local: factor PreAllocate and SetSparse to lib/file

lib/file/preallocate_unix.go

@@ -0,0 +1,51 @@
+//+build linux
+
+package file
+
+import (
+	"os"
+	"sync/atomic"
+
+	"github.com/rclone/rclone/fs"
+	"golang.org/x/sys/unix"
+)
+
+var (
+	fallocFlags = [...]uint32{
+		unix.FALLOC_FL_KEEP_SIZE,                             // Default
+		unix.FALLOC_FL_KEEP_SIZE | unix.FALLOC_FL_PUNCH_HOLE, // for ZFS #3066
+	}
+	fallocFlagsIndex int32
+)
+
+// PreAllocate the file for performance reasons
+func PreAllocate(size int64, out *os.File) error {
+	if size <= 0 {
+		return nil
+	}
+	index := atomic.LoadInt32(&fallocFlagsIndex)
+again:
+	if index >= int32(len(fallocFlags)) {
+		return nil // Fallocate is disabled
+	}
+	flags := fallocFlags[index]
+	err := unix.Fallocate(int(out.Fd()), flags, 0, size)
+	if err == unix.ENOTSUP {
+		// Try the next flags combination
+		index++
+		atomic.StoreInt32(&fallocFlagsIndex, index)
+		fs.Debugf(nil, "preAllocate: got error on fallocate, trying combination %d/%d: %v", index, len(fallocFlags), err)
+		goto again
+
+	}
+	// FIXME could be doing something here
+	// if err == unix.ENOSPC {
+	// 	log.Printf("No space")
+	// }
+	return err
+}
+
+// SetSparse makes the file be a sparse file
+func SetSparse(out *os.File) error {
+	return nil
+}