Feature/line seeker

.travis.yml

@@ -17,3 +17,4 @@ matrix:
 install:
   - go get gopkg.in/fsnotify.v1
   - go get gopkg.in/tomb.v1
+  - go get github.com/stoicperlman/fls

cmd/gotail/gotail.go

@@ -6,6 +6,7 @@ package main
 import (
 	"flag"
 	"fmt"
+	"io"
 	"os"
 
 	"github.com/hpcloud/tail"
@@ -36,7 +37,7 @@ func main() {
 	}
 
 	if n != 0 {
-		config.Location = &tail.SeekInfo{-n, os.SEEK_END}
+		config.LineLocation = &tail.SeekInfo{-n, io.SeekEnd}
 	}
 
 	done := make(chan bool)

tail.go

@@ -18,6 +18,7 @@ import (
 	"github.com/hpcloud/tail/ratelimiter"
 	"github.com/hpcloud/tail/util"
 	"github.com/hpcloud/tail/watch"
+	"github.com/stoicperlman/fls"
 	"gopkg.in/tomb.v1"
 )
 
@@ -57,12 +58,13 @@ type logger interface {
 // Config is used to specify how a file must be tailed.
 type Config struct {
 	// File-specifc
-	Location    *SeekInfo // Seek to this location before tailing
-	ReOpen      bool      // Reopen recreated files (tail -F)
-	MustExist   bool      // Fail early if the file does not exist
-	Poll        bool      // Poll for file changes instead of using inotify
-	Pipe        bool      // Is a named pipe (mkfifo)
-	RateLimiter *ratelimiter.LeakyBucket
+	Location     *SeekInfo // Seek to this location before tailing
+	LineLocation *SeekInfo // Seek to this line number before tailing
+	ReOpen       bool      // Reopen recreated files (tail -F)
+	MustExist    bool      // Fail early if the file does not exist
+	Poll         bool      // Poll for file changes instead of using inotify
+	Pipe         bool      // Is a named pipe (mkfifo)
+	RateLimiter  *ratelimiter.LeakyBucket
 
 	// Generic IO
 	Follow      bool // Continue looking for new lines (tail -f)
@@ -105,6 +107,10 @@ func TailFile(filename string, config Config) (*Tail, error) {
 		util.Fatal("cannot set ReOpen without Follow.")
 	}
 
+	if config.Location != nil && config.LineLocation != nil {
+		util.Fatal("Location and LineLocation cannot be set at the same time")
+	}
+
 	t := &Tail{
 		Filename: filename,
 		Lines:    make(chan *Line),
@@ -246,6 +252,34 @@ func (tail *Tail) tailFileSync() {
 			tail.Killf("Seek error on %s: %s", tail.Filename, err)
 			return
 		}
+	} else if tail.LineLocation != nil {
+		lineFile := fls.LineFile(tail.file)
+		buf := make([]byte, 1)
+
+		_, err := lineFile.Seek(-1, io.SeekEnd)
+		if err != nil {
+			tail.Killf("Seek error on %s: %s", tail.Filename, err)
+			return
+		}
+
+		_, err = lineFile.Read(buf)
+		if err != nil {
+			tail.Killf("Seek error on %s: %s", tail.Filename, err)
+			return
+		}
+
+		// if file ends in newline don't count it in lines
+		// to read from end (mimics unix tail command)
+		correction := int64(1)
+		if string(buf) == "\n" {
+			correction = 0
+		}
+
+		_, err = lineFile.SeekLine(tail.LineLocation.Offset+correction, tail.LineLocation.Whence)
+		if err != nil && err != io.EOF {
+			tail.Killf("Seek error on %s: %s", tail.Filename, err)
+			return
+		}
 	}
 
 	tail.openReader()

tail_test.go

@@ -8,6 +8,7 @@ package tail
 
 import (
 	_ "fmt"
+	"io"
 	"io/ioutil"
 	"os"
 	"strings"
@@ -208,6 +209,66 @@ func TestLocationMiddle(t *testing.T) {
 	tailTest.Cleanup(tail, true)
 }
 
+func TestLineLocationFull(t *testing.T) {
+	tailTest := NewTailTest("line-location-full", t)
+	tailTest.CreateFile("test.txt", "hello\nworld\n")
+	tail := tailTest.StartTail("test.txt", Config{Follow: true, LineLocation: nil})
+	go tailTest.VerifyTailOutput(tail, []string{"hello", "world"}, false)
+
+	// Delete after a reasonable delay, to give tail sufficient time
+	// to read all lines.
+	<-time.After(100 * time.Millisecond)
+	tailTest.RemoveFile("test.txt")
+	tailTest.Cleanup(tail, true)
+}
+
+func TestLineLocationFullDontFollow(t *testing.T) {
+	tailTest := NewTailTest("line-location-full-dontfollow", t)
+	tailTest.CreateFile("test.txt", "hello\nworld\n")
+	tail := tailTest.StartTail("test.txt", Config{Follow: false, LineLocation: nil})
+	go tailTest.VerifyTailOutput(tail, []string{"hello", "world"}, false)
+
+	// Add more data only after reasonable delay.
+	<-time.After(100 * time.Millisecond)
+	tailTest.AppendFile("test.txt", "more\ndata\n")
+	<-time.After(100 * time.Millisecond)
+
+	tailTest.Cleanup(tail, true)
+}
+
+func TestLineLocationEnd(t *testing.T) {
+	tailTest := NewTailTest("line-location-end", t)
+	tailTest.CreateFile("test.txt", "hello\nworld\n")
+	tail := tailTest.StartTail("test.txt", Config{Follow: true, LineLocation: &SeekInfo{0, io.SeekEnd}})
+	go tailTest.VerifyTailOutput(tail, []string{"more", "data"}, false)
+
+	<-time.After(100 * time.Millisecond)
+	tailTest.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)
+	tailTest.RemoveFile("test.txt")
+	tailTest.Cleanup(tail, true)
+}
+
+func TestLineLocationMiddle(t *testing.T) {
+	// Test reading from middle.
+	tailTest := NewTailTest("line-location-middle", t)
+	tailTest.CreateFile("test.txt", "hello\nworld\n")
+	tail := tailTest.StartTail("test.txt", Config{Follow: true, LineLocation: &SeekInfo{-1, io.SeekEnd}})
+	go tailTest.VerifyTailOutput(tail, []string{"world", "more", "data"}, false)
+
+	<-time.After(100 * time.Millisecond)
+	tailTest.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)
+	tailTest.RemoveFile("test.txt")
+	tailTest.Cleanup(tail, true)
+}
+
 // The use of polling file watcher could affect file rotation
 // (detected via renames), so test these explicitly.