qcow: Add a zero_cluster method to raw file

Zeroing a cluster will be done from more than one place in qcow.rs soon,
add a helper to reduce duplication.

Change-Id: Idb40539f8e4ed2338fc84c0d53b37c913f2d90fe
Signed-off-by: Dylan Reid <dgreid@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1697122
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
(cherry picked from crosvm commit 13c219139504d0a191948fd205c835a2505cadba)
Signed-off-by: Rob Bradford <robert.bradford@intel.com>

qcow/src/qcow.rs

@@ -1065,11 +1065,7 @@ impl QcowFile {
     fn get_new_cluster(&mut self) -> std::io::Result<u64> {
         // First use a pre allocated cluster if one is available.
         if let Some(free_cluster) = self.avail_clusters.pop() {
-            let cluster_size = self.raw_file.cluster_size() as usize;
-            self.raw_file
-                .file_mut()
-                .seek(SeekFrom::Start(free_cluster))?;
-            self.raw_file.file_mut().write_zeroes(cluster_size)?;
+            self.raw_file.zero_cluster(free_cluster)?;
             return Ok(free_cluster);
         }
 

qcow/src/qcow_raw_file.rs

@@ -7,6 +7,7 @@ use std::io::{self, BufWriter, Seek, SeekFrom};
 use std::mem::size_of;
 
 use byteorder::{BigEndian, ReadBytesExt, WriteBytesExt};
+use vmm_sys_util::WriteZeroes;
 
 /// A qcow file. Allows reading/writing clusters and appending clusters.
 #[derive(Debug)]
@@ -133,6 +134,14 @@ impl QcowRawFile {
     pub fn cluster_offset(&self, address: u64) -> u64 {
         address & self.cluster_mask
     }
+
+    /// Zeros out a cluster in the file.
+    pub fn zero_cluster(&mut self, address: u64) -> io::Result<()> {
+        let cluster_size = self.cluster_size as usize;
+        self.file.seek(SeekFrom::Start(address))?;
+        self.file.write_zeroes(cluster_size)?;
+        Ok(())
+    }
 }
 
 impl Clone for QcowRawFile {