add read_start option

plugins/inputs/tail/README.md

@@ -59,6 +59,15 @@ See the [CONFIGURATION.md][CONFIGURATION.md] for more details.
   ## Read file from beginning.
   # from_beginning = false
 
+  ## Read file method (defaults to 'save-offset-or-end').
+  ## After setting this, the "from_beginning" option will be ignored.
+  ## The following methods are available:
+  ##   save-offset-or-end -- start reading from the persisted offset of the file or, if no offset persisted, start from the end (default) 
+  ##   save-offset-or-beginning -- start reading from the persisted offset of the file or, if no offset persisted, start from the beginning
+  ##   end  -- start reading from the end of the file ignoring any saved offset
+  ##   beginning -- start reading from the beginning of the file ignoring any saved offset
+  # read_start = "save-offset-or-end"
+
   ## Whether file is a named pipe
   # pipe = false
 

plugins/inputs/tail/sample.conf

@@ -15,6 +15,15 @@
   ## Read file from beginning.
   # from_beginning = false
 
+  ## Read file method (defaults to 'save-offset-or-end').
+  ## After setting this, the "from_beginning" option will be ignored.
+  ## The following methods are available:
+  ##   save-offset-or-end -- start reading from the persisted offset of the file or, if no offset persisted, start from the end (default) 
+  ##   save-offset-or-beginning -- start reading from the persisted offset of the file or, if no offset persisted, start from the beginning
+  ##   end  -- start reading from the end of the file ignoring any saved offset
+  ##   beginning -- start reading from the beginning of the file ignoring any saved offset
+  # read_start = "save-offset-or-end"
+
   ## Whether file is a named pipe
   # pipe = false
 

plugins/inputs/tail/tail.go

@@ -41,6 +41,7 @@ type semaphore chan empty
 type Tail struct {
 	Files               []string `toml:"files"`
 	FromBeginning       bool     `toml:"from_beginning"`
+	ReadStart           string   `toml:"read_start"`
 	Pipe                bool     `toml:"pipe"`
 	WatchMethod         string   `toml:"watch_method"`
 	MaxUndeliveredLines int      `toml:"max_undelivered_lines"`
@@ -162,6 +163,54 @@ func (t *Tail) Start(acc telegraf.Accumulator) error {
 	return err
 }
 
+func (t *Tail) getSeekInfo(file string, fromBeginning bool) (*tail.SeekInfo, error) {
+	var seek *tail.SeekInfo
+
+	seekStart := &tail.SeekInfo{
+		Whence: 0,
+		Offset: 0,
+	}
+	seekEnd := &tail.SeekInfo{
+		Whence: 2,
+		Offset: 0,
+	}
+
+	if fromBeginning && t.ReadStart == "" {
+		return seekStart, nil
+	}
+
+	switch t.ReadStart {
+	case "end":
+		seek = seekEnd
+	case "beginning":
+		seek = seekStart
+	case "", "save-offset-or-end":
+		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 = seekEnd
+		}
+	case "save-offset-or-beginning":
+		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 = seekStart
+		}
+	default:
+		return nil, errors.New("invalid 'read_start' setting")
+	}
+
+	return seek, nil
+}
+
 func (t *Tail) tailNewFiles(fromBeginning bool) error {
 	var poll bool
 	if t.WatchMethod == "poll" {
@@ -180,20 +229,9 @@ 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, err := t.getSeekInfo(file, fromBeginning)
+			if err != nil {
+				return err
 			}
 
 			tailer, err := tail.TailFile(file,
@@ -379,7 +417,7 @@ 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

@@ -9,8 +9,7 @@ import (
 	"time"
 
 	"github.com/google/go-cmp/cmp"
-	"github.com/stretchr/testify/require"
-
+	"github.com/influxdata/tail"
 	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/config"
 	"github.com/influxdata/telegraf/metric"
@@ -19,6 +18,7 @@ import (
 	"github.com/influxdata/telegraf/plugins/parsers/influx"
 	"github.com/influxdata/telegraf/plugins/parsers/json"
 	"github.com/influxdata/telegraf/testutil"
+	"github.com/stretchr/testify/require"
 )
 
 func newInfluxParser() (telegraf.Parser, error) {
@@ -800,3 +800,124 @@ 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
+		readStart     string
+		expected      *tail.SeekInfo
+	}{
+		{
+			name:          "Read from beginning when from_beginning set to true",
+			offsets:       map[string]int64{"test.log": 100},
+			file:          "test.log",
+			fromBeginning: true,
+			expected: &tail.SeekInfo{
+				Whence: 0,
+				Offset: 0,
+			},
+		},
+		{
+			name:      "Read from beginning when read_start set to beginning",
+			offsets:   map[string]int64{"test.log": 100},
+			file:      "test.log",
+			readStart: "beginning",
+			expected: &tail.SeekInfo{
+				Whence: 0,
+				Offset: 0,
+			},
+		},
+		{
+			name:      "Read from end when read_start set to end",
+			offsets:   map[string]int64{"test.log": 100},
+			file:      "test.log",
+			readStart: "end",
+			expected: &tail.SeekInfo{
+				Whence: 2,
+				Offset: 0,
+			},
+		},
+		{
+			name:      "Read from end when offset not exists and read_start set to save-offset-or-end",
+			offsets:   map[string]int64{},
+			file:      "test.log",
+			readStart: "save-offset-or-end",
+			expected: &tail.SeekInfo{
+				Whence: 2,
+				Offset: 0,
+			},
+		},
+		{
+			name:      "Read from offset when offset exists and read_start set to save-offset-or-end",
+			offsets:   map[string]int64{"test.log": 100},
+			file:      "test.log",
+			readStart: "save-offset-or-end",
+			expected: &tail.SeekInfo{
+				Whence: 0,
+				Offset: 100,
+			},
+		},
+		{
+			name:      "Read from start when offset not exists and read_start set to save-offset-or-start",
+			offsets:   map[string]int64{},
+			file:      "test.log",
+			readStart: "save-offset-or-beginning",
+			expected: &tail.SeekInfo{
+				Whence: 0,
+				Offset: 0,
+			},
+		},
+		{
+			name:      "Read from offset when offset exists and read_start set to save-offset-or-end",
+			offsets:   map[string]int64{"test.log": 100},
+			file:      "test.log",
+			readStart: "save-offset-or-beginning",
+			expected: &tail.SeekInfo{
+				Whence: 0,
+				Offset: 100,
+			},
+		},
+		{
+			name:          "Ignore from_beginning when read_start is set",
+			offsets:       map[string]int64{"test.log": 100},
+			file:          "test.log",
+			fromBeginning: true,
+			readStart:     "save-offset-or-beginning",
+			expected: &tail.SeekInfo{
+				Whence: 0,
+				Offset: 100,
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			logger := &testutil.CaptureLogger{}
+			tt := NewTestTail()
+			tt.Log = logger
+			tt.ReadStart = test.readStart
+			tt.FromBeginning = test.fromBeginning
+
+			require.NoError(t, tt.Init())
+			tt.offsets = test.offsets
+
+			seekInfo, err := tt.getSeekInfo(test.file, test.fromBeginning)
+			require.NoError(t, err)
+			require.Equal(t, test.expected, seekInfo)
+		})
+	}
+
+	t.Run("Return error when read_start is invalid", func(t *testing.T) {
+		logger := &testutil.CaptureLogger{}
+		tt := NewTestTail()
+		tt.Log = logger
+		tt.ReadStart = "invalid"
+
+		require.NoError(t, tt.Init())
+		_, err := tt.getSeekInfo("test.log", false)
+		require.Error(t, err)
+	})
+}