ESQL: can't increase refCount on already released object [ConstantNullBlock[...]]

[alex-spies]

Elasticsearch Version

main, 8.16.3

Installed Plugins

No response

Java Version

N/A

OS Version

N/A

Problem Description

Queries with union types of type boolean/keyword can lead to re-using an already released constant null block in the value source reader operator. This causes an illegal state exception and returns a 500 to the user.

The error may be in ComputeBlockLoaderFactory.constantNulls() (here), where we do not take into account that a constant null block that was once handed out may already have been consumed and closed downstream.

Steps to Reproduce

./gradlew run

curl -u elastic-admin:elastic-password -H "Content-Type: application/json" "127.0.0.1:9200/test1" -XPUT -d '{
  "mappings": {
        "properties": {"truefalse1": {"type": "boolean"}, "truefalse2": {"type": "boolean"}}}}'

curl -u elastic-admin:elastic-password -H "Content-Type: application/json" "127.0.0.1:9200/test2" -XPUT -d '{
  "mappings": {
        "properties": {"truefalse1": {"type": "keyword"}, "truefalse2": {"type": "keyword"}}}}'

curl -u elastic-admin:elastic-password -H "Content-Type: application/json" "127.0.0.1:9200/test3" -XPUT -d '{
  "mappings": {
        "properties": {"truefalse1": {"type": "keyword"}, "truefalse2": {"type": "keyword"}}}}'

curl -u elastic:password -HContent-Type:application/json 'localhost:9200/test1/_doc?refresh' -d '{"truefalse1": null}'

curl -u elastic:password -HContent-Type:application/json 'localhost:9200/test2/_doc?refresh' -d '{"truefalse1": null}' 

curl -u elastic:password -HContent-Type:application/json 'localhost:9200/test3/_doc?refresh' -d '{"foo": 1}'

curl -u elastic-admin:elastic-password -H "Content-Type: application/json" 'localhost:9200/_query?format=txt&error_trace' -d '
{
  "query": "from test* | eval t1 = truefalse1::boolean, t2 = truefalse2::boolean | keep t1, t2"
}'

Logs (if relevant)

No response

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)