Use systemd.timer instead of a cron job

The rational is that we need to spread the load on the mirrors
that apt update and unattended-upgrades cause. To do so, we
leverage the RandomizeDelay feature of systemd. The other advantage
is that the timer is not run at a fixed daily.daily time but
instead every 24h. This also fixes the problem that the randomized
deplay in the current apt.cron.daily causes other cron jobs to
be deplayed.

A compatibility cron job is also provided for systems that do not
use systemd.

Note that the time is fired two times a day, but the logic inside
of apt.systemd.daily will ensure (via stamp files) that the
servers are hit at most every 24h. Firing two times a day helps
with the worst case update time and it also helps with systems
that are not always on.

LP: #246381, #727685
Closes: #600262, #709675, #663290

1 files changed, 33 insertions, 0 deletions

diff --git a/debian/apt.apt-compat.cron.daily b/debian/apt.apt-compat.cron.daily
new file mode 100644
index 000000000..1ea843008
--- /dev/null
+++ b/debian/apt.apt-compat.cron.daily
@@ -0,0 +1,33 @@
+#!/bin/sh
+
+set -e
+
+# Systemd systems use a systemd timer unit which is preferable to
+# run. We want to randomize the apt update and unattended-upgrade
+# runs as much as possible to avoid hitting the mirrors all at the
+# same time. The systemd time is better at this than the fixed
+# cron.daily time
+if [ -d /run/systemd/system ]; then
+    exit 0
+fi
+
+# sleep for a random interval of time (default 30min)
+# (some code taken from cron-apt, thanks)
+random_sleep()
+{
+    RandomSleep=1800
+    eval $(apt-config shell RandomSleep APT::Periodic::RandomSleep)
+    if [ $RandomSleep -eq 0 ]; then
+	return
+    fi
+    if [ -z "$RANDOM" ] ; then
+        # A fix for shells that do not have this bash feature.
+	RANDOM=$(( $(dd if=/dev/urandom bs=2 count=1 2> /dev/null | cksum | cut -d' ' -f1) % 32767 ))
+    fi
+    TIME=$(($RANDOM % $RandomSleep))
+    sleep $TIME
+}
+
+# run daily job
+random_sleep
+exec /usr/lib/apt/apt.systemd.daily