We currently try to detect a replaced node so to insert it to
endpoints_to_remove when it has no owned tokens left.
However, for each token we first generate a multimap using
get_endpoint_to_token_map_for_reading().
There are 2 problems with that:
1. unless the replaced node owns a single token, this map will not
be empty after erasing one token out of it, since the
token metadata has not changed yet (this is done later with
update_normal_tokens(owned_tokens, endpoint)).
2. generating this map for each token is inefficient, turning this
algorithm complexity to quadratic in the number of tokens...
This change copies the current token_to_endpoint map
to temporary map and erases replaced tokens from it,
while maintaining a set of candidates_for_removal.
After traversing all replaced tokens, we check again
the `token_to_endpoint_map` erasing from `candidates_for_removal`
any endpoint that still owns tokens.
The leftover candidates are endpoints the own no tokens
and so they are added to `hosts_to_remove`.
Fixes#12082
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Closes#12141
c61083852c