Scripting: JSON parsing and writing in watcher

[stu-elastic]

• Json.load(String)
• Json.dump(Object)
• Json.dump(Object, boolean).

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Watcher)

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Scripting)

[jdconrad]

LGTM. Would prefer @rjernst look too since I wrote a few of the tests.

[jdconrad]

@stu-elastic Thanks for separating the docs out into a different PR. Still LGTM.

[stu-elastic]

@elasticmachine run elasticsearch-ci/2

[rjernst]

This looks good, but we also need to consider how this is documented for users. There should be something in the painless reference or watcher docs?

[rjernst]

LGTM

[stu-elastic]

@elasticmachine run elasticsearch-ci/2

[stu-elastic]

Master (8.0): f43e52d
7.x (7.10): 8a61b95

[agone1]

Hello @stu-elastic !

Is there any chance that same functionality would be added to other painless context beside watcher?

[djmcgreal-cc]

Seems weird just to have it in Watcher. Specifically I was hoping to use it in painless execute API to allow unit testing of the result. Current behaviour prints out a Java .toString which is pretty impossible to parse correctly in clients.

[stu-elastic]

Hi @agone1 and @djmcgreal-cc.

We're being cautious about rolling out this feature in other contexts because parsing JSON can create a lot of objects leading to GC pressure if it's run in, say, an aggregation.

@agone1 What contexts are you interested in? Can you cut a new issue for discussion?

@djmcgreal-cc Good point, I've added it (and some other per-context whitelists) to the execute api in #67035.