Implicit event stream payload members

[drganjoo]

Summary

• Fix response payload ownership for operations with distinct input/output stream types
• Handle implicit event payload members (members with neither @eventHeader nor @eventPayload)

Ownership

payloadMetadata() used operationShape.isInputEventStream(model) to determine takesOwnership for both request and response serializers. For operations where the input has a @streaming event stream but the output has a non-streaming union payload, the response serializer incorrectly generated fn(payload: Option<T>) (owned), then passed the owned value to a function expecting &T.

For example:

  operation DuplexStreamWithDistinctStreams {
      input: DuplexInput
      output: DuplexOutput
  }

  structure DuplexInput {
      @httpPayload
      stream: EventStream        // @streaming union
  }

  structure DuplexOutput {
      @httpPayload
      stream: SingletonStream    // non-streaming union
  }

The response serializer checked isInputEventStream (true, because of EventStream) to decide ownership for the output's SingletonStream, causing a type mismatch in the generated Rust.

Fix: Check isInputEventStream for requests and isOutputEventStream for responses.

Implicit Payload

Per the https://smithy.io/2.0/spec/streaming.html#eventpayload-trait, event struct members with neither @eventHeader nor @eventPayload are collectively serialized as a protocol-specific document in the message body. The unmarshaller handled @eventHeader and @eventPayload members but never parsed the body for these implicit members, leaving their set_* builder methods as dead code (fails with -D warnings).

There are three valid member combinations in an event struct:

│               Case                │                 Example                  │                          Handled by                          │
├───────────────────────────────────┼──────────────────────────────────────────┼──────────────────────────────────────────────────────────────┤
│ All @eventPayload/@eventHeader    │ @eventHeader header + @eventPayload data │ renderUnmarshallEventPayload + renderUnmarshallEventHeader   │
├───────────────────────────────────┼──────────────────────────────────────────┼──────────────────────────────────────────────────────────────┤
│ No traits on any member           │ fieldA + fieldB (all implicit)           │ payloadOnly branch — parses entire body as protocol document │
├───────────────────────────────────┼──────────────────────────────────────────┼──────────────────────────────────────────────────────────────┤
│ @eventHeader + trait-less members │ @eventHeader header + payload (implicit) │ Was broken — headers parsed, body ignored                    │
└───────────────────────────────────┴──────────────────────────────────────────┴──────────────────────────────────────────────────────────────┘

For case 3, the wire format looks like:
Event stream message headers:

    header = "my-header-value"          <-- from @eventHeader

Event stream message body (JSON):

    {"payload": "my-payload-value"}     <-- implicit members as JSON document

Fix: In the else branch, after handling @eventPayload and @eventHeader members, detect implicit members and parse them from the message body using the existing structure parser into the builder.

[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• Server Test (ignoring whitespace)
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.

[jlizen]

lgtm

[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• Server Test (ignoring whitespace)
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.

[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• Server Test (ignoring whitespace)
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.

[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• Server Test (ignoring whitespace)
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.