Bug #41565: mds: detect MDS<->MDS messages that are not versioned
mds: Add inter MDS messages to the corpus and enforce versioning
Now that the current and the new inter-MDS messages are guaranteed to be of type MMDSOp. MMDSOp could be used to pass information between MDSs in the future too.
But MMDSOp doesn't enforce versioning. As an MDS of release n should be able to talk to at least an MDS of release n-2, it'd be important to have the inter MDS messages collected in corpus to make sure the decoders of "n" release are able to to read the messages emitted by "n-2" release. MMDSOp does not add any runtime/compile time error for "unversioned" inter MDS messages, it just guarantees same type. But if we add the MDS messages to corpus, we will be able to test the backward compatibility of the dencoder of them.
See the discussion: https://github.com/ceph/ceph/pull/31726#pullrequestreview-351937077.