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

lib/fs/basicfs_copy_range_sendfile.go

@@ -0,0 +1,69 @@
+// 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/.
+
+// +build !windows,!darwin
+
+package fs
+
+import (
+	"io"
+	"syscall"
+)
+
+func init() {
+	registerCopyRangeImplementation(CopyRangeMethodSendFile, copyRangeImplementationForBasicFile(copyRangeSendFile))
+}
+
+func copyRangeSendFile(src, dst basicFile, srcOffset, dstOffset, size int64) error {
+	// Check that the destination file has sufficient space
+	if fi, err := dst.Stat(); err != nil {
+		return err
+	} else if fi.Size() < dstOffset+size {
+		if err := dst.Truncate(dstOffset + size); err != nil {
+			return err
+		}
+	}
+
+	// Record old dst offset.
+	oldDstOffset, err := dst.Seek(0, io.SeekCurrent)
+	if err != nil {
+		return err
+	}
+	defer func() { _, _ = dst.Seek(oldDstOffset, io.SeekStart) }()
+
+	// Seek to the offset we expect to write
+	if oldDstOffset != dstOffset {
+		if n, err := dst.Seek(dstOffset, io.SeekStart); err != nil {
+			return err
+		} else if n != dstOffset {
+			return io.ErrUnexpectedEOF
+		}
+	}
+
+	for size > 0 {
+		// From the MAN page:
+		//
+		// If offset is not NULL, then it points to a variable holding the file offset from which sendfile() will start
+		// reading data from in_fd. When sendfile() returns, this variable will be set to the offset of the byte
+		// following the last byte that was read. If offset is not NULL, then sendfile() does not modify the current
+		// file offset of in_fd; otherwise the current file offset is adjusted to reflect the number of bytes read from
+		// in_fd.
+		n, err := syscall.Sendfile(int(dst.Fd()), int(src.Fd()), &srcOffset, int(size))
+		if n == 0 && err == nil {
+			err = io.ErrUnexpectedEOF
+		}
+		if err != nil && err != syscall.EAGAIN {
+			return err
+		}
+		// Handle case where err == EAGAIN and n == -1 (it's not clear if that can happen)
+		if n > 0 {
+			size -= int64(n)
+		}
+	}
+
+	_, err = dst.Seek(oldDstOffset, io.SeekStart)
+	return err
+}