[Upgrade Details] Fix corruption when writing upgrade marker file (#3948

) (#3951)

* Truncate marker file before writing

* Add unit test for truncation

(cherry picked from commit 8f543bb)

Co-authored-by: Shaunak Kashyap <[email protected]>

internal/pkg/agent/application/upgrade/marker_access_common.go

@@ -10,7 +10,7 @@ import (
 )
 
 func writeMarkerFileCommon(markerFile string, markerBytes []byte, shouldFsync bool) error {
-	f, err := os.OpenFile(markerFile, os.O_WRONLY|os.O_CREATE, 0600)
+	f, err := os.OpenFile(markerFile, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
 	if err != nil {
 		return fmt.Errorf("failed to open upgrade marker file for writing: %w", err)
 	}

internal/pkg/agent/application/upgrade/marker_access_common_test.go

@@ -0,0 +1,56 @@
+// Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+// or more contributor license agreements. Licensed under the Elastic License;
+// you may not use this file except in compliance with the Elastic License.
+
+package upgrade
+
+import (
+	"math/rand"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestWriteMarkerFileWithTruncation(t *testing.T) {
+	tmpDir := t.TempDir()
+	testMarkerFile := filepath.Join(tmpDir, markerFilename)
+
+	// Write a long marker file
+	err := writeMarkerFileCommon(testMarkerFile, randomBytes(40), true)
+	require.NoError(t, err)
+
+	// Get length of file
+	fileInfo, err := os.Stat(testMarkerFile)
+	require.NoError(t, err)
+	originalSize := fileInfo.Size()
+
+	// Write a shorter marker file
+	err = writeMarkerFileCommon(testMarkerFile, randomBytes(25), true)
+	require.NoError(t, err)
+
+	// Get length of file
+	fileInfo, err = os.Stat(testMarkerFile)
+	require.NoError(t, err)
+	newSize := fileInfo.Size()
+
+	// Make sure shorter file has is smaller in length than
+	// the original long marker file
+	require.Less(t, newSize, originalSize)
+}
+
+func randomBytes(length int) []byte {
+	chars := []rune("ABCDEFGHIJKLMNOPQRSTUVWXYZÅÄÖ" +
+		"abcdefghijklmnopqrstuvwxyzåäö" +
+		"0123456789" +
+		"~=+%^*/()[]{}/!@#$?|")
+
+	var b []byte
+	for i := 0; i < length; i++ {
+		rune := chars[rand.Intn(len(chars))]
+		b = append(b, byte(rune))
+	}
+
+	return b
+}