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

lib/fs/basicfs_copy_range_copyfilerange.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/.
+
+// +build linux
+
+package fs
+
+import (
+	"io"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+func init() {
+	registerCopyRangeImplementation(CopyRangeMethodCopyFileRange, copyRangeImplementationForBasicFile(copyRangeCopyFileRange))
+}
+
+func copyRangeCopyFileRange(src, dst basicFile, srcOffset, dstOffset, size int64) error {
+	for size > 0 {
+		// From MAN page:
+		//
+		// If off_in is not NULL, then off_in must point to a buffer that
+		// specifies the starting offset where bytes from fd_in will be read.
+		// 	The file offset of fd_in is not changed, but off_in is adjusted
+		// appropriately.
+		//
+		// Also, even if explicitly not stated, the same is true for dstOffset
+		n, err := unix.CopyFileRange(int(src.Fd()), &srcOffset, int(dst.Fd()), &dstOffset, int(size), 0)
+		if n == 0 && err == nil {
+			return 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)
+		}
+	}
+	return nil
+}