dynamodbattribute.Decoder panics on decoding map with custom-typed key

[DiabuFR]

Please fill out the sections below to help us address your issue.

Version of AWS SDK for Go?

1.25.3

Version of Go (go version)?

go version go1.11.2 windows/amd64

What issue did you see?

dynamodbattribute.Decoder panics when unmarshaling to a Map with a custom key type:

panic: reflect.Value.SetMapIndex: value of type string is not assignable to type dynamodbattribute.Key
goroutine 20 [running]:
testing.tRunner.func1(0xc0001aa100)
	C:/Go/src/testing/testing.go:792 +0x38e
panic(0x74e500, 0xc0000435c0)
	C:/Go/src/runtime/panic.go:513 +0x1c7
reflect.Value.assignTo(0x74e500, 0xc0000435a0, 0x98, 0x7ca720, 0x19, 0x74db80, 0x0, 0x0, 0x0, 0x0)
	C:/Go/src/reflect/value.go:2269 +0x453
reflect.Value.SetMapIndex(0x75f4c0, 0xc000076088, 0x195, 0x74e500, 0xc0000435a0, 0x98, 0x74e500, 0xc0000435b0, 0x198)
	C:/Go/src/reflect/value.go:1526 +0xb5
github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute.(*Decoder).decodeMap(0xc000187da0, 0xc000187d70, 0x75f4c0, 0xc000076088, 0x195, 0x0, 0x75f4c0)
	E:/GoWS/src/github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute/decode.go:497 +0x463
github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute.(*Decoder).decode(0xc000187da0, 0xc000047860, 0x7419a0, 0xc000076088, 0x16, 0x0, 0x0, 0x0, 0x77d020, 0xc000187d01)
	E:/GoWS/src/github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute/decode.go:186 +0x2e5
github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute.(*Decoder).Decode(0xc000187da0, 0xc000047860, 0x7419a0, 0xc000076088, 0x0, 0x0, 0x0, 0x7ad160, 0x7a8301)
	E:/GoWS/src/github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute/decode.go:151 +0x1c3
github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute.UnmarshalMap(0xc000187d70, 0x7419a0, 0xc000076088, 0x8, 0xc000198978)
	E:/GoWS/src/github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute/decode.go:88 +0xa6
...

Steps to reproduce

func TestDecodeMapWithCustomKeyType(t *testing.T) {
	type Key string

	aKey := "aKey"
	aValue := "aValue"

	decodedMap := map[Key]string{}

	attrs := map[string]*dynamodb.AttributeValue{
		aKey: {S: aws.String(aValue)},
	}
	if err := UnmarshalMap(attrs, &decodedMap); err != nil {
		t.Fatal(err)
	}
	if val, ok := decodedMap[Key(aKey)]; !ok {
		t.Fatal("expected key not found in decoded map")
	} else if val != aValue {
		t.Fatal("unexpected value for key in decoded map")
	}
}

Interpretation of what's happening

In decodeMap (decode.go:492), the underlying type of key (reflect.Value) is always a string because we create it from k (string):

for k, av := range avMap {
	key := reflect.ValueOf(k) // <= Here
	elem := reflect.New(v.Type().Elem()).Elem()
	if err := d.decode(av, elem, tag{}); err != nil {
		return err
	}
	v.SetMapIndex(key, elem)
}

Proposed fix

Instead, we should create key based on the type of the map v and set its string value:

for k, av := range avMap {
	key := reflect.New(v.Type().Key()).Elem()
	key.SetString(k)
	elem := reflect.New(v.Type().Elem()).Elem()
	if err := d.decode(av, elem, tag{}); err != nil {
		return err
	}
	v.SetMapIndex(key, elem)
}

[skmcgrail]

Thanks for reporting this bug @qProust. I have created a pull request to incorporate the outlined change. PR: #2870