This is required for a reef client to work with a higher revision MDS, since essentially, this happens:

reef(client):

if (version >=17) {
decode(bal_rank_mask, p);
}

and higher-revision MDS (say, upcoming squid):

version = 17
encode(version, bl);
...
...
encode(max_xattr_size, bl);
encode(bal_rank_mask, bl);

The client incorrectly decodes max_xattr_size (type: uint64_t) into
bal_rank_mask (type: string).

This situation ended up due to a couple of reasons:

• the kclient patchset hanlding max_xattr_size was merged early on and another MDS side change that bumped the MDSMap encoding version to 17 got merged in the midst (PR mds: add mds_bal_rank_mask config option #43284). Details in comment:

    https://github.com/ceph/ceph/pull/46357#issuecomment-1293556227
  

• The reef backport for PR mds: prevent clients from exceeding the xattrs key/value limits #46357 got delayed (and, reef branched out).

Which means reef(18.2.0) user-space clients are broken with higher version MDSs.

Fixes: https://tracker.ceph.com/issues/63713

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