From b803413bc8a6ffc9c21bbeda35fb2b83baa6daa7 Mon Sep 17 00:00:00 2001
From: Dustin Spicuzza <dustin@virtualroadside.com>
Date: Wed, 10 Oct 2018 11:20:46 -0400
Subject: [PATCH] Tailer: add option to start tailing at specified seek offset

---
 tailer/fileTailer.go      | 35 ++++++++++++++++++++++++++++-------
 tailer/fileTailer_test.go | 26 ++++++++++++++++++++++++++
 2 files changed, 54 insertions(+), 7 deletions(-)

diff --git a/tailer/fileTailer.go b/tailer/fileTailer.go
index 2e5f8db1..f6d8e353 100644
--- a/tailer/fileTailer.go
+++ b/tailer/fileTailer.go
@@ -44,18 +44,39 @@ func (f *fileTailer) Errors() chan Error {
 	return f.errors
 }
 
+func getSeekArgs(readall bool) (bool, int64, int) {
+	if readall {
+		return false, 0, 0
+	} else {
+		return true, 0, io.SeekEnd
+	}
+}
+
 func RunFseventFileTailer(path string, readall bool, failOnMissingFile bool, logger simpleLogger) Tailer {
-	return runFileTailer(path, readall, failOnMissingFile, logger, NewFseventWatcher)
+	seek, offset, whence := getSeekArgs(readall)
+	return runFileTailer(path, failOnMissingFile, logger, seek, offset, whence, NewFseventWatcher)
+}
+
+func RunFseventFileTailerWithSeek(path string, failOnMissingFile bool, logger simpleLogger, offset int64, whence int) Tailer {
+	return runFileTailer(path, failOnMissingFile, logger, true, offset, whence, NewFseventWatcher)
 }
 
 func RunPollingFileTailer(path string, readall bool, failOnMissingFile bool, pollIntervall time.Duration, logger simpleLogger) Tailer {
+	seek, offset, whence := getSeekArgs(readall)
+	makeWatcher := func(abspath string, _ *File) (Watcher, error) {
+		return NewPollingWatcher(abspath, pollIntervall)
+	}
+	return runFileTailer(path, failOnMissingFile, logger, seek, offset, whence, makeWatcher)
+}
+
+func RunPollingFileTailerWithSeek(path string, failOnMissingFile bool, pollIntervall time.Duration, logger simpleLogger, offset int64, whence int) Tailer {
 	makeWatcher := func(abspath string, _ *File) (Watcher, error) {
 		return NewPollingWatcher(abspath, pollIntervall)
 	}
-	return runFileTailer(path, readall, failOnMissingFile, logger, makeWatcher)
+	return runFileTailer(path, failOnMissingFile, logger, true, offset, whence, makeWatcher)
 }
 
-func runFileTailer(path string, readall bool, failOnMissingFile bool, logger simpleLogger, makeWatcher func(string, *File) (Watcher, error)) Tailer {
+func runFileTailer(path string, failOnMissingFile bool, logger simpleLogger, seek bool, seekOffset int64, seekWhence int, makeWatcher func(string, *File) (Watcher, error)) Tailer {
 	if logger == nil {
 		logger = &nilLogger{}
 	}
@@ -71,7 +92,7 @@ func runFileTailer(path string, readall bool, failOnMissingFile bool, logger sim
 		closed: false,
 	}
 
-	file, abspath, err := openLogfile(path, readall, failOnMissingFile) // file may be nil if failOnMissingFile is false and the file doesn't exist yet.
+	file, abspath, err := openLogfile(path, failOnMissingFile, seek, seekOffset, seekWhence) // file may be nil if failOnMissingFile is false and the file doesn't exist yet.
 	if err != nil {
 		go func(err error) {
 			writeError(errors, done, err, "failed to initialize file system watcher for %v", path)
@@ -173,7 +194,7 @@ func runFileTailer(path string, readall bool, failOnMissingFile bool, logger sim
 }
 
 // may return *File == nil if the file does not exist and failOnMissingFile == false
-func openLogfile(path string, readall bool, failOnMissingFile bool) (*File, string, error) {
+func openLogfile(path string, failOnMissingFile bool, seek bool, seekOffset int64, seekWhence int) (*File, string, error) {
 	abspath, err := filepath.Abs(path)
 	if err != nil {
 		return nil, "", err
@@ -182,8 +203,8 @@ func openLogfile(path string, readall bool, failOnMissingFile bool) (*File, stri
 	if err != nil && (failOnMissingFile || !os.IsNotExist(err)) {
 		return nil, "", err
 	}
-	if !readall && file != nil {
-		_, err = file.Seek(0, io.SeekEnd)
+	if seek && file != nil {
+		_, err = file.Seek(seekOffset, seekWhence)
 		if err != nil {
 			if file != nil {
 				file.Close()
diff --git a/tailer/fileTailer_test.go b/tailer/fileTailer_test.go
index 01fbff24..f5bcc677 100644
--- a/tailer/fileTailer_test.go
+++ b/tailer/fileTailer_test.go
@@ -16,6 +16,7 @@ package tailer
 
 import (
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
 	"os/user"
@@ -211,6 +212,31 @@ func TestFileMissingOnStartup(t *testing.T) {
 	expect(t, log, tail.Lines(), "test line 2", 1*time.Second)
 }
 
+func TestFileSeek(t *testing.T) {
+	const logfileName = "grok_exporter_seek_logfile.log"
+	log := NewTestRunLogger(400)
+	tmpDir := mkTmpDirOrFail(t)
+	defer cleanUp(t, tmpDir)
+	var logfile = fmt.Sprintf("%s%c%s", tmpDir, os.PathSeparator, logfileName)
+
+	logFileWriter := newLogFileWriter(t, logfile, closeFileAfterEachLine)
+	logFileWriter.writeLine(t, log, "test line 1")
+
+	tail := RunFseventFileTailerWithSeek(logfile, true, log, 12, io.SeekStart)
+	defer tail.Close()
+
+	// We don't expect errors. However, start a go-routine listening on
+	// the tailer's errorChannel in case something goes wrong.
+	go func() {
+		for err := range tail.Errors() {
+			t.Errorf("Tailer failed: %v", err.Error()) // Cannot call t.Fatalf() in other goroutine.
+		}
+	}()
+
+	logFileWriter.writeLine(t, log, "test line 2")
+	expect(t, log, tail.Lines(), "test line 2", 1*time.Second)
+}
+
 func TestShutdownDuringSyscall(t *testing.T) {
 	runTestShutdown(t, "shutdown while the watcher is hanging in the blocking kevent() or syscall.Read() call")
 }