Auto follow patterns with 'follow_index_pattern' fail to replicate data streams #81751
Description
A auto follow patterns with a rename pattern fails to replicate a data stream:
{
"pattern": {
"remote_cluster": "leader_cluser",
"leader_index_patterns": ["*"],
"follow_index_pattern": "{{leader_index}}_copy"
}
}
The follow error can then occur:
java.lang.IllegalArgumentException: a backing index name in the local and remote cluster must remain the same
The problem is that only the backing index being replicated is renamed, but not the data stream. This then fails internal validation that checks that name of backing indices contains the name of the data stream.
Maybe we can drop this validation (since there are cases when we already allow for this, for example when migrating an alias to data stream) or we should also rename the data stream when this happens (so that naming is what the validation logic expect it to be).