Allow omitting the Content-Length header on unary requests

[sevlaan]

Fixes #438.

ConnectUnaryChannelHandler sets a Content-Length header on every unary request sent through NIOHTTPClient. NIOHTTPClient speaks HTTP/2 only, where the header is optional: DATA frames are self-delimiting and END_STREAM marks the end of the body. grpc-go and grpc-swift do not send it.

It becomes a problem behind a gRPC proxy that forwards incoming metadata to its own outgoing calls (a common way to propagate auth and tracing headers). Content-Length is not on grpc-go's reserved-header list, so it is forwarded too, and the next hop gets a length that does not match its own body. Envoy then resets the stream with PROTOCOL_ERROR before the handler runs. Moving a client from grpc-swift to connect-swift is enough to trigger this, with no server change.

There is no way to suppress the header from the application today: it is added below the interceptor chain, and the only hook above it is subclassing NIOHTTPClient, which is not viable across a module boundary.

Change

A nested NIOHTTPClient.ContentLength option, passed as the last initializer parameter:

public init(
    host: String,
    port: Int? = nil,
    timeout: TimeInterval? = nil,
    contentLength: ContentLength = .sent
)

.sent is the default and keeps today's behaviour, so nothing changes for existing users. It follows the shape of the existing ProtocolClientConfig.UnaryGET option.

Tests

ConnectNIO had no unit test target, so this adds one: ConnectLibraryTests now also depends on ConnectNIO (one line in Package.swift), and ConnectUnaryChannelHandlerTests drives the handler with an EmbeddedChannel — no server or sockets. Three cases: the header is present by default, absent with .omitted, and Host plus caller headers survive in both modes.

swift build and the new tests pass locally, and swiftlint lint is clean. I could not run make testconformance here (no Go toolchain on this machine), so I am relying on CI for it — the default path is unchanged, so I do not expect any movement there.

Happy to switch to a plain Bool or another spelling if you prefer.

🤖 Generated with Claude Code