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

lib/fs/basicfs_copy_range.go

@@ -0,0 +1,24 @@
+// Copyright (C) 2020 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
+
+import (
+	"syscall"
+)
+
+type copyRangeImplementationBasicFile func(src, dst basicFile, srcOffset, dstOffset, size int64) error
+
+func copyRangeImplementationForBasicFile(impl copyRangeImplementationBasicFile) copyRangeImplementation {
+	return func(src, dst File, srcOffset, dstOffset, size int64) error {
+		srcFile, srcOk := src.(basicFile)
+		dstFile, dstOk := dst.(basicFile)
+		if !srcOk || !dstOk {
+			return syscall.ENOTSUP
+		}
+		return impl(srcFile, dstFile, srcOffset, dstOffset, size)
+	}
+}