Problem/Motivation
The use case is very simple: you have an entity type which is not yet bundleable but was installed. In a subsequent effort, a bundle key and base field is added and the updates are run. However, the check for determining if there are changes between the installed version and the new definition fails due to wrong data used for doing this.
The main culprit is SqlContentEntityStorageSchema::requiresEntityStorageSchemaChanges which calls getEntitySchema() on the new entity type definition. The problem is that the SqlContentEntityStorageSchema, when instantiated, uses the active (installed) field storage definitions and getEntitySchema() throws an exception due to a missing field which is expected due to the table mapping using the new entity definition to determine the list of fields. So essentially we are comparing the new entity definition but using installed field storage definitions.
See SqlContentEntityStorageSchema:978 for the exception and SqlContentEntityStorageSchema:955 for where the table mapping is created using the new entity type definition together with the installed field storage definitions.
Proposed resolution
I would imagine is to do like elsewhere and when calling getEntitySchema() for this purpose, to use the correct, i.e. fresh field definitions.
Remaining tasks
Work the patch.
User interface changes
None
API changes
None
Data model changes
None
