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python/machine: handle "fast" QEMU terminations

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In the case that the QEMU process actually launches -- but then dies so
quickly that we can't establish a QMP connection to it -- QEMUMachine
currently calls _post_shutdown() assuming that it never launched the VM
process.

This isn't true, though: it "merely" may have failed to establish a QMP
connection and the process is in the middle of its own exit path.

If we don't wait for the subprocess, the caller may get a bogus `None`
return for .exitcode(). This behavior was observed from
device-crash-test; after the switch to Async QMP, the timings were
changed such that it was now seemingly possible to witness the failure
of "vm.launch()" *prior* to the exitcode becoming available.

The semantic of the `_launched` property is changed in this
patch. Instead of representing the condition "launch() executed
successfully", it will now represent "has forked a child process
successfully". This way, wait() when called in the exit path won't
become a no-op.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-id: 20211118204620.1897674-6-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>
This commit is contained in:
John Snow 2021-11-18 15:46:18 -05:00
parent b1ca991993
commit 1611e6cf4e
1 changed files with 12 additions and 7 deletions
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19
python/qemu/machine/machine.py
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@ -349,9 +349,6 @@ class QEMUMachine:
Called to cleanup the VM instance after the process has exited.
May also be called after a failed launch.
"""
# Comprehensive reset for the failed launch case:
self._early_cleanup()
try:
self._close_qmp_connection()
except Exception as err: # pylint: disable=broad-except
@ -400,9 +397,16 @@ class QEMUMachine:
try:
self._launch()
self._launched = True
except:
self._post_shutdown()
# We may have launched the process but it may
# have exited before we could connect via QMP.
# Assume the VM didn't launch or is exiting.
# If we don't wait for the process, exitcode() may still be
# 'None' by the time control is ceded back to the caller.
if self._launched:
self.wait()
else:
self._post_shutdown()
LOG.debug('Error launching VM')
if self._qemu_full_args:
@ -426,6 +430,7 @@ class QEMUMachine:
stderr=subprocess.STDOUT,
shell=False,
close_fds=False)
self._launched = True
self._post_launch()
def _close_qmp_connection(self) -> None:
@ -457,8 +462,8 @@ class QEMUMachine:
"""
Perform any cleanup that needs to happen before the VM exits.
May be invoked by both soft and hard shutdown in failover scenarios.
Called additionally by _post_shutdown for comprehensive cleanup.
This method may be called twice upon shutdown, once each by soft
and hard shutdown in failover scenarios.
"""
# If we keep the console socket open, we may deadlock waiting
# for QEMU to exit, while QEMU is waiting for the socket to