GODRIVER-3574 Align BSON interface slice decoding with json package.

[qingyang-hu]

GODRIVER-3574

Summary

1. Align BSON decoding for interface slice with json package.
   • Fix the inconsistent behavior from v1 when the pointed target value contains pre-assigned concrete values.
   • Extend the targeted slice when the capacity is insufficient.
2. Clarify the error message in the case of an unsettable value.

Background & Motivation

[prestonvasquez]

@qingyang-hu We should add the test described in the ticket to to the mongo/ package. Is there a way to update the test to be specific to the bson package (i.e extend TestDefaultValueDecoders)?

[qingyang-hu]

@qingyang-hu We should add the test described in the ticket to to the mongo/ package. Is there a way to update the test to be specific to the bson package (i.e extend TestDefaultValueDecoders)?

Sorry, I forgot to label this PR in draft. It's still in progress. I also need to update the error message wording.

[prestonvasquez]

Nice work 👍

[prestonvasquez]

Could we rename this to "overwriting prepopulated slice?"

[prestonvasquez]

Could we rename this to "overwriting prepopulated slice?"

[prestonvasquez]

This change is really nuanced. Can we add a comment here noting why we do this? "If an element is unsettable and allocated, it must be overwritten."

[prestonvasquez]

I also suggest making the following changes to make this more readable:

isInterfaceSlice := eType.Kind() == reflect.Interface && val.Len() > 0

// If this is not an interface slice with pre-populated elements, we can look
// up the decoder for eType once.
var vDecoder ValueDecoder
if !isInterfaceSlice {
	vDecoder, err = dc.LookupDecoder(eType)
	if err != nil {
		return nil, err
	}
}

for {
	// ...
	if isInterfaceSlice && idx < val.Len() {
		// Decode into an existing interface{} slot.
		
		// ...
	} else {
		// For non-interface slices, or if we've exhuasted the pre-populated
		// slots, we create a fresh value.
		if vDecoder == nil {
			// ...
		}

		//...
	}
}