[systemd-devel] Synchronization Between Services at Shutdown

[Andrei Borzenkov]

В Wed, 1 Apr 2015 18:22:50 -0700
Kurt von Laven <kurt at endlessm.com> пишет:

> Hello folks,
> 
> I am trying to ensure that my system DBus service has time to handle a DBus
> method call before it shuts down. My DBus service is implemented as a
> system systemd service; let's call it service A. It runs a single-threaded,
> synchronous event loop that listens for DBus method calls and handles them
> appropriately. This event loop also listens for SIGTERM. When SIGTERM is
> received, service A currently terminates the event loop, performs any
> necessary writes to disk, and releases its resources. I am assuming that
> when the system is shut down cleanly, systemd initially sends SIGTERM to
> systemd services and only sends SIGKILL to abort it abruptly if it takes
> too long to shut down when asked nicely.
> 
> I have another system systemd service, call it service B, that runs an
> event loop listening for SIGTERM. When service B receives a SIGTERM, it
> makes the aforementioned DBus method call and then flushes the system bus.
> Service B specifies Requires=
> <http://www.freedesktop.org/software/systemd/man/systemd.unit.html#Requires=>Service
> A and After=
> <http://www.freedesktop.org/software/systemd/man/systemd.unit.html#Before=>Service
> A. Service A makes no mention of service B in its unit configuration file.
> Does systemd make any sort of guarantee that service A will get a chance to
> handle the DBus method call before receiving SIGTERM, or does my code as
> written have a race condition?
> 

systemd will not initiate stopping of A before B has stopped. If it
does, this should be considered a bug. So it is up to service B to
handle termination correctly. Note that systemd will send SIGKILL after
timeout if service has not terminated.