XPath Vars in Case Search

[esoergel]

Product Description

Add a new magic case search parameter prefix, _xpath_var_ which can be used to declare template variables for use in _xpath_query expressions.

The evaluation layers are:

• Process expressions as xpath in formplayer, which returns strings
• Interpolate those strings with the provided variables (python string templating syntax)
• Interpret those strings as CSQL expressions, returning an elasticsearch filter.

This seems like a pretty easy way to break up the massive xpath expressions USH projects use. I'd be interested to hear what app builders think of it before merging. It seemed easier to build than to write up and get feedback on in the abstract (only took a few hours of work). Some thoughts I have from brief testing are:

• I don't love adding another layer of evaluation, especially as this doesn't solve all of the complexity problems
• When testing it out, I realized that I really want the ability to use variables before the xpath evaluation step. This might be doable if we instead treat this as a macro expansion kind of thing during the build step, but that's a different approach entirely.
• At present, variable declarations can't reference other variable declarations. That might be fine, but we could certainly consider adding that capability if it sounds useful. If default search filters have a deterministic ordering (I'm not certain whether that's the case), we could just interpolate top to bottom. Otherwise we'd have to do dependency tree stuff, which sounds nasty.
• I still think a template function in xpath would be a huge win for relatively low effort. This operates similarly, but after the xpath evaluation is complete, so without that available context and a bit less cleanly, in my opinion.

Technical Summary

Feature Flag

Safety Assurance

Safety story

Automated test coverage

QA Plan

Rollback instructions

• This PR can be reverted after deploy with no further considerations

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