This stems from a time where index requests were directly forwarded to
TransportReplicationAction. Nowadays they are wrapped in a BulkShardRequest, and this logic is
obsolete.
In contrast to prior PR (#49647), this PR also fixes (see b3697cc) a situation where the previous
index expression logic had an interesting side effect. For bulk requests (which had resolveIndex
= false), the reroute phase was waiting for the index to appear in case where it was not present,
and for all other replication requests (resolveIndex = true) it would right away throw an
IndexNotFoundException while resolving the name and exit. With #49647, every replication
request was now waiting for the index to appear, which was problematic when the given index
had just been deleted (e.g. deleting a follower index while it's still receiving requests from the
leader, where these requests would now wait up to a minute for the index to appear). This PR
now adds b3697cc on top of that prior PR to make sure to reestablish some of the prior behavior
where the reroute phase waits for the bulk request for the index to appear. That logic was in
place to ensure that when an index was created and not all nodes had learned about it yet, that
the bulk would not fail somewhere in the reroute phase. This is now only restricted to the
situation where the current node has an older cluster state than the one that coordinated the
bulk request (which checks that the index is present). This also means that when an index is
deleted, we will no longer unnecessarily wait up to the timeout for the index o appear, and
instead fail the request.
Closes#20279
c2d316a22f