Merge pull request #12 from ActiveState/better_location

Ability to seek to anywhere in the file before tailing

cmd/gotail/gotail.go

@@ -9,9 +9,10 @@ import (
 	"os"
 )
 
-func args2config() tail.Config {
+func args2config() (tail.Config, int64) {
 	config := tail.Config{Follow: true}
-	flag.IntVar(&config.Location, "n", 0, "tail from the last Nth location")
+	n := int64(0)
+	flag.Int64Var(&n, "n", 0, "tail from the last Nth location")
 	flag.BoolVar(&config.Follow, "f", false, "wait for additional data to be appended to the file")
 	flag.BoolVar(&config.ReOpen, "F", false, "follow, and track file rename/rotation")
 	flag.BoolVar(&config.Poll, "p", false, "use polling, instead of inotify")
@@ -19,16 +20,20 @@ func args2config() tail.Config {
 	if config.ReOpen {
 		config.Follow = true
 	}
-	return config
+	return config, n
 }
 
 func main() {
-	config := args2config()
+	config, n := args2config()
 	if flag.NFlag() < 1 {
 		fmt.Println("need one or more files as arguments")
 		os.Exit(1)
 	}
 
+	if n != 0 {
+		config.Location = &tail.SeekInfo{-n, os.SEEK_END}
+	}
+
 	done := make(chan bool)
 	for _, filename := range flag.Args() {
 		go tailFile(filename, config, done)

tail.go

@@ -24,9 +24,14 @@ type Line struct {
 	// log line itself.
 }
 
+type SeekInfo struct {
+	Offset int64
+	Whence int // os.SEEK_*
+}
+
 // Config is used to specify how a file must be tailed.
 type Config struct {
-	Location    int   // Tail from last N lines (tail -n)
+	Location    *SeekInfo // Seek before tailing
 	Follow      bool      // Continue looking for new lines (tail -f)
 	ReOpen      bool      // Reopen recreated files (tail -F)
 	MustExist   bool      // Fail early if the file does not exist
@@ -55,10 +60,6 @@ type Tail struct {
 // invoke the `Wait` or `Err` method after finishing reading from the
 // `Lines` channel.
 func TailFile(filename string, config Config) (*Tail, error) {
-	if !(config.Location == 0 || config.Location == -1) {
-		panic("only 0/-1 values are supported for Location.")
-	}
-
 	if config.ReOpen && !config.Follow {
 		panic("cannot set ReOpen without Follow.")
 	}
@@ -143,8 +144,9 @@ func (tail *Tail) tailFileSync() {
 	}
 
 	// Seek to requested location on first open of the file.
-	if tail.Location == 0 {
+	if tail.Location != nil {
-		_, err := tail.file.Seek(0, 2) // Seek to the file end
+		_, err := tail.file.Seek(tail.Location.Offset, tail.Location.Whence)
+		// log.Printf("Seeked %s - %+v\n", tail.Filename, tail.Location)
 		if err != nil {
 			tail.Killf("Seek error on %s: %s", tail.Filename, err)
 			return

tail_test.go

@@ -43,7 +43,7 @@ func TestMustExist(t *testing.T) {
 func TestMaxLineSize(_t *testing.T) {
 	t := NewTailTest("maxlinesize", _t)
 	t.CreateFile("test.txt", "hello\nworld\nfin\nhe")
-	tail := t.StartTail("test.txt", Config{Follow: true, Location: -1, MaxLineSize: 3})
+	tail := t.StartTail("test.txt", Config{Follow: true, Location: nil, MaxLineSize: 3})
 	go t.VerifyTailOutput(tail, []string{"hel", "lo", "wor", "ld", "fin", "he"})
 
 	// Delete after a reasonable delay, to give tail sufficient time
@@ -56,7 +56,7 @@ func TestMaxLineSize(_t *testing.T) {
 func TestLocationFull(_t *testing.T) {
 	t := NewTailTest("location-full", _t)
 	t.CreateFile("test.txt", "hello\nworld\n")
-	tail := t.StartTail("test.txt", Config{Follow: true, Location: -1})
+	tail := t.StartTail("test.txt", Config{Follow: true, Location: nil})
 	go t.VerifyTailOutput(tail, []string{"hello", "world"})
 
 	// Delete after a reasonable delay, to give tail sufficient time
@@ -69,7 +69,7 @@ func TestLocationFull(_t *testing.T) {
 func TestLocationFullDontFollow(_t *testing.T) {
 	t := NewTailTest("location-full-dontfollow", _t)
 	t.CreateFile("test.txt", "hello\nworld\n")
-	tail := t.StartTail("test.txt", Config{Follow: false, Location: -1})
+	tail := t.StartTail("test.txt", Config{Follow: false, Location: nil})
 	go t.VerifyTailOutput(tail, []string{"hello", "world"})
 
 	// Add more data only after reasonable delay.
@@ -83,7 +83,7 @@ func TestLocationFullDontFollow(_t *testing.T) {
 func TestLocationEnd(_t *testing.T) {
 	t := NewTailTest("location-end", _t)
 	t.CreateFile("test.txt", "hello\nworld\n")
-	tail := t.StartTail("test.txt", Config{Follow: true, Location: 0})
+	tail := t.StartTail("test.txt", Config{Follow: true, Location: &SeekInfo{0, os.SEEK_END}})
 	go t.VerifyTailOutput(tail, []string{"more", "data"})
 
 	<-time.After(100 * time.Millisecond)
@@ -96,6 +96,23 @@ func TestLocationEnd(_t *testing.T) {
 	tail.Stop()
 }
 
+func TestLocationMiddle(_t *testing.T) {
+	// Test reading from middle.
+	t := NewTailTest("location-end", _t)
+	t.CreateFile("test.txt", "hello\nworld\n")
+	tail := t.StartTail("test.txt", Config{Follow: true, Location: &SeekInfo{-6, os.SEEK_END}})
+	go t.VerifyTailOutput(tail, []string{"world", "more", "data"})
+
+	<-time.After(100 * time.Millisecond)
+	t.AppendFile("test.txt", "more\ndata\n")
+
+	// Delete after a reasonable delay, to give tail sufficient time
+	// to read all lines.
+	<-time.After(100 * time.Millisecond)
+	t.RemoveFile("test.txt")
+	tail.Stop()
+}
+
 func _TestReOpen(_t *testing.T, poll bool) {
 	var name string
 	if poll {
@@ -107,7 +124,7 @@ func _TestReOpen(_t *testing.T, poll bool) {
 	t.CreateFile("test.txt", "hello\nworld\n")
 	tail := t.StartTail(
 		"test.txt",
-		Config{Follow: true, ReOpen: true, Poll: poll, Location: -1})
+		Config{Follow: true, ReOpen: true, Poll: poll})
 
 	go t.VerifyTailOutput(tail, []string{"hello", "world", "more", "data", "endofworld"})
 
@@ -157,7 +174,7 @@ func _TestReSeek(_t *testing.T, poll bool) {
 	t.CreateFile("test.txt", "a really long string goes here\nhello\nworld\n")
 	tail := t.StartTail(
 		"test.txt",
-		Config{Follow: true, ReOpen: false, Poll: poll, Location: -1})
+		Config{Follow: true, ReOpen: false, Poll: poll})
 
 	go t.VerifyTailOutput(tail, []string{
 		"a really long string goes here", "hello", "world", "h311o", "w0r1d", "endofworld"})
@@ -193,7 +210,6 @@ func TestRateLimiting(_t *testing.T) {
 	t.CreateFile("test.txt", "hello\nworld\nagain\n")
 	config := Config{
 		Follow:    true,
-		Location:  -1,
 		LimitRate: 2}
 	tail := t.StartTail("test.txt", config)
 	// TODO: also verify that tail resumes after the cooloff period.