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/[email protected] | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/examples/[email protected] b/examples/[email protected]
index d95eb83..946de44 100644
--- a/examples/[email protected]
+++ b/examples/[email protected]
@@ -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
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