feature/dynamodb/attributevalue: Inconsistent struct field name marshaled

[jasdel]

The DynamoDB's attributevalue module's caching of a struct type's resolved reflect.Type values and field names based on struct tags does not consider the TagKey option when creating the cache entry. This causes the attributevalue module to cache the first occurrence of a struct type and the TagKey that type was cached with. If subsequent usages of that struct type are used with the (un)marshalers the original cached version will be used. Even if subsequent usages are configured for different TagKey options.

The following items posed in #1486 demonstrate this issue.

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Originally posted by @moogacs in #1486 (comment)

I have encountered the same bug which json tag is not respected when marshalling a struct and it has inconsistent behaviour that it's sometimes respected and sometimes not

go version v1.16
dynamodb v1.9.0 
aws sdk v1.11.1

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Originally posted by @moogacs in #1486 (comment)

To proof what we are talking about, here a demonstration of different behaviours for the same pkg

case 1 :
IDLE case expected behavior using custom marshaler
https://go.dev/play/p/zoqHVu5tVjN

case 2 :
2 go routines do the same thing but one of them using custom tag marshaler and other is not and they lead to the same results
https://go.dev/play/p/HvpkhgDUl63

case 3 :
simply case 2 without go routines and it seems the custom tag marshaler doesn't work and affected by the other one
https://go.dev/play/p/ruZGAaWAgtx

all what you need to look at the printed keys Data and UserID

[jasdel]

Thanks for clarifying the behavior and the situation that you're running into @moogacs. The concurrency example was very helpful in demonstrating this issue. I have a hunch this bug is attributed to the attribute value marshaler field caching of a struct field. This cache is a sync map of the parsed struct fields for the first usage of structs with the DDB attribute value (un)marshalers.

The DDB expression module uses the attributevalue marshalers under the hood, but doesn't have a way to specify alternative TagKeys. So I think what is happening here is that the expressions module calling the attributevalue marshaler, is causing the unexpected field names to be cached for the struct type.

We'll be investigating the best way to fix this. There are two parts that need to be investigated in order to fix this issue I think.
1.) expressions package may need a way to configure the TagKey to be used.
2.) attributevalue's fieldCache needs to keep track of the options the struct type is being used with, e.g. TagKey. Most likely the map key for the cache should be something like the following:

type fieldCacheKey struct{
    typ reflect.Type
    opts structFieldOptions
}

[jasdel]

This will potentially be an issue with the v1 SDK as well.

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