Currently, it is not allowed to first create a new index in the read-only repository, and then in the read-write repository. Trying to do so will fail with "OakConstraint0001: /oak:index/.../:oak:mount-readOnlyV1-index-data[]: The primary type null does not exist"
OAK-7917: oak-lucene/src/test/java/org/apache/jackrabbit/oak/composite - CompositeNodeStoreLuceneIndexTest.java tryAddIndexInReadWriteWithIndexExistinginReadOnly line 112.
It would be better to allow this use case, to reduce the possibility of problems.
We should specially test with lucene indexes, but also with property indexes. (If that's more complicated, we can concentrate on the lucene case first.)