[PATCH] examples/unicorn@.service: note the NonBlocking flag

[Eric Wong <e@80x24.org>]

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
---
 examples/unicorn@.service | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/examples/unicorn@.service b/examples/unicorn@.service
index d95eb83..946de44 100644
--- a/examples/unicorn@.service
+++ b/examples/unicorn@.service
@@ -14,7 +14,14 @@ After = unicorn.socket
 # bundler users must use the "--keep-file-descriptors" switch, here:
 # ExecStart = bundle exec --keep-file-descriptors unicorn -c ...
 ExecStart = /usr/bin/unicorn -c /path/to/unicorn.conf.rb /path/to/config.ru
+
+# NonBlocking MUST be true if using socket activation with unicorn.
+# Otherwise, there's a small window in-between when the non-blocking
+# flag is set by us and our accept4 call where systemd can momentarily
+# make the socket blocking, causing us to block on accept4:
+NonBlocking = true
 Sockets = unicorn.socket
+
 KillSignal = SIGQUIT
 User = nobody
 Group = nogroup
-- 
EW