user/dev discussion of public-inbox itself
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From: Eric Wong <e@80x24.org>
To: meta@public-inbox.org
Subject: [PATCH] examples/*@.service: sockets MUST be NonBlocking
Date: Sun, 30 Jun 2019 07:41:46 +0000
Message-ID: <20190630074146.GA16199@dcvr> (raw)

For users running multiple (-nntpd@1, -nntpd@2) instances of
either -httpd or -nntpd via systemd to implement zero-downtime
restarts; it's possible for a listen socket to become blocking
for a moment during an accept syscall and cause a daemons to
get stuck in a blocking accept() during
PublicInbox::Listener::event_step (event_read in previous
versions).

Since O_NONBLOCK is a file description flag, systemd clearing
O_NONBLOCK momentarily (before PublicInbox::Listener::new
re-enables it) creates a window for another instance of our
daemon to get stuck in accept().

cf. systemd.service(5)
---
  Additional warnings and tests for this race coming;
  but I need food + sleep.

  I kinda wished I'd pushed for accept4(..., SOCK_DONTWAIT):
  https://lore.kernel.org/lkml/20150513023712.GA4206@dcvr.yhbt.net/

 examples/public-inbox-httpd@.service | 5 +++++
 examples/public-inbox-nntpd@.service | 5 +++++
 examples/unsubscribe-psgi@.service   | 5 +++++
 3 files changed, 15 insertions(+)

diff --git a/examples/public-inbox-httpd@.service b/examples/public-inbox-httpd@.service
index 56117ef..e811da4 100644
--- a/examples/public-inbox-httpd@.service
+++ b/examples/public-inbox-httpd@.service
@@ -20,7 +20,12 @@ ExecStartPre = /bin/mkdir -p -m 1777 /tmp/.pub-inline
 ExecStart = /usr/local/bin/public-inbox-httpd \
 -1 /var/log/public-inbox/httpd.out.log
 StandardError = syslog
+
+# NonBlocking is REQUIRED to avoid a race condition if running
+# simultaneous services
+NonBlocking = true
 Sockets = public-inbox-httpd.socket
+
 KillSignal = SIGQUIT
 User = nobody
 Group = nogroup
diff --git a/examples/public-inbox-nntpd@.service b/examples/public-inbox-nntpd@.service
index 62202c2..a879841 100644
--- a/examples/public-inbox-nntpd@.service
+++ b/examples/public-inbox-nntpd@.service
@@ -20,7 +20,12 @@ ExecStartPre = /bin/mkdir -p -m 1777 /tmp/.pub-inline
 ExecStart = /usr/local/bin/public-inbox-nntpd \
 -1 /var/log/public-inbox/nntpd.out.log
 StandardError = syslog
+
+# NonBlocking is REQUIRED to avoid a race condition if running
+# simultaneous services
+NonBlocking = true
 Sockets = public-inbox-nntpd.socket
+
 KillSignal = SIGQUIT
 User = nobody
 Group = nogroup
diff --git a/examples/unsubscribe-psgi@.service b/examples/unsubscribe-psgi@.service
index acc29e8..c8721fb 100644
--- a/examples/unsubscribe-psgi@.service
+++ b/examples/unsubscribe-psgi@.service
@@ -12,7 +12,12 @@ After = unsubscribe-psgi.socket
 # any PSGI server ought to work,
 # but public-inbox-httpd supports socket activation like unsubscribe.milter
 ExecStart = /usr/local/bin/public-inbox-httpd -W0 /etc/unsubscribe.psgi
+
+# NonBlocking is REQUIRED to avoid a race condition if running
+# simultaneous services
+NonBlocking = true
 Sockets = unsubscribe-psgi.socket
+
 # we need to modify the mlmmj spool
 User = mlmmj
 KillMode = process
-- 
EW


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user/dev discussion of public-inbox itself

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