tests/avocado: Fix console data loss

Occasionally some avocado tests will fail waiting for console line
despite the machine running correctly. Console data goes missing, as can
be seen in the console log. This is due to _console_interaction calling
makefile() on the console socket each time it is invoked, which must be
losing old buffer contents when going out of scope.

It is not enough to makefile() with buffered=0. That helps significantly
but data loss is still possible. My guess is that readline() has a line
buffer even when the file is in unbuffered mode, that can eat data.

Fix this by providing a console file that persists for the life of the
console.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: "Daniel P. Berrangé" <berrange@redhat.com>
Message-Id: <20230912131340.405619-1-npiggin@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: John Snow <jsnow@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230914155422.426639-9-alex.bennee@linaro.org>

python/qemu/machine/machine.py

@@ -191,6 +191,7 @@ class QEMUMachine:
             self.sock_dir, f"{self._name}.con"
         )
         self._console_socket: Optional[socket.socket] = None
+        self._console_file: Optional[socket.SocketIO] = None
         self._remove_files: List[str] = []
         self._user_killed = False
         self._quit_issued = False
@@ -509,6 +510,11 @@ class QEMUMachine:
         # If we keep the console socket open, we may deadlock waiting
         # for QEMU to exit, while QEMU is waiting for the socket to
         # become writable.
+        if self._console_file is not None:
+            LOG.debug("Closing console file")
+            self._console_file.close()
+            self._console_file = None
+
         if self._console_socket is not None:
             LOG.debug("Closing console socket")
             self._console_socket.close()
@@ -874,12 +880,25 @@ class QEMUMachine:
         Returns a socket connected to the console
         """
         if self._console_socket is None:
+            LOG.debug("Opening console socket")
             self._console_socket = console_socket.ConsoleSocket(
                 self._console_address,
                 file=self._console_log_path,
                 drain=self._drain_console)
         return self._console_socket
 
+    @property
+    def console_file(self) -> socket.SocketIO:
+        """
+        Returns a file associated with the console socket
+        """
+        if self._console_file is None:
+            LOG.debug("Opening console file")
+            self._console_file = self.console_socket.makefile(mode='rb',
+                                                              buffering=0,
+                                                              encoding='utf-8')
+        return self._console_file
+
     @property
     def temp_dir(self) -> str:
         """

tests/avocado/avocado_qemu/__init__.py

@@ -137,7 +137,7 @@ def _console_interaction(test, success_message, failure_message,
     assert not keep_sending or send_string
     if vm is None:
         vm = test.vm
-    console = vm.console_socket.makefile(mode='rb', encoding='utf-8')
+    console = vm.console_file
     console_logger = logging.getLogger('console')
     while True:
         if send_string: