Currently, the topology coordinator has the
`topology::transition_state::publish_cdc_generation` state responsible
for publishing the already created CDC generations to the user-facing
description tables. This process cannot fail as it would cause some CDC
updates to be missed. On the other hand, we would like to abort the
`publish_cdc_generation` state when bootstrap aborts. Of course, we
could also wait until handling this state finishes, even in the case of
the bootstrap abort, but that would be inefficient. We don't want to
unnecessarily block topology operations by publishing CDC generations.
The solution proposed by this PR is to remove the
`publish_cdc_generation` state completely and introduce a new background
fiber of the topology coordinator -- `cdc_generation_publisher` -- that
continually publishes committed CDC generations.
Apart from introducing the CDC generation publisher, we add
`test_cdc_generation_publishing.py` that verifies its correctness and we
adapt other CDC tests to the new changes.
Fixes#15194Closes#15281
* github.com:scylladb/scylladb:
test: test_cdc: introduce wait_for_first_cdc_generation
test: move cdc_streams_check_and_repair check
test: add test_cdc_generation_publishing
docs: remove information about publish_cdc_generation
raft topology: introduce the CDC generation publisher
system_keyspace: load unpublished_cdc_generations to topology
raft topology: mark committed CDC generations as unpublished
raft topology: add unpublished_cdc_generations to system.topology
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