static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset,
    uint64_t *host_offset, uint64_t *bytes, QCowL2Meta **m)
{
    BDRVQcowState *s = bs->opaque;
    int l2_index;
    uint64_t *l2_table;
    uint64_t entry;
    unsigned int nb_clusters;
    int ret;

    uint64_t alloc_cluster_offset;

    trace_qcow2_handle_alloc(qemu_coroutine_self(), guest_offset, *host_offset,
                             *bytes);
    assert(*bytes > 0);

    /*
     * Calculate the number of clusters to look for. We stop at L2 table
     * boundaries to keep things simple.
     */
    nb_clusters =
        size_to_clusters(s, offset_into_cluster(s, guest_offset) + *bytes);

    l2_index = offset_to_l2_index(s, guest_offset);
    nb_clusters = MIN(nb_clusters, s->l2_size - l2_index);

    /* Find L2 entry for the first involved cluster */
    ret = get_cluster_table(bs, guest_offset, &l2_table, &l2_index);
    if (ret < 0) {
        return ret;
    }

    entry = be64_to_cpu(l2_table[l2_index]);

    /* For the moment, overwrite compressed clusters one by one */
    if (entry & QCOW_OFLAG_COMPRESSED) {
        nb_clusters = 1;
    } else {
        nb_clusters = count_cow_clusters(s, nb_clusters, l2_table, l2_index);
    }

    /* This function is only called when there were no non-COW clusters, so if
     * we can't find any unallocated or COW clusters either, something is
     * wrong with our code. */
    assert(nb_clusters > 0);

    ret = qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
    if (ret < 0) {
        return ret;
    }

    /* Allocate, if necessary at a given offset in the image file */
    alloc_cluster_offset = start_of_cluster(s, *host_offset);
    ret = do_alloc_cluster_offset(bs, guest_offset, &alloc_cluster_offset,
                                  &nb_clusters);
    if (ret < 0) {
        goto fail;
    }

    /* Can't extend contiguous allocation */
    if (nb_clusters == 0) {
        *bytes = 0;
        return 0;
    }

    /*
     * Save info needed for meta data update.
     *
     * requested_sectors: Number of sectors from the start of the first
     * newly allocated cluster to the end of the (possibly shortened
     * before) write request.
     *
     * avail_sectors: Number of sectors from the start of the first
     * newly allocated to the end of the last newly allocated cluster.
     *
     * nb_sectors: The number of sectors from the start of the first
     * newly allocated cluster to the end of the area that the write
     * request actually writes to (excluding COW at the end)
     */
    int requested_sectors =
        (*bytes + offset_into_cluster(s, guest_offset))
        >> BDRV_SECTOR_BITS;
    int avail_sectors = nb_clusters
                        << (s->cluster_bits - BDRV_SECTOR_BITS);
    int alloc_n_start = offset_into_cluster(s, guest_offset)
                        >> BDRV_SECTOR_BITS;
    int nb_sectors = MIN(requested_sectors, avail_sectors);
    QCowL2Meta *old_m = *m;

    *m = g_malloc0(sizeof(**m));

    **m = (QCowL2Meta) {
        .next           = old_m,

        .alloc_offset   = alloc_cluster_offset,
        .offset         = start_of_cluster(s, guest_offset),
        .nb_clusters    = nb_clusters,
        .nb_available   = nb_sectors,

        .cow_start = {
            .offset     = 0,
            .nb_sectors = alloc_n_start,
        },
        .cow_end = {
            .offset     = nb_sectors * BDRV_SECTOR_SIZE,
            .nb_sectors = avail_sectors - nb_sectors,
        },
    };
    qemu_co_queue_init(&(*m)->dependent_requests);
    QLIST_INSERT_HEAD(&s->cluster_allocs, *m, next_in_flight);

    *host_offset = alloc_cluster_offset + offset_into_cluster(s, guest_offset);
    *bytes = MIN(*bytes, (nb_sectors * BDRV_SECTOR_SIZE)
                         - offset_into_cluster(s, guest_offset));
    assert(*bytes != 0);

    return 1;

fail:
    if (*m && (*m)->nb_clusters > 0) {
        QLIST_REMOVE(*m, next_in_flight);
    }
    return ret;
}