[Backport 5.4] mv: handle different ERMs for base and view table

[mergify]

When calculating the base-view mapping while the topology is changing, we may encounter a situation where the base table noticed the change in its effective replication map while the view table hasn't, or vice-versa. This can happen because the ERM update may be performed during the preemption between taking the base ERM and view ERM, or, due to f2ff701, the update may have just been performed partially when we are taking the ERMs.

Until now, we assumed that the ERMs are synchronized while calling finding the base-view endpoint mapping, so in particular, we were using the topology from the base's ERM to check the datacenters of all endpoints. Now that the ERMs are more likely to not be the same, we may try to get the datacenter of a view endpoint that doesn't exist in the base's topology, causing us to crash.

This is fixed in the following patch by using the view table's topology for endpoints coming from the view ERM. The mapping resulting from the call might now be a temporary mapping between endpoints in different topologies, but it still maps base and view replicas 1-to-1.

Fixes: #17786
Fixes: #18709

(cherry picked from commit ed95782)

Refs #18816

[mergify]

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Changes to be committed:
	new file:   test/topology_custom/test_mv_topology_change.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   db/view/view.cc
	both modified:   service/storage_service.cc

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[wmitros]

Looks like this backport is not needed after all. The crash was made possible in 9b656ec which uses get_datacenter which doesn't sanitize whether the endpoint we're getting the datacenter of is in its topology, and the crash was made more likely by f2ff701 which introduces a preemption point in the erm update code. As neither is in 5.4, I'm closing this PR