Method read_clusters

block/src/qcow/backing.rs:84–140

Resolve cluster mappings via metadata then read allocated clusters with pread64.

(&self, address: u64, buf: &mut [u8])

    /// Resolve cluster mappings via metadata then read allocated clusters
    /// with pread64.
    fn read_clusters(&self, address: u64, buf: &mut [u8]) -> io::Result<()> {
        let total_len = buf.len();
        let has_backing = self.backing_file.is_some();

        let mappings = self
            .metadata
            .map_clusters_for_read(address, total_len, has_backing)?;

        let mut buf_offset = 0usize;
        for mapping in mappings {
            match mapping {
                ClusterReadMapping::Zero { length } => {
                    buf[buf_offset..buf_offset + length as usize].fill(0);
                    buf_offset += length as usize;
                }
                ClusterReadMapping::Allocated {
                    offset: host_offset,
                    length,
                } => {
                    pread_exact(
                        self.data_fd.as_raw_fd(),
                        &mut buf[buf_offset..buf_offset + length as usize],
                        host_offset,
                    )?;
                    buf_offset += length as usize;
                }
                ClusterReadMapping::Compressed {
                    host_offset,
                    compressed_size,
                    cluster_offset,
                    length,
                } => {
                    let compressed =
                        pread_alloc(self.data_fd.as_raw_fd(), host_offset, compressed_size)?;
                    let decompressed = decompress_cluster(
                        &compressed,
                        self.cluster_size as usize,
                        &*self.decoder,
                    )?;
                    buf[buf_offset..buf_offset + length]
                        .copy_from_slice(&decompressed[cluster_offset..cluster_offset + length]);
                    buf_offset += length;
                }
                ClusterReadMapping::Backing {
                    offset: backing_offset,
                    length,
                } => {
                    self.backing_file.as_ref().unwrap().read_at(
                        backing_offset,
                        &mut buf[buf_offset..buf_offset + length as usize],
                    )?;
                    buf_offset += length as usize;
                }
            }
        }
        Ok(())
    }
}