Add StopAtEOF to stop tailing when the end of the file is reached

tail.go

@@ -4,6 +4,7 @@ package tail
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"github.com/ActiveState/tail/ratelimiter"
 	"github.com/ActiveState/tail/util"
@@ -136,6 +137,14 @@ func (tail *Tail) Stop() error {
 	return tail.Wait()
 }
 
+// StopAtEOF stops tailing as soon as the end of the file is reached.
+func (tail *Tail) StopAtEOF() error {
+	tail.Kill(errStopAtEOF)
+	return tail.Wait()
+}
+
+var errStopAtEOF = errors.New("tail: stop at eof")
+
 func (tail *Tail) close() {
 	close(tail.Lines)
 	if tail.file != nil {
@@ -209,6 +218,8 @@ func (tail *Tail) tailFileSync() {
 
 	tail.openReader()
 
+	var stopAtEOF bool
+
 	// Read line by line.
 	for {
 		line, err := tail.readLine()
@@ -228,14 +239,16 @@ func (tail *Tail) tailFileSync() {
 				case <-tail.Dying():
 					return
 				}
-				err = tail.seekEnd()
+				if err := tail.seekEnd(); err != nil {
-				if err != nil {
 					tail.Kill(err)
 					return
 				}
 			}
+			if stopAtEOF && err == io.EOF {
+				return
+			}
 		} else if err == io.EOF {
-			if !tail.Follow {
+			if stopAtEOF || !tail.Follow {
 				return
 			}
 			// When EOF is reached, wait for more data to become
@@ -256,6 +269,10 @@ func (tail *Tail) tailFileSync() {
 
 		select {
 		case <-tail.Dying():
+			if tail.Err() == errStopAtEOF {
+				stopAtEOF = true
+				continue
+			}
 			return
 		default:
 		}

tail_test.go

@@ -54,6 +54,25 @@ func TestStop(t *testing.T) {
 	Cleanup()
 }
 
+func TestStopAtEOF(_t *testing.T) {
+	t := NewTailTest("maxlinesize", _t)
+	t.CreateFile("test.txt", "hello\nthere\nworld\n")
+	tail := t.StartTail("test.txt", Config{Follow: true, Location: nil})
+
+	// read "hello"
+	<-tail.Lines
+
+	done := make(chan struct{})
+	go func() {
+		<-time.After(100 * time.Millisecond)
+		t.VerifyTailOutput(tail, []string{"there", "world"})
+		close(done)
+	}()
+	tail.StopAtEOF()
+	<-done
+	Cleanup()
+}
+
 func TestMaxLineSize(_t *testing.T) {
 	t := NewTailTest("maxlinesize", _t)
 	t.CreateFile("test.txt", "hello\nworld\nfin\nhe")