In the TestDowngradeUpgradeCluster case, the brand-new cluster is using
simple-config-changer, which means that entries has been committed
before leader election and these entries will be applied when etcdserver
starts to receive apply-requests. The simple-config-changer will mark
the `confState` dirty and the storage backend precommit hook will update
the `confState`.
For the new cluster, the storage version is nil at the beginning. And
it will be v3.5 if the `confState` record has been committed. And it
will be >v3.5 if the `storageVersion` record has been committed.
When the new cluster is ready, the leader will set init cluster version
with v3.6.x. And then it will trigger the `monitorStorageVersion` to
update the `storageVersion` to v3.6.x. If the `confState` record has
been updated before cluster version update, we will get storageVersion
record.
If the storage backend doesn't commit in time, the
`monitorStorageVersion` won't update the version because of `cannot
detect storage schema version: missing confstate information`.
And then we file the downgrade request before next round of
`monitorStorageVersion`(per 4 second), the cluster version will be
v3.5.0 which is equal to the `UnsafeDetectSchemaVersion`'s result.
And we won't see that `The server is ready to downgrade`.
It is easy to reproduce the issue if you use cpuset or taskset to limit
in two cpus.
So, we should wait for the new cluster's storage ready before downgrade
request.
Fixes: #14540
Signed-off-by: Wei Fu <fuweid89@gmail.com>
