GrokTests unit tests fail with "Unable to find pattern [] in Grok's pattern dictionary"

[alpar-t]

Example build failure

https://scans.gradle.com/s/iepue6nvnpjco/tests/vmr3zh2fll5n6-jtrpes23gv6dm

Reproduction line

does not reproduce locally

./gradlew :libs:elasticsearch-grok:test --tests "org.elasticsearch.grok.GrokTests.testDatePattern" -Dtests.seed=7783EF4A23023948 -Dtests.security.manager=true -Dtests.locale=fur -Dtests.timezone=Europe/Kiev -Dcompiler.java=12 -Druntime.java=12
HomeTestsGrokTests » testDatePatternClose test output (esc)

Example relevant log:

java.lang.IllegalArgumentException: Unable to find pattern [] in Grok's pattern dictionary

Frequency

reproduces ~ 6 times in the past 30 days

/cc @talevy

[elasticmachine]

Pinging @elastic/es-search

[elasticmachine]

Pinging @elastic/es-core-features

[jimczi]

It is related to the ingest grok processor, @talevy can you take a look and reassign if needed ?

[talevy]

I investigated this issue in the past and wasn't sure how to resolve it. I'll take a look.

[andrershov]

"testExponentialExpressions" failed for the same reason on CI:

https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+7.x+multijob-unix-compatibility/os=ubuntu-16.04&&immutable/188/console