Warn or fail if runtime field doesn't have emit

[nik9000]

At this point runtime fields require you to call emit to return values. As much as it'd be nice to auto-emit in some obvious cases, we don't now. So maybe we should fail to compile scripts that don't ever call emit. Or at return a warning or something. These scripts are almost certainly an accident. Possibly one from copying one of kibana's "scripted fields".

I'm imagining something like this being ok:

if (doc['a'].value < 10) {
  emit(doc['b'].value * 10);
}

It calls emit in an if. But something like this:

doc['b'].value * 10;

would fail. Or return a warning. Or something. Because its never going to result in a value at all. It can't because you never invoke the emit function in any code path.

[elasticmachine]

Pinging @elastic/es-search (Team:Search)

[javanna]

I tend to see the proposed solution as a way to work around the fact that we don't automatically emit values when we could. I would much rather prefer that we rather focus first on auto-emitting.

[nik9000]

Would you prefer that we leave things as they are for six mon

[javanna]

We have discussed this with the team, we said we are ok going ahead with it. I also opened #73536 to track the need for automatically emitting values. Like I commented there, failing when emit is forgotten, if we can detect it accurately, would be a good improvement, but we also want to raise the importance of automatically emitting values when possible.

[elasticmachine]

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

[rjernst]

Both warning, as well as auto emitting, would be difficult to do in painless in a script agnostic way. It's not impossible (similar in some ways to auto return, as far as detection), but it would require some new designs to designate what gets added to the tree, perhaps by generalizing auto return. Remember that emit is not a language feature, it is just a method exposed on runtime field script classes.

I've added this for discussion by the scripting team.

[stu-elastic]

A straightforward way of doing this would be to track the methods called by a script and expose that to the caller. The caller (rt fields in this case) can then inspect the methods and fails if emit is not among them.