Previously the SetConsistentIndex() is called during the apply workflow,
but it's outside the db transaction. If a commit happens between SetConsistentIndex
and the following apply workflow, and etcd crashes for whatever reason right
after the commit, then etcd commits an incomplete transaction to db.
Eventually etcd runs into the data inconsistency issue.
In this commit, we move the SetConsistentIndex into a txPostLockHook, so
it will be executed inside the transaction lock.
When performing the downgrade operation, users can confirm whether each member
is ready to be downgraded using the field 'storageVersion'. If it's equal to the
'target version' in the downgrade command, then it's ready to be downgraded;
otherwise, the etcd member is still in progress of processing the db file.
To avoid inconsistant behavior during cluster upgrade we are feature
gating persistance behind cluster version. This should ensure that
all cluster members are upgraded to v3.6 before changing behavior.
To allow backporting this fix to v3.5 we are also introducing flag
--experimental-enable-lease-checkpoint-persist that will allow for
smooth upgrade in v3.5 clusters with this feature enabled.
Storage version should follow cluster version. During upgrades this
should be immidiate as storage version can be always upgraded as storage
is backward compatible. During downgrades it will be delayed and will
require time for incompatible changes to be snapshotted.
As storage version change can happen long after cluster is running, we
need to add a step during bootstrap to validate if loaded data can be
understood by migrator.
During review of: https://github.com/etcd-io/etcd/pull/12988 spotted
that PUT is actially writing to v3-backend.
If we are replaying WAL log, it might happened that backend's
applied_index is > than the WAL's log entry. In such situation we should
skip applying on backend V3.
I think both the methods (setVersion, setMembersAttributes) are in
practice idempotent so its not that 'serious' problem, but for
formal correctness adding the proper checks.
