The feature check in `enable_features_on_startup` loads the list
of features that were enabled previously, goes over every one of them
and checks whether each feature is considered supported and whether
there is a corresponding `gms::feature` object for it (i.e. the feature
is "registered"). The second part of the check is unnecessary
and wrong. A feature can be marked as supported but its `gms::feature`
object not be present anymore: after a feature is supported for long
enough (i.e. we only support upgrades from versions that support the
feature), we can consider such a feature to be deprecated.
When a feature is deprecated, its `gms::feature` object is removed and
the feature is always considered enabled which allows to remove some
legacy code. We still consider this feature to be supported and
advertise it in gossip, for the sake of the old nodes which, even
though they always support the feature, they still check whether other
nodes support it.
The problem with the check as it is now is that it disallows moving
features to the disabled list. If one tries to do it, they will find
out that upgrading the node to the new version does not work:
`enable_features_on_startup` will load the feature, notice that it is
not "registered" (there is no `gms::feature` object for it) and fail
to boot.
This commit fixes the problem by modifying `enable_features_on_startup`
not to look at the registered features list at all. In addition to
this, some other small cleanups are performed:
- "LARGE_COLLECTION_DETECTION" is removed from the deprecated features
list. For some reason, it was put there when the feature was being
introduced. It does not break anything because there is
a `gms::feature` object for it, but it's slightly confusing
and therefore is removed.
- The comment in `supported_feature_set` that invites developers to add
features there as they are introduced is removed. It is no longer
necessary to do so because registered features are put there
automatically. Deprecated features should still be put there,
as indicated as another comment.
Fortunately, this issue does not break any upgrades as of now - since
we added enabled cluster feature persisting, no features were
deprecated, and we only add registered features to the persisted feature
list.
An error injection and a regression test is added.
Closes#14701
* github.com:scylladb/scylladb:
topology_custom: add deprecated features test
feature_service: add error injection for deprecated cluster feature
feature_service: move error injection check to helper function
feature_service: handle deprecated features correctly in feature check
8bc42f54d4