[Canvas] Fix: avoid server crash when no value is found

[monfera]

Summary

Closes #29035

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[ppisljar]

how is it possible that the heap[threadId] does not exist ?

[monfera]

I don't fully grok it at the moment, as mentioned in chat I'd like to learn more about the heap concept here, will reach out to @w33ble who's most familiar with the code.

The error was caused by acting on an object that isn't in this lookup table, so it's safe to avoid that and with that, the server crash.

[w33ble]

This came up in one of Peter's PRs too, but he thought at the time it was related to some other change. I think @chrisdavies did an awesome job explaining what happens in this comment

What's happening is that worker.js is getting lots of calls to onFunctionNotFound, each with the same threadId, but with different params

This happens because something like mapColumn will map over every row, as you can see in the example Stacey provided in the original issue

Once the first one fails, the threadId is removed from the heap in thread/index.js, so the subsequent calls fail to find any heap context for that threadId.

And that's why it was blowing up. In Stacey's example, she was using doesntexist (an invalid function) as the function to run in mapColumn. That rejects, clears the heap, and subsequent heap actions blow up because the id is no longer valid.

[chrisdavies]

LGTM. This does fix the Node crash, but as mentioned, it's a stop-gap until we can figure out how it's happening. I have several hunches that I'm going to look into.