Crash when sending empty text/plain request body (precondition(!bytesToWrite.isEmpty)) for generated client operation

[timbms]

Description

Summary

When a generated Swift OpenAPI client sends an empty string as a text/plain request body, the request crashes at runtime with:

Precondition failed: writePendingBytes(_:) must be called with non-empty bytes

This appears to come from the OpenAPI runtime/URLSession transport path (HTTPBodyOutputStreamBridge).

Actual behavior

App crashes with precondition failure:
writePendingBytes(_:) must be called with non-empty bytes.

Reproduction

OpenAPI snippet

paths:
  /items/{itemname}:
    post:
      operationId: sendItemCommand
      parameters:
        - name: itemname
          in: path
          required: true
          schema:
            type: string
      requestBody:
        required: true
        content:
          text/plain:
            schema:
              type: string
      responses:
        "200":
          description: OK

let path = Operations.sendItemCommand.Input.Path(itemname: "MyItem")
let body = Operations.sendItemCommand.Input.Body.plainText(.init(""))
let query = Operations.sendItemCommand.Input.Query()
_ = try await client.sendItemCommand(path: path, query: query, body: body)

Package version(s)

swift-openapi-runtime: 1.8.0
swift-openapi-urlsession: 1.0.2

Expected behavior

No crash.
Either:

• send a valid request with Content-Length: 0, or
• return a normal thrown error if empty body is unsupported.

Environment

Xcode: 26.3 (17C529)
swift-openapi-runtime: 1.8.0
swift-openapi-urlsession: 1.0.2
Generated by swift-openapi-generator

Additional information

No response

[guoye-zhang]

@czechboy0 @simonjbeaumont What's the contract between the OpenAPI generator and the transport? Should we just remove the precondition and return from the function?

[simonjbeaumont]

IMHO the transport should send a request with an empty body. It's what the transport has been told to do by the runtime layer. Whether that's a sensible thing or not is probably not the concern of the transport layer. If it results in an error from the server, then fine, it can propagate that back to the user via the runtime. WDYT @czechboy0

[timbms]

IMHO the transport should send a request with an empty body.

That's exactly what I need: sending a "command" is used here to send a string and indeed the string can be empty, therefore I need to pass an empty body

[czechboy0]

I agree, we should just send an empty body. An HTTPBody is legal, so we can't really constrain it just for this case.

[guoye-zhang]

I agree that the request body can be empty, but should iteration return an empty chunk or nil directly?

[czechboy0]

I think nil directly.

[guoye-zhang]

In that case, the precondition in this code is fine right? Is the bug in openapi-runtime or the client code?

[timbms]

It crashes on the precondition

@guoye-zhang If you want, I can send you my repro project.

[guoye-zhang]

I understand that it crashed in the precondition, but the fix might not be just removing the precondition, but to fix the code path that led to the precondition

[TAKeanice]

Thank you for triaging this. What could you suggest as a workaround for the time until there is a fix?