fix: fix from_beginning options behavior

plugins/inputs/tail/README.md

@@ -56,7 +56,7 @@ See the [CONFIGURATION.md][CONFIGURATION.md] for more details.
   ##
   files = ["/var/mymetrics.out"]
 
-  ## Read file from beginning.
+  ## Read file from beginning for new discovered files (without a persisted offset).
   # from_beginning = false
 
   ## Whether file is a named pipe

plugins/inputs/tail/tail.go

@@ -162,6 +162,32 @@ func (t *Tail) Start(acc telegraf.Accumulator) error {
 	return err
 }
 
+func (t *Tail) getSeekInfo(file string, fromBeginning bool) *tail.SeekInfo {
+	if t.Pipe {
+		return nil
+	}
+
+	if offset, ok := t.offsets[file]; ok {
+		t.Log.Debugf("Using offset %d for %q", offset, file)
+		return &tail.SeekInfo{
+			Whence: 0,
+			Offset: offset,
+		}
+	}
+
+	if fromBeginning {
+		return &tail.SeekInfo{
+			Whence: 0,
+			Offset: 0,
+		}
+	}
+
+	return &tail.SeekInfo{
+		Whence: 2,
+		Offset: 0,
+	}
+}
+
 func (t *Tail) tailNewFiles(fromBeginning bool) error {
 	var poll bool
 	if t.WatchMethod == "poll" {
@@ -180,21 +206,7 @@ func (t *Tail) tailNewFiles(fromBeginning bool) error {
 				continue
 			}
 
-			var seek *tail.SeekInfo
-			if !t.Pipe && !fromBeginning {
-				if offset, ok := t.offsets[file]; ok {
-					t.Log.Debugf("Using offset %d for %q", offset, file)
-					seek = &tail.SeekInfo{
-						Whence: 0,
-						Offset: offset,
-					}
-				} else {
-					seek = &tail.SeekInfo{
-						Whence: 2,
-						Offset: 0,
-					}
-				}
-			}
+			seek := t.getSeekInfo(file, fromBeginning)
 
 			tailer, err := tail.TailFile(file,
 				tail.Config{
@@ -379,7 +391,7 @@ func (t *Tail) receiver(parser telegraf.Parser, tailer *tail.Tail) {
 
 func (t *Tail) Stop() {
 	for _, tailer := range t.tailers {
-		if !t.Pipe && !t.FromBeginning {
+		if !t.Pipe {
 			// store offset for resume
 			offset, err := tailer.Tell()
 			if err == nil {

plugins/inputs/tail/tail_test.go

@@ -9,6 +9,8 @@ import (
 	"time"
 
 	"github.com/google/go-cmp/cmp"
+	"github.com/influxdata/tail"
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
 	"github.com/influxdata/telegraf"
@@ -800,3 +802,61 @@ func TestStatePersistence(t *testing.T) {
 	require.True(t, ok, "state is not a map[string]int64")
 	require.Equal(t, expectedState, actualState)
 }
+
+func TestGetSeekInfo(t *testing.T) {
+	tests := []struct {
+		name string
+		offsets       map[string]int64
+		file          string
+		fromBeginning bool
+		expected      *tail.SeekInfo
+	}{
+		{
+			name:    "Offset exists",
+			offsets: map[string]int64{"test.log": 100},
+			file:    "test.log",
+			expected: &tail.SeekInfo{
+				Whence: 0,
+				Offset: 100,
+			},
+		},
+		{
+			name:          "Ignore from beginning for already discoverd files",
+			offsets:       map[string]int64{"test.log": 100},
+			file:          "test.log",
+			fromBeginning: true,
+			expected: &tail.SeekInfo{
+				Whence: 0,
+				Offset: 100,
+			},
+		},
+		{
+			name:          "From beginning",
+			file:          "test.log",
+			fromBeginning: true,
+			expected: &tail.SeekInfo{
+				Whence: 0,
+				Offset: 0,
+			},
+		},
+		{
+			name: "Default case",
+			file: "test.log",
+			expected: &tail.SeekInfo{
+				Whence: 2,
+				Offset: 0,
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			tail := NewTestTail()
+			tail.offsets = test.offsets
+			logger := &testutil.CaptureLogger{}
+			tail.Log = logger
+			seekInfo := tail.getSeekInfo(test.file, test.fromBeginning)
+			assert.Equal(t, test.expected, seekInfo)
+		})
+	}
+}