Node blocked due to malformed JSON on _count API

[Matthour]

Elasticsearch version:
5.5.1

Plugins installed:
ingest-geoip, ingest-user-agent, x-pack

JVM version:
openjdk version "1.8.0_141"
OpenJDK Runtime Environment (build 1.8.0_141-b16)
OpenJDK 64-Bit Server VM (build 25.141-b16, mixed mode)

OS version:
Linux e012087cca9f 4.9.36-moby #1 SMP Wed Jul 12 15:29:07 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

Description of the problem including expected versus actual behavior:
_count API doesn't return a 400 status "parsing_exception" error when sending data matching ".*" regex pattern.

Steps to reproduce:

docker run -p 9200:9200 -e "http.host=0.0.0.0" -e "transport.host=127.0.0.1" -d docker.elastic.co/elasticsearch/elasticsearch:5.5.1

curl -XGET -u elastic:changeme -H 'Content-Type: application/json' http://localhost:9200/*/_count -d '""'

# (don't forget to kill the container to save your CPU :-)

FYI:

• _search works with the same data ("") returning a 400 status "parsing_exception" error
• _count returns same error on data like a, ", etc. but the problem occurs with data matching ".*" regex pattern

[cbuescher]

@Matthour this is indeed interesting, I just tried it on 5.2.2 and there _count and _search still give the same respone (a 400 with "parse_exception") when the request body is "".
Starting with 5.3.0, _search still returns 400 ("parse_exception") but _count doesn't return.