Accept zero payloads when decoding a Void input

[k3zi]

Motivation

The Swift SDK encodes a Void input as a single empty payload, whereas the other SDKs send zero payloads for no input. In one place, UpdateWithStart, the Swift SDK does the right thing and sends zero payloads. But on decode the worker rejects zero payloads for Void.self: convertPayloads(as: Void.self) requires exactly one. So a Void update sent via UpdateWithStart is rejected before acceptance, and the client sees notFound even though the workflow runs fine.

Making the encode consistently send zero payloads would break older workers that still expect the payload, so it is left as a separate change.

Modifications

In PayloadConverter.convertPayloads, treat a single Void as carrying no payload and ignore the input payload count, matching the other SDKs, which ignore extra input payloads.

Result

A Void update, signal, or query sent with zero payloads is accepted instead of dropped. The encode is unchanged, so it is safe to roll out in any client / worker order.

Other Issues (will file separately)

• When the input cannot be decoded, the worker rejects the update and the client surfaces notFound, which does not match the other SDKs / APIs that return the conversion failure; the real error is lost.
• The UpdateWithStart client does not use the update outcome returned in the first ExecuteMultiOperation response; it discards it and makes an extra PollWorkflowExecutionUpdate call. The other SDKs reuse the first response.
• Encode Void as zero payloads, matching the other SDKs, instead of a single empty payload. This is a breaking wire change because older workers require the payload, so it needs its own change and a version bump.

Test Plan

updateWithStartVoidInput and updateWithStartVoidInputZeroPayloads in WorkflowUpdateTests.swift; swift test passes.

[FranzBusch]

Can you link where the other SDKs are ignoring single payloads? Also do I understand this correct that in the current SDK this is not a problem but when decoding workflows started by older SDKs?

[k3zi]

They aren't ignoring single payloads per-se. They ignore any extra payloads delivered beyond what they are decoding for and only error when not enough were delivered.

In addition we are incorrectly encoding Void as a payload, when it is sent as zero payloads by other SDKs. I.e. Void / () is overloaded in Swift to mean the absence of a type, so it should be encoded as zero payloads, but we encode it as a single zero-byte payload.

.NET - WorkflowInstance.cs

It is intentional that we discard extra input arguments that the signature doesn't accept.

Ruby - workflow_instance.rb

We want to discard extra arguments if we can.

The change in this PR only fixes the Void sent as empty / zero-byte payload case but I can adjust to be general instead if we want to match all SDKs now in that regard. It would be moving towards leniency at least so not so breaking of a change.