Write files in O_SYNC for fs backend to protect against machine crashes (#9434)

cmd/fs-v1-helpers.go

@@ -321,7 +321,7 @@ func fsCreateFile(ctx context.Context, filePath string, reader io.Reader, buf []
 		return 0, err
 	}
 
-	writer, err := lock.Open(filePath, os.O_CREATE|os.O_WRONLY, 0666)
+	writer, err := lock.Open(filePath, os.O_CREATE|os.O_WRONLY|os.O_SYNC, 0666)
 	if err != nil {
 		return 0, osErrToFSFileErr(err)
 	}

pkg/ioutil/append-file_nix.go

@@ -25,7 +25,7 @@ import (
 
 // AppendFile - appends the file "src" to the file "dst"
 func AppendFile(dst string, src string) error {
-	appendFile, err := os.OpenFile(dst, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0666)
+	appendFile, err := os.OpenFile(dst, os.O_WRONLY|os.O_APPEND|os.O_CREATE|os.O_SYNC, 0666)
 	if err != nil {
 		return err
 	}