_field_caps API fails to return results for an index that contains a rank_feature field

[astefan]

For a simple index containing a rank_feature field in its mappings:

{ "mappings": { "properties": { "branches": { "type": "keyword" }, "rank" : { "type": "rank_feature" }}}}

the call to _field_caps (/_field_caps?fields=*) fails silently and responds with

{
    "indices": [
        "test"
    ],
    "fields": {}
}

but, upon close inspection and debugging, the following error is getting thrown when trying to establish if the rank field is aggregatable:

[elasticsearch] org.elasticsearch.transport.RemoteTransportException: [node-0][127.0.0.1:9300][indices:data/read/field_caps[index][s]]
[elasticsearch] Caused by: java.lang.UnsupportedOperationException: [rank_feature] fields do not support sorting, scripting or aggregating
[elasticsearch]         at org.elasticsearch.index.mapper.RankFeatureFieldMapper$RankFeatureFieldType.fielddataBuilder(RankFeatureFieldMapper.java:167) ~[?:?]
[elasticsearch]         at org.elasticsearch.index.mapper.MappedFieldType.isAggregatable(MappedFieldType.java:303) ~[elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.action.fieldcaps.TransportFieldCapabilitiesIndexAction.shardOperation(TransportFieldCapabilitiesIndexAction.java:92) ~[elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.action.fieldcaps.TransportFieldCapabilitiesIndexAction.shardOperation(TransportFieldCapabilitiesIndexAction.java:47) ~[elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.action.support.single.shard.TransportSingleShardAction$1.doRun(TransportSingleShardAction.java:112) ~[elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:769) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
[elasticsearch]         at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [?:?]
[elasticsearch]         at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [?:?]
[elasticsearch]         at java.lang.Thread.run(Thread.java:834) [?:?]

It seems the fielddataBuilder(String fullyQualifiedIndexName) method throws an UnsupportedOperationException while MappedFieldTypes isAggregatable() method catches an IllegalArgumentException when calling fielddataBuilder(). Should isAggregatable deal with UnsupportedOperationExceptions, as well, or fielddataBuidler should have a different logic?

This is related to #44320 where the above failure makes SQL to be unable to execute a simple query.

[elasticmachine]

Pinging @elastic/es-search

[jtibshirani]

I merged #44370 to address the bug. The solution we went with changed fielddataBuilder to throw IllegalArgumentException to be consistent with the method contract.

[acbox]

FYI I hit this bug after upgrading ELK stack to 7.x, the _field_caps API is used by Kibana to create index patterns. It results in all non-default fields for an index containing a single rank_feature field being stripped from the index pattern and thus not being searchable in Kibana.