Several of our classes can be modeled as state machines, where each states has a set of states it is allowed to transition to.

These state machines are often implemented as enum classes, where the set of legal state transitions is, at times, undocumented and unenforced.

Having a generic CRTP state machine class would allow us to:

• have a standard way of defining state machines, making our classes easier to understand
• document and enforce the validity of certain state transitions
• customize the handling of illegal state transitions
• discover existing bugs related to illegal state transitions, and customize our handling of them
• make our classes' state machine part of the class' type itself, rather than a private: implementation detail
• reason about other classes based on the states they present

Some examples of existing state machines:

OplogSyncState

• ✓ documented in replication_coordinator.h
•    not enforced

PrimaryOnlyService::State

•     not documented
•     not enforced

ReplicationCoordinatorImpl::ConfigState

• ✓ documented in replication_coordinator_impl.h
•     not enforced

Transaction::TransactionState

•     not documented
• ✓ enforced

AbstractAsyncComponent::State

• ✓ documented in abstract_async_component.h
•     not enforced

SyncSourceResolver::State

• ✓ documented in sync_source_resolver.h
•     not enforced

BackgroundSync::ProducerState

• ✓ documented in bgsync.h
•     not enforced