fix(checker): Do not fetch extra metric data before lastCheck.Timestamp

lastCheck.Timestamp denotes a most recent point in time when a trigger
was checked. While checking trigger, we are interested only in metric
points after lastCheck.Timestamp, because all previous points should
have been checked during the previous check. There is no obvious reason
to fetch more historical data points. At least, there is no test that
shows why we should do that.

Why is this bad? Because we don't have any limits on TTL value set by
user. Trying to fetch 1e15 metric points from a data source leads to OOM
death of all checker instances.

Close #190

checker/trigger_checker.go

@@ -58,7 +58,7 @@ func MakeTriggerChecker(triggerID string, dataBase moira.Database, logger moira.
 		metrics:  metrics.GetCheckMetrics(&trigger),
 		source:   source,
 
-		from:  calculateFrom(lastCheck.Timestamp, trigger.TTL),
+		from:  lastCheck.Timestamp,
 		until: until,
 
 		triggerID: triggerID,
@@ -98,10 +98,3 @@ func getTTLState(triggerTTLState *moira.TTLState) moira.TTLState {
 	}
 	return moira.TTLStateNODATA
 }
-
-func calculateFrom(lastCheckTimestamp, triggerTTL int64) int64 {
-	if triggerTTL != 0 {
-		return lastCheckTimestamp - triggerTTL
-	}
-	return lastCheckTimestamp - 600
-}

checker/trigger_checker_test.go

@@ -113,7 +113,7 @@ func TestInitTriggerChecker(t *testing.T) {
 			ttl:       trigger.TTL,
 			ttlState:  *trigger.TTLState,
 			lastCheck: &lastCheck,
-			from:      lastCheck.Timestamp - ttl,
+			from:      lastCheck.Timestamp,
 			until:     actual.until,
 		}
 		So(*actual, ShouldResemble, expected)
@@ -139,7 +139,7 @@ func TestInitTriggerChecker(t *testing.T) {
 				State:     moira.StateOK,
 				Timestamp: actual.until - 3600,
 			},
-			from:  actual.until - 3600 - ttl,
+			from:  actual.until - 3600,
 			until: actual.until,
 		}
 		So(*actual, ShouldResemble, expected)
@@ -168,7 +168,7 @@ func TestInitTriggerChecker(t *testing.T) {
 				State:     moira.StateOK,
 				Timestamp: actual.until - 3600,
 			},
-			from:  actual.until - 3600 - 600,
+			from:  actual.until - 3600,
 			until: actual.until,
 		}
 		So(*actual, ShouldResemble, expected)
@@ -190,7 +190,7 @@ func TestInitTriggerChecker(t *testing.T) {
 			ttl:       0,
 			ttlState:  moira.TTLStateNODATA,
 			lastCheck: &lastCheck,
-			from:      lastCheck.Timestamp - 600,
+			from:      lastCheck.Timestamp,
 			until:     actual.until,
 		}
 		So(*actual, ShouldResemble, expected)