"Fix" capnproto serialisation of undefined values, closes #371

[mrbbot]

Previously passing compatibilityDate: undefined or compatibilityFlags: undefined would throw.

This is because the undefineds would make it to the encodeCapnpStruct function:

miniflare/packages/tre/src/plugins/core/index.ts

Lines 172 to 173 in b0e1d06

	compatibilityDate: options.compatibilityDate,
	compatibilityFlags: options.compatibilityFlags,

encodeCapnpStruct distinguishes between { compatibilityDate: undefined } and {}. Calling tre/src/runtime/config/sserve-conf.capnp.js/Worker#setCompatibilityDate(value) with an undefined value throws.

The problem is that undefined may be needed in some cases to signal that a function should be called, e.g. sserve-conf.capnp.js/Worker_Binding_Type#setR2Bucket().

This "fix" checks if the function actually accepts a value before calling it. Because we don't have runtime type information, this is done by looking at the function's source code dynamically. 🤮

Writing out this description has given me an idea for an alternative. We could replace the undefineds in sserve-conf.ts/Worker_Binding_Type with a special Symbol to signal that we wanted to call the function with no arguments. Then just ignore all undefined values in encodeCapnpStruct? What do you think?

[mrbbot]

Rethinking this today, I not sure I want to ship this fix at the moment. Moving this PR to draft until I've got a better solution. 👍

[penalosa]

Rethinking this today, I not sure I want to ship this fix at the moment. Moving this PR to draft until I've got a better solution. 👍

There's definitely something about parsing JS strings that makes me uneasy... Is there any specific reason we don't have runtime type information? It looks like function encodeCapnpStruct(obj: any, struct: Struct) types obj as any, but it's passed as config: Config? Why can't that type information be used?

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[penalosa]

Looks good! My one concern is the typing of obj: any—is there any reason that's not able to be fully typed?