implement wrapper type for custom message param json marshalling

[frrist]

fixes #819

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 31.8%. Comparing base (a473ac6) to head (93aae7e).
Report is 295 commits behind head on master.

Additional details and impacted files

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##           master    #821   +/-   ##
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  Coverage    31.8%   31.8%           
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  Files          39      39           
  Lines        3777    3777           
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  Hits         1202    1202           
  Misses       2429    2429           
  Partials      146     146           

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[iand]

While I think this would work it's feels like it's more complex than it needs to be.

How about this alternate approach. Assuming that all message params are simple structs then you could use reflection to convert the param struct to an equivalent map (like is done on line 380 in this PR). If the value of a param field is a bitfield then you wrap it in a struct with its own custom marshaler. Then you just marshal the map.

This approach relies on message params being simple structs without nested maps of other structs, which is valid as far as I can tell.

[iand]

It would be a lot clearer if you made this a first class function:

type marshaler func(interface{}) ([]byte, error)

func MarshalWithOverrides(v interface{}, overrides map[reflect.Type]marshaler) []byte, error {
  ...
}

The function then uses paramWraperType as an internal implementation detail.

The marshaler returns an error so you can replace the panics with error returns in your bitfield marshaler.

Then you can call it like this:

var bitfieldCountMarshaler = func(v interface{}) ([]byte, error) {
   ...
}

b, err := MarshalWithOverrides(b, map[reflect.Type]marshaler{
   reflect.TypeOf(bitfield.BitField{}): bitfieldCountMarshaler,
})

[placer14]

Given:

Old:
{"Recoveries": [{"Sectors": {"rle": "[10816,31,1407,1,2,23,1,4,1,1,1405,1,3,26,1,1,1,1,521,1,3,915,58,1,2,1343]", "_type": "bitfield", "elemcount": "2349"}, "Deadline": 6, "Partition": 0}]}

I would have made this struct match the existing schema:

var ret = struct {
  Count uint64 `json:elemcount`
  RLE []uint64 `json:rle`
  Type string `json:_type`
}{ Type: "bitfield" }