Support tailing files created after startup in tail input (#4704)

plugins/inputs/tail/tail.go

@@ -25,7 +25,7 @@ type Tail struct {
 	Pipe          bool
 	WatchMethod   string
 
-	tailers []*tail.Tail
+	tailers map[string]*tail.Tail
 	parser  parsers.Parser
 	wg      sync.WaitGroup
 	acc     telegraf.Accumulator
@@ -74,7 +74,10 @@ func (t *Tail) Description() string {
 }
 
 func (t *Tail) Gather(acc telegraf.Accumulator) error {
-	return nil
+	t.Lock()
+	defer t.Unlock()
+
+	return t.tailNewFiles(true)
 }
 
 func (t *Tail) Start(acc telegraf.Accumulator) error {
@@ -82,9 +85,14 @@ func (t *Tail) Start(acc telegraf.Accumulator) error {
 	defer t.Unlock()
 
 	t.acc = acc
+	t.tailers = make(map[string]*tail.Tail)
 
+	return t.tailNewFiles(t.FromBeginning)
+}
+
+func (t *Tail) tailNewFiles(fromBeginning bool) error {
 	var seek *tail.SeekInfo
-	if !t.Pipe && !t.FromBeginning {
+	if !t.Pipe && !fromBeginning {
 		seek = &tail.SeekInfo{
 			Whence: 2,
 			Offset: 0,
@@ -103,6 +111,11 @@ func (t *Tail) Start(acc telegraf.Accumulator) error {
 			t.acc.AddError(fmt.Errorf("E! Error Glob %s failed to compile, %s", filepath, err))
 		}
 		for file, _ := range g.Match() {
+			if _, ok := t.tailers[file]; ok {
+				// we're already tailing this file
+				continue
+			}
+
 			tailer, err := tail.TailFile(file,
 				tail.Config{
 					ReOpen:    true,
@@ -114,16 +127,15 @@ func (t *Tail) Start(acc telegraf.Accumulator) error {
 					Logger:    tail.DiscardingLogger,
 				})
 			if err != nil {
-				acc.AddError(err)
+				t.acc.AddError(err)
 				continue
 			}
 			// create a goroutine for each "tailer"
 			t.wg.Add(1)
 			go t.receiver(tailer)
-			t.tailers = append(t.tailers, tailer)
+			t.tailers[file] = tailer
 		}
 	}
-
 	return nil
 }