defaults for non-optional mutable members

[snosenzo]

Public-Facing Changes

• non-optional fields for mutable structs and unions now receive default values

Description

Most of this is done in the DeserializationInfoCache since we generally want to cache default values for complex types.
Primitive fields receive default values per spec. Default values for fixed size arrays are given
Default unions use a default case if one exists, otherwise they use the case selected by the default value of the switch case type.
Default structs are filled with non-optional members' default values.

Reference: https://www.omg.org/spec/DDS-XTypes/1.3/PDF
7.2.2.4.4.4.7

FG-6554

[achim-k]

My main question is, if we can't compute default values at MessageReader instantiation time (when creating the deserialization info cache). This PR seems to compute (or lookup from a map) default values during message deserialization time which could be avoided when pre-computing them.

[achim-k]

Not sure exactly what this function does, but I don't think you need it. You should be able to just call getFieldDefault recursively if required. Maybe something like this:

  public getFieldDefault(deserInfo: FieldDeserializationInfo): unknown {
    if (deserInfo.defaultValue != undefined) {
      return deserInfo.defaultValue;
    }

    if (deserInfo.typeDeserInfo.type === "primitive") {
      deserInfo.defaultValue = PRIMITIVE_DEFAULT_VALUE_GETTERS.get(deserInfo.type);
      return deserInfo.defaultValue;
    } else if (deserInfo.typeDeserInfo.type === "array-primitive") {
      deserInfo.defaultValue = [];
      return deserInfo.defaultValue;
    } else if (deserInfo.typeDeserInfo.type === "struct") {
      const defaultValue: Record<string, unknown> = {};
      for (const field of deserInfo.typeDeserInfo.fields) {
        defaultValue[field.name] = this.getFieldDefault(field);
      }
      deserInfo.defaultValue = defaultValue;
      return deserInfo.defaultValue;
    } else if (deserInfo.typeDeserInfo.type === "union") {
      // not sure if the union case should be handled here. At this point the type of the union should be resolved already? 
    }

    throw new Error();
  }

[snosenzo]

I know it's a bit verbose, but I wanted to be encapsulate the mutation of the deserInfo in its own function and be clear in the function name that it was updateing the variable. Anyway this is definitely confusing, going to make another pass at it and try to consolidate the complex info handling

[snosenzo]

alright updated to remove this function. hopefully it's a bit more clear now.

[achim-k]

I think you can just pass value: unknown here instead of a getter function?

[snosenzo]

I use getValue instead of value here because I want to prevent assigning all of the values to the same reference if the value is not a primitive.

[achim-k]

Can that be a <string, unknown> map instead?

[snosenzo]

It can, but because this is how the default values are accessed by complex types as well I think it's preferable to keep them consistent.

[achim-k]

in what cases would this be bad?

[snosenzo]

If a message contains several instances of a single default value, and the user of the library mutates a single one of those values, then all of the instances will be changed because they point to the same reference. This also includes default values across all messages read by an instance of this parser.
We don't do this in studio, and it is an added cost to instantiate a new instance of this value each time, but I think that's what users of this library will expect and it will save time fighting against potential errors that could come from manipulating (accidentally or otherwise) these objects sharing the same reference.