all: Add copy-on-write filesystem support (fixes #4271) (#6746)

lib/fs/filesystem_copy_range_allwithfallback.go

@@ -0,0 +1,21 @@
+// Copyright (C) 2019 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package fs
+
+func init() {
+	registerCopyRangeImplementation(CopyRangeMethodAllWithFallback, copyRangeAllWithFallback)
+}
+
+func copyRangeAllWithFallback(src, dst File, srcOffset, dstOffset, size int64) error {
+	var err error
+	for _, method := range []CopyRangeMethod{CopyRangeMethodIoctl, CopyRangeMethodCopyFileRange, CopyRangeMethodSendFile, CopyRangeMethodStandard} {
+		if err = CopyRange(method, src, dst, srcOffset, dstOffset, size); err == nil {
+			return nil
+		}
+	}
+	return err
+}