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It's racy otherwise when starting simultaneous instanced units.
Without specifying NonBlocking=true, systemd will clear the
O_NONBLOCK flag every time it starts a new service instance.
There's a small window where systemd can clear O_NONBLOCK
immediately after it's set by Ruby (or kgio):
unicorn@1 |systemd |unicorn@2
---------------------------+----------------+--------------------
F_SETFL, O_NONBLOCK|O_RDWR | | (not running, yet)
|F_SETFL, O_RDWR |
|fork |
| exec unicorn@2 |
accept4(...) # blocks! | | (now started by systemd)
| |F_SETFL,O_NONBLOCK|O_RDWR
| |accept4(...) non-blocking
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By default, systemd kills every process in the control group
when stopping a service. While it ought to be harmless to
signal workers, some Rack applications (and perhaps further
subprocesses) can misbehave when interrupted by a signal.
Ensure we only hit the master on graceful shutdown to avoid
tickling bugs in Rack apps.
This is the reason we switched to having the master send
"fake" signals for workers beginning with unicorn 4.8.0
back in 2013/2014.
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"bundle exec" alone is not suitable for use with systemd-style
socket activation due to Ruby 2.0+ behavior of setting close-on-exec
for file descriptors above 2. However, the "--keep-file-descriptors"
option was added to bundler 1.4.0 to workaround this Ruby 2.0 change
and may be used to prevent Ruby 2.0+ from closing file descriptors
on exec.
Thanks to Amir Yalon and Christos Trochalakis for bringing up
this issue on the mailing list:
http://bogomips.org/unicorn-public/1457824748.3666627.547425122.2A828B07@webmail.messagingengine.com/
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Since we have init scripts, we ought to have the equivalent for
systemd users who cannot upgrade via the normal SIGUSR2 mechanism;
but can use multiple services: "unicorn@1" + h"unicorn@2" to
accomplish the same thing.
Based on examples by Christos Trochalakis <yatiohi@ideopolis.gr>
ref:
http://bogomips.org/unicorn-public/20150708130821.GA1361@luke.ws.skroutz.gr/
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