From 7787ea2dcc55f37f1baffe1a89266aa93a7cf14c Mon Sep 17 00:00:00 2001
From: scier <g.sergeykhegay@gmail.com>
Date: Wed, 27 Feb 2019 16:33:38 -0800
Subject: [PATCH] Create log file in append mode (#5497)

---
 logger/logger.go      | 14 ++++----------
 logger/logger_test.go | 23 +++++++++++++++++++++++
 2 files changed, 27 insertions(+), 10 deletions(-)

diff --git a/logger/logger.go b/logger/logger.go
index 7ad1c8069..6250dedd6 100644
--- a/logger/logger.go
+++ b/logger/logger.go
@@ -50,16 +50,10 @@ func SetupLogging(debug, quiet bool, logfile string) {
 
 	var oFile *os.File
 	if logfile != "" {
-		if _, err := os.Stat(logfile); os.IsNotExist(err) {
-			if oFile, err = os.Create(logfile); err != nil {
-				log.Printf("E! Unable to create %s (%s), using stderr", logfile, err)
-				oFile = os.Stderr
-			}
-		} else {
-			if oFile, err = os.OpenFile(logfile, os.O_APPEND|os.O_WRONLY, os.ModeAppend); err != nil {
-				log.Printf("E! Unable to append to %s (%s), using stderr", logfile, err)
-				oFile = os.Stderr
-			}
+		var err error
+		if oFile, err = os.OpenFile(logfile, os.O_CREATE|os.O_APPEND|os.O_WRONLY, os.ModeAppend|0644); err != nil {
+			log.Printf("E! Unable to open %s (%s), using stderr", logfile, err)
+			oFile = os.Stderr
 		}
 	} else {
 		oFile = os.Stderr
diff --git a/logger/logger_test.go b/logger/logger_test.go
index 09c7c82eb..a721cbba7 100644
--- a/logger/logger_test.go
+++ b/logger/logger_test.go
@@ -64,6 +64,29 @@ func TestAddDefaultLogLevel(t *testing.T) {
 	assert.Equal(t, f[19:], []byte("Z I! TEST\n"))
 }
 
+func TestWriteToTruncatedFile(t *testing.T) {
+	tmpfile, err := ioutil.TempFile("", "")
+	assert.NoError(t, err)
+	defer func() { os.Remove(tmpfile.Name()) }()
+
+	SetupLogging(true, false, tmpfile.Name())
+	log.Printf("TEST")
+
+	f, err := ioutil.ReadFile(tmpfile.Name())
+	assert.NoError(t, err)
+	assert.Equal(t, f[19:], []byte("Z I! TEST\n"))
+
+	tmpf, err := os.OpenFile(tmpfile.Name(), os.O_TRUNC, 0644)
+	assert.NoError(t, err)
+	assert.NoError(t, tmpf.Close())
+
+	log.Printf("SHOULD BE FIRST")
+
+	f, err = ioutil.ReadFile(tmpfile.Name())
+	assert.NoError(t, err)
+	assert.Equal(t, f[19:], []byte("Z I! SHOULD BE FIRST\n"))
+}
+
 func BenchmarkTelegrafLogWrite(b *testing.B) {
 	var msg = []byte("test")
 	var buf bytes.Buffer