Lack of coverage in RPC v2 CBOR protocol test suite

[david-perez]

I'll use this issue to track lack of coverage in the RPC v2 CBOR protocol test suite: https://github.com/smithy-lang/smithy/tree/main/smithy-protocol-tests/model/rpcv2Cbor; I'll update the issue description as I find issues when finishing my implementation.

• union shapes
  • Containing members targeting Unit.
  • Containing all members targeting Unit.
  • Ensuring that unknown variants are rejected by the server but accepted by the client.
  • Testing both fixed-length and variable-length map encodings.
• structure shapes
  • Test unknown members are ignored (RpcV2CborExtraFieldsInTheBodyShouldBeSkippedByClients, RpcV2CborExtraFieldsInTheBodyShouldBeSkippedByServers).
  • Test that clients and servers reject when encountering additional tokens after deserializing the top-level structure (although we can only really test servers with @httpMalformedRequestTests due to the lack of HTTP malformed response testing #2334). We have to test both when the modeled operation input has no members (empty input) and when it does have some members.
    • We're lacking coverage here in all protocols: Servers incorrectly accept any request body when the modeled operation input is empty or non-existent smithy-rs#3723
• Forbid X-Amzn-Target header in requests.
• Forbid X-Amzn-ErrorType header in responses.
• Test operation error serialization, ensuring __type is serialized as expected.
  • Ensure that __type field is only added by servers in top-level structure shapes corresponding to error responses, not by clients, and not if the error shape is reused elsewhere.
• Server routing tests, rejecting requests with:
  • unroutable URIs (operation not found).
  • missing or invalid Smithy-Protocol and Content-Type headers.
  • forbidden headers (x-amz-target or x-amzn-target).
  • invalid HTTP verbs (only POST is allowed).
• There's no @httpMalformedRequestTests.
  • Constraint traits (including @required).
  • Content-Type and Accept header checking.
  • Test that a malformed CBOR data item gets rejected.
  • Test that servers reject upon encountering first null value in a non-@sparse list/map.
• Test response HTTP status codes, honoring the only supported HTTP binding trait, @httpError.

[mullermp]

Should there be a test similar to "RpcV2CborDeserializesDenseSetMapAndSkipsNull" for lists?

[drganjoo]

Ignore unknown members

We have the following tests to check servers and clients ignore unknown members:

For clients:RpcV2CborExtraFieldsInTheBodyShouldBeSkippedByClients

{_
    "byteValue": 5,
    "doubleValue": 1.889_3,
    "falseBooleanValue": false,
    "floatValue": 7.625_2,
    "extraObject": {_
        "indefiniteLengthMap": {_ "withAnArray": [_ 1, 2, 3]},
        "definiteLengthMap": {
            "withADefiniteArray": [1, 2, 3],
            "andSomeIndefiniteLengthString": "that has, been chunked on comma",
            "normalString": "foo",
        },
        "shortValue": 9999_1,
        "someOtherField": "this should be skipped",
    },
    "integerValue": 256_1,
    "longValue": 9873_1,
    "shortValue": 9898_1,
    "stringValue": "simple",
    "trueBooleanValue": true,
    "blobValue": h'666f6f',
}

For servers: RpcV2CborExtraFieldsInTheBodyShouldBeSkippedByServers:

{_
    "byteValue": 5,
    "doubleValue": 1.889_3,
    "falseBooleanValue": false,
    "floatValue": 7.625_2,
    "extraObject": {_
        "indefiniteLengthMap": {_ "withAnArray": [_ 1, 2, 3]},
        "definiteLengthMap": {
            "withADefiniteArray": [1, 2, 3],
            "andSomeIndefiniteLengthString": "that has, been chunked on comma",
            "normalString": "foo",
        },
        "shortValue": 9999_1,
        "someOtherField": "this should be skipped",
    },
    "integerValue": 256_1,
    "longValue": 9873_1,
    "shortValue": 9898_1,
    "stringValue": "simple",
    "trueBooleanValue": true,
    "blobValue": h'666f6f',
}