Always touch snapshpot in checkpoint_daemon

* Remove check on checkpoint signature and always touch snapshot
  even if there were no transations since the previous checkpoint
* Fix timeout check

Fixup #2780

src/box/lua/checkpoint_daemon.lua

@@ -44,10 +44,6 @@ local function process(self)
 
     local checkpoints = box.internal.gc.info().checkpoints
     local last_checkpoint = checkpoints[#checkpoints]
-    if last_checkpoint.signature == box.info.signature then
-        log.debug('snapshot %d already exists', last_checkpoint.signature)
-        return false
-    end
 
     local last_snap = fio.pathjoin(box.cfg.memtx_dir,
             string.format('%020d.snap', last_checkpoint.signature))
@@ -56,7 +52,7 @@ local function process(self)
         log.error("can't stat %s: %s", last_snap, errno.strerror())
         return false
     end
-    if snstat.mtime <= fiber.time() + daemon.checkpoint_interval then
+    if snstat.mtime + daemon.checkpoint_interval <= fiber.time() then
         return snapshot()
     end
 end

test/xlog/checkpoint_daemon.result

@@ -241,3 +241,25 @@ daemon.control == nil
 ---
 - true
 ...
+-- gh-2780 check that scheduled snapshots are performed
+PERIOD = 0.03
+---
+...
+if jit.os ~= 'Linux' then PERIOD = 1.5 end
+---
+...
+box.cfg{ checkpoint_interval = PERIOD}
+---
+...
+fiber.sleep(3 * PERIOD)
+---
+...
+-- check that it's not first snapshot
+test_run:grep_log("default", "saving snapshot", 400)
+---
+- null
+...
+test_run:grep_log("default", "making snapshot", 400)
+---
+- making snapshot
+...

test/xlog/checkpoint_daemon.test.lua

@@ -112,3 +112,12 @@ box.cfg{ checkpoint_count = 2, checkpoint_interval = 0}
 daemon.next_snapshot_time
 daemon.fiber == nil
 daemon.control == nil
+
+-- gh-2780 check that scheduled snapshots are performed
+PERIOD = 0.03
+if jit.os ~= 'Linux' then PERIOD = 1.5 end
+box.cfg{ checkpoint_interval = PERIOD}
+fiber.sleep(3 * PERIOD)
+-- check that it's not first snapshot
+test_run:grep_log("default", "saving snapshot", 400)
+test_run:grep_log("default", "making snapshot", 400)
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