On 02/05/2015 08:58 AM, Max Reitz wrote: > qcow2_alloc_bytes() is a function with insufficient error handling and > an unnecessary goto. This patch rewrites it. > > Signed-off-by: Max Reitz <mre...@redhat.com> > --- > v2: > - s/free_cluster_index/free_byte_index/ [Eric] > - added an assertion at the start of the function that > s->free_byte_offset is either 0 or points to the tail of a cluster > (but never to the start) > - use ROUND_UP() instead of start_of_cluster() + cluster_size [Eric] > - added an assertion that s->free_byte_offset is set before using it > [Eric]
Looks nicer.
> + cluster_end = ROUND_UP(s->free_byte_offset, s->cluster_size);
> + if (!s->free_byte_offset || cluster_end != new_cluster) {
Here, I can prove that the left side of the || makes no difference to
the outcome (using just the right half is sufficient, since
!s->free_byte_offset implies cluster_end == 0, but new_cluster will be
non-zero because we never allocate the header cluster). I'd be fine if
you wanted to respin that and add the comment for the micro-optimized
shorter code; but for maintenance purposes, it's easier to rely on the
explicit condition (even if redundant) than to think about whether a
proof listed in a comment is correct, so I'm also fine leaving things as is.
> + s->free_byte_offset = new_cluster;
At this point, s->free_byte_offset violates the precondition if we just
allocated a cluster, so we have to make sure that we restore the
precondition before exiting...
> + if (offset_into_cluster(s, s->free_byte_offset)) {
> + int ret = qcow2_update_cluster_refcount(bs,
> + s->free_byte_offset >> s->cluster_bits, 1,
> + QCOW2_DISCARD_NEVER);
> + if (ret < 0) {
> + if (new_cluster > 0) {
> + qcow2_free_clusters(bs, new_cluster, s->cluster_size,
> + QCOW2_DISCARD_OTHER);
> + }
> + return ret;
...this early exit only happens if s->free_byte_offset was pointing to a
tail, and there are no other early exits, with the final exit properly
restoring the precondition.
Reviewed-by: Eric Blake <ebl...@redhat.com>
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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