ALTER tablets KS executes in 2 steps:
1. ALTER KS's cql handler forms a global topo req, and saves data required to execute this req,
2. global topo req is executed by topo coordinator, which reads data attached to the req.
The KS name is among the data attached to the req. There's a time window between these steps where a to-be-altered KS could have been DROPped, which results in topo coordinator forever trying to ALTER a non-existing KS. In order to avoid it, the code has been changed to first check if a to-be-altered KS exists, and if it's not the case, it doesn't perform any schema/tablets mutations, but just removes the global topo req from the coordinator's queue.
BTW. just adding this extra check resulted in broader than expected changes, which is due to the fact that the code is written badly and needs to be refactored - an effort that's already planned under #19126
(I suggest to disable displaying whitespace differences when reviewing this PR).
Fixes: scylladb/scylladb#19576Closesscylladb/scylladb#19666
* github.com:scylladb/scylladb:
tests: ensure ALTER tablets KS doesn't crash if KS doesn't exist
cql: refactor rf_change indentation
Prevent ALTERing non-existing KS with tablets
a3d53bd224