block: introduce trait BlockBackend for block types

This commit introduces the trait `BlockBackend` with generic ops
including read, write and seek, which can be used for common I/O
interfaces for the block types without using `DiskFile` and `AsyncIo`.

Signed-off-by: Yu Li <liyu.yukiteru@bytedance.com>

block/src/qcow/mod.rs

@@ -12,6 +12,7 @@ use crate::qcow::{
     refcount::RefCount,
     vec_cache::{CacheMap, Cacheable, VecCache},
 };
+use crate::BlockBackend;
 use byteorder::{BigEndian, ReadBytesExt, WriteBytesExt};
 use libc::{EINVAL, ENOSPC, ENOTSUP};
 use remain::sorted;
@@ -1555,6 +1556,12 @@ impl SeekHole for QcowFile {
     }
 }
 
+impl BlockBackend for QcowFile {
+    fn size(&self) -> std::result::Result<u64, crate::Error> {
+        Ok(self.virtual_size())
+    }
+}
+
 // Returns an Error if the given offset doesn't align to a cluster boundary.
 fn offset_is_cluster_boundary(offset: u64, cluster_bits: u32) -> Result<()> {
     if offset & ((0x01 << cluster_bits) - 1) != 0 {

block/src/qcow/raw_file.rs

@@ -8,6 +8,7 @@
 //
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
+use crate::BlockBackend;
 use libc::c_void;
 use std::alloc::{alloc_zeroed, dealloc, Layout};
 use std::convert::TryInto;
@@ -343,6 +344,12 @@ impl SeekHole for RawFile {
     }
 }
 
+impl BlockBackend for RawFile {
+    fn size(&self) -> std::result::Result<u64, crate::Error> {
+        Ok(self.metadata().map_err(crate::Error::RawFileError)?.len())
+    }
+}
+
 impl Clone for RawFile {
     fn clone(&self) -> Self {
         RawFile {