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

lib/fs/filesystem_copy_range_standard.go

@@ -0,0 +1,45 @@
+// 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
+
+import (
+	"io"
+)
+
+func init() {
+	registerCopyRangeImplementation(CopyRangeMethodStandard, copyRangeStandard)
+}
+
+func copyRangeStandard(src, dst File, srcOffset, dstOffset, size int64) error {
+	const bufSize = 4 << 20
+
+	buf := make([]byte, bufSize)
+
+	// TODO: In go 1.15, we should use file.ReadFrom that uses copy_file_range underneath.
+
+	// ReadAt and WriteAt does not modify the position of the file.
+	for size > 0 {
+		if size < bufSize {
+			buf = buf[:size]
+		}
+		n, err := src.ReadAt(buf, srcOffset)
+		if err != nil {
+			if err == io.EOF {
+				return io.ErrUnexpectedEOF
+			}
+			return err
+		}
+		if _, err = dst.WriteAt(buf[:n], dstOffset); err != nil {
+			return err
+		}
+		srcOffset += int64(n)
+		dstOffset += int64(n)
+		size -= int64(n)
+	}
+
+	return nil
+}