Use O_EXCL when creating file as we never overwrite an existing file (#7189)

cmd/posix.go

@@ -1105,8 +1105,9 @@ func (s *posix) WriteAll(volume, path string, buf []byte) (err error) {
 		return errFaultyDisk
 	}
 
-	// Create file if not found
-	w, err := s.openFile(volume, path, os.O_CREATE|os.O_SYNC|os.O_WRONLY)
+	// Create file if not found. Note that it is created with os.O_EXCL flag as the file
+	// always is supposed to be created in the tmp directory with a unique file name.
+	w, err := s.openFile(volume, path, os.O_CREATE|os.O_SYNC|os.O_WRONLY|os.O_EXCL)
 	if err != nil {
 		return err
 	}

cmd/xl-v1-object_test.go

@@ -323,6 +323,11 @@ func TestHealing(t *testing.T) {
 		t.Fatal("HealObject failed")
 	}
 
+	err = os.RemoveAll(path.Join(fsDirs[0], bucket, object, "xl.json"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	// Write xl.json with different modtime to simulate the case where a disk had
 	// gone down when an object was replaced by a new object.
 	xlMetaOutDated := xlMetaPreHeal