are *definitely* freed in
         # _post_shutdown()!
         # pylint: disable=consider-using-with
@@ -368,6 +375,9 @@ def _pre_launch(self) -> None:
     def _post_launch(self) -> None:
         if self._sock_pair:
             self._sock_pair[0].close()
+        if self._cons_sock_pair:
+            self._cons_sock_pair[0].close()
+
         if self._qmp_connection:
             if self._sock_pair:
                 self._qmp.connect()
@@ -518,6 +528,11 @@ def _early_cleanup(self) -> None:
             self._console_socket.close()
             self._console_socket = None
 
+        if self._cons_sock_pair:
+            self._cons_sock_pair[0].close()
+            self._cons_sock_pair[1].close()
+            self._cons_sock_pair = None
+
     def _hard_shutdown(self) -> None:
         """
         Perform early cleanup, kill the VM, and wait for it to terminate.
@@ -878,10 +893,19 @@ def console_socket(self) -> socket.socket:
         Returns a socket connected to the console
         """
         if self._console_socket is None:
+            if not self._console_set:
+                raise QEMUMachineError(
+                    "Attempt to access console socket with no connection")
+            assert self._cons_sock_pair is not None
+            # os.dup() is used here for sock_fd because otherwise we'd
+            # have two rich python socket objects that would each try to
+            # close the same underlying fd when either one gets garbage
+            # collected.
             self._console_socket = console_socket.ConsoleSocket(
-                self._console_address,
+                sock_fd=os.dup(self._cons_sock_pair[1].fileno()),
                 file=self._console_log_path,
                 drain=self._drain_console)
+            self._cons_sock_pair[1].close()
         return self._console_socket
 
     @property
-- 
2.41.0