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

lib/fs/filesystem_copy_range.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 (
+	"syscall"
+
+	"github.com/syncthing/syncthing/lib/sync"
+)
+
+var (
+	copyRangeMethods = make(map[CopyRangeMethod]copyRangeImplementation)
+	mut              = sync.NewMutex()
+)
+
+type copyRangeImplementation func(src, dst File, srcOffset, dstOffset, size int64) error
+
+func registerCopyRangeImplementation(copyMethod CopyRangeMethod, impl copyRangeImplementation) {
+	mut.Lock()
+	defer mut.Unlock()
+
+	l.Debugln("Registering " + copyMethod.String() + " copyRange method")
+
+	copyRangeMethods[copyMethod] = impl
+}
+
+// CopyRange tries to use the specified method to copy data between two files.
+// Takes size bytes at offset srcOffset from the source file, and copies the data to destination file at offset
+// dstOffset. If required, adjusts the size of the destination file to fit that much data.
+//
+// On Linux/BSD you can ask it to use ioctl and copy_file_range system calls, which if the underlying filesystem supports
+// it tries referencing existing data in the source file, instead of making a copy and taking up additional space.
+//
+// CopyRange does its best to have no effect on src and dst file offsets (copy operation should not affect it).
+func CopyRange(copyMethod CopyRangeMethod, src, dst File, srcOffset, dstOffset, size int64) error {
+	if impl, ok := copyRangeMethods[copyMethod]; ok {
+		return impl(src, dst, srcOffset, dstOffset, size)
+	}
+
+	return syscall.ENOTSUP
+}