block: qcow: Add pread_alloc and decompress_cluster helpers

Add two reusable helpers for the compressed cluster read path:

- pread_alloc(fd, offset, len) allocates a buffer and fills it with
  pread_exact, returning the owned Vec.
- decompress_cluster(compressed, cluster_size, decoder) allocates the
  output buffer, decodes via the Decoder trait, and validates that the
  decoder produced exactly cluster_size bytes.

These will be used by QcowSync, QcowAsync, Qcow2Backing, and the
legacy QcowFile.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/qcow/mod.rs

@@ -5,7 +5,7 @@
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
 pub(crate) mod backing;
-mod decoder;
+pub(crate) mod decoder;
 mod header;
 pub(crate) mod metadata;
 pub(crate) mod qcow_raw_file;

block/src/qcow_common.rs

@@ -14,6 +14,8 @@ use std::cmp::min;
 use std::os::fd::RawFd;
 use std::{io, ptr, slice};
 
+use crate::qcow::decoder::Decoder;
+
 // -- Position independent I/O helpers --
 //
 // Duplicated file descriptors share the kernel file description and thus the
@@ -44,6 +46,33 @@ pub fn pread_exact(fd: RawFd, buf: &mut [u8], offset: u64) -> io::Result<()> {
     Ok(())
 }
 
+/// Allocate a buffer and pread exactly `len` bytes at `offset`.
+pub fn pread_alloc(fd: RawFd, offset: u64, len: usize) -> io::Result<Vec<u8>> {
+    let mut buf = vec![0u8; len];
+    pread_exact(fd, &mut buf, offset)?;
+    Ok(buf)
+}
+
+/// Decompress a full QCOW2 cluster from compressed data.
+///
+/// Returns a `cluster_size` byte buffer with the decompressed cluster
+/// content. Fails if the decoder does not produce exactly `cluster_size`
+/// bytes.
+pub fn decompress_cluster(
+    compressed: &[u8],
+    cluster_size: usize,
+    decoder: &dyn Decoder,
+) -> io::Result<Vec<u8>> {
+    let mut decompressed = vec![0u8; cluster_size];
+    let n = decoder
+        .decode(compressed, &mut decompressed)
+        .map_err(|_| io::Error::from_raw_os_error(libc::EIO))?;
+    if n != cluster_size {
+        return Err(io::Error::from_raw_os_error(libc::EIO));
+    }
+    Ok(decompressed)
+}
+
 /// Write all bytes to fd at offset, looping on short writes.
 pub fn pwrite_all(fd: RawFd, buf: &[u8], offset: u64) -> io::Result<()> {
     let mut total = 0usize;