checkpoint daemon doesn't randomise checkpoint interval

[sergepetrenko]

Tarantool version: 2.1, 2.2, 2.3

Bug description:
#4432 resolved the issue after box.snapshot(), but it turns out that
checkpoint_daemon doesn't randomise checkpoint time at all.
This was introduced in 4c04808

Take a look at the logs. Note that before the manual box.snapshot() checkpoints are
made every 10 seconds. Issuing box.snapshot() reschedules a checkpoint to a random interval,
but the next automatic reschedule sets the interval to 10 seconds again.

2019-10-28 15:25:10.360 [17665] main/104/checkpoint_daemon I> scheduled next checkpoint for Mon Oct 28 15:25:20 2019
2019-10-28 15:25:20.362 [17665] snapshot/101/main I> saving snapshot `./00000000000000000035.snap.inprogress'
2019-10-28 15:25:20.367 [17665] snapshot/101/main I> done
2019-10-28 15:25:20.368 [17665] main/104/checkpoint_daemon I> scheduled next checkpoint for Mon Oct 28 15:25:30 2019
2019-10-28 15:25:30.362 [17665] snapshot/101/main I> saving snapshot `./00000000000000000036.snap.inprogress'
2019-10-28 15:25:30.367 [17665] snapshot/101/main I> done
2019-10-28 15:25:30.368 [17665] main/104/checkpoint_daemon I> scheduled next checkpoint for Mon Oct 28 15:25:40 2019

tarantool> box.snapshot()

2019-10-28 15:25:36.991 [17665] main/104/checkpoint_daemon I> scheduled next checkpoint for Mon Oct 28 15:25:53 2019
2019-10-28 15:25:36.991 [17665] snapshot/101/main I> saving snapshot `./00000000000000000037.snap.inprogress'
2019-10-28 15:25:36.996 [17665] snapshot/101/main I> done
2019-10-28 15:25:53.995 [17665] main/104/checkpoint_daemon I> scheduled next checkpoint for Mon Oct 28 15:26:03 2019

[kostja]

What is not randomized? You should not re-set the interval after each checkpoint, otherwise you'r ebound to to have collisions in a running setup once in a while. A collision can be easily fixed with box.snapshot(). This is an imperfect solution but the right one is kostja#1

[sergepetrenko]

@kostja Makes sense, thanks. Closing then.