Helmut Grohne wrote:
Please continue to CC me in reply.
On 28.01.2013, at 07:02, Ben Finney wrote:
The ‘DaemonContext’ class is designed to allow overriding behaviour through
subclassing and the ‘super()’ mechanism, so I'm not sure what you would
need monkey-patching for. Can you give a concrete example which can't be
addressed by a custom ‘DaemonContext’ subclass?
I gave this example in my previous mail. It was about some initialization to
be done after daemonizing and only then terminating the parent. This is
currently hard or impossible to achieve with DaemonContext. An attempt to do so
would subclass DaemonContext and provide an own open method. But it cannot call
DaemonContext.open, because that would terminate the parent process. So my
subclass has to copy most of DaemonContext.open or monkey-patch the original
one. If you disagree maybe you can fill in the missing bits in the pseudo code
below?
class CustomDaemonContext(DaemonContext):
def finish(self, error_message=None):
if error_message:
pass # Let the parent print the error message and exit non-zero.
else:
pass # Let the parent exit with a zero status.
context = CustomDaemonContext()
initial_setup()
with context:
try:
fork_worker_child()
except WorkerSpawnFailure:
context.finish("failed to spawn worker")
else:
context.finish()
do_main_program()
Please observe that the worker cannot be spawned within initial_setup, because
the
fork performed in DaemonContext.open would detach that child from the process
and
receiving SIGCHLD would no longer work.
I see two ways around this, both enhancements to daemon.py:
- have __init__ accept a parent_callback parameter which will be called
after the child is forked but before exiting
- have a stub method (such as .finished()) that is always called after
the child is forked but before the parent exits, which can then be
overridden in subclasses
I would prefer the first option as that would make subclassing unnecessary.
--
~Ethan~
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