Fix flow timestamp update

CHANGELOG.asciidoc

@@ -61,6 +61,8 @@ https://github.com/elastic/beats/compare/v6.0.0-beta1...master[Check the HEAD di
 
 *Packetbeat*
 
+- Update flow timestamp on each packet being received. {issue}4895[4895]
+
 *Winlogbeat*
 
 ==== Added

packetbeat/flows/table.go

@@ -58,6 +58,8 @@ func (t *flowMetaTable) get(id *FlowID, counter *counterReg) Flow {
 }
 
 func (t *flowTable) get(id *FlowID, counter *counterReg) Flow {
+	ts := time.Now()
+
 	t.mutex.Lock()
 	defer t.mutex.Unlock()
 
@@ -66,13 +68,14 @@ func (t *flowTable) get(id *FlowID, counter *counterReg) Flow {
 	if bf == nil || !bf.isAlive() {
 		debugf("create new flow")
 
-		bf = newBiFlow(id.rawFlowID.clone(), time.Now(), id.dir)
+		bf = newBiFlow(id.rawFlowID.clone(), ts, id.dir)
 		t.table[string(bf.id.flowID)] = bf
 		t.flows.append(bf)
 	} else if bf.dir != id.dir {
 		dir = flowDirReversed
 	}
 
+	bf.ts = ts
 	stats := bf.stats[dir]
 	if stats == nil {
 		stats = newFlowStats(counter)

packetbeat/tests/system/test_0060_flows.py

@@ -1,5 +1,6 @@
 from packetbeat import (BaseTest, FLOWS_REQUIRED_FIELDS)
 from pprint import PrettyPrinter
+from datetime import datetime
 import six
 
 
@@ -11,6 +12,10 @@ def check_fields(flow, fields):
         assert flow[k] == v
 
 
+def parse_timestamp(ts):
+    return datetime.strptime(ts, "%Y-%m-%dT%H:%M:%S.%fZ")
+
+
 class Test(BaseTest):
 
     def test_mysql_flow(self):
@@ -43,6 +48,10 @@ def test_mysql_flow(self):
             'dest.stats.net_bytes_total': 181133,
         })
 
+        start_ts = parse_timestamp(objs[0]['start_time'])
+        last_ts = parse_timestamp(objs[0]['last_time'])
+        assert last_ts > start_ts
+
     def test_memcache_udp_flow(self):
         self.render_config_template(
             flows=True,