Add Expression Function to Read File

[mfleader]

Changes introduced with this PR

• Add a function to the expression language to read a file into a string, and return that string
• Add unit tests

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[jaredoconnell]

Seems straightforward. It doesn't include any of our ideas for improving the security by pre-validating permission to access files outside of the working directory, but more prerequisites are required to make that work.

Let's see if Webb or Dave have any comments.

[webbnh]

On its face, the change looks good (except possibly for a nit or two).

I'm not sure what other constraints we want to place on file access. Is there a canonical list of permitted/disallowed locations that we should be checking?

[mfleader]

I'm not sure what other constraints we want to place on file access. Is there a canonical list of permitted/disallowed locations that we should be checking?

That's a great question. I do not know what we should disallow.

[webbnh]

I'm not sure what other constraints we want to place on file access. Is there a canonical list of permitted/disallowed locations that we should be checking?

That's a great question. I do not know what we should disallow.

We cannot enforce a policy which we do not have. Ship it! (Unless you want to tweak the nits, first....)

[webbnh]

🚢

[dbutenhof]

This is cool. I can't help wondering if it'd be useful to be able to read an object, e.g. from a YAML file, perhaps even with reference to a schema for validation; but even if that's desirable, it can wait.