games/crates/common/game-realtime-webtransport-lib/src/webtransport_wasm.rs

// file: crates/common/game-realtime-webtransport-lib/src/webtransport_wasm.rs
// version: 3

const CERTIFICATE_HASH_SIZE: usize = 32;
const FRAME_PROTOCOL_ERROR_CODE: u32 = 0x10;
const FRAME_TOO_LARGE_ERROR_CODE: u32 = 0x11;
const PRIMARY_FRAME_HEADER_SIZE: usize = 4;
const SEND_FAILURE_ERROR_CODE: u32 = 0x12;
const SEND_TIMEOUT_ERROR_CODE: u32 = 0x13;
const STREAM_CANCELLED_ERROR_CODE: u32 = 0x14;
const TRACING_TARGET: &str = "games::realtime::webtransport";

/// SHA-256 fingerprint of one certificate accepted by the browser WebTransport client.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct WebTransportCertificateHash {
    bytes: [u8; CERTIFICATE_HASH_SIZE],
}

impl WebTransportCertificateHash {
    /// Creates a fingerprint from an already-computed SHA-256 digest.
    #[must_use]
    pub fn from_sha256(bytes: [u8; CERTIFICATE_HASH_SIZE]) -> Self {
        return Self { bytes };
    }

    /// Returns the exact 32-byte SHA-256 digest.
    #[must_use]
    pub fn as_bytes(&self) -> &[u8; CERTIFICATE_HASH_SIZE] {
        return &self.bytes;
    }
}

/// Browser WebTransport endpoint, certificate pin and reliable-path configuration.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct WebTransportClientConfig {
    endpoint: url::Url,
    certificate_hash: WebTransportCertificateHash,
    transport: crate::WebTransportConfig,
}

impl WebTransportClientConfig {
    /// Parses and validates a secure WebTransport endpoint with one pinned SHA-256 certificate fingerprint.
    pub fn new(endpoint: &str, certificate_hash: WebTransportCertificateHash) -> Result<Self, game_realtime_transport_lib::TransportError> {
        let parsed = match url::Url::parse(endpoint) {
            Ok(value) => value,
            Err(error) => return Err(invalid_configuration(error.to_string())),
        };
        if parsed.scheme() != "https" {
            return Err(invalid_configuration("WebTransport endpoint scheme must be https"));
        }
        if parsed.host().is_none() {
            return Err(invalid_configuration("WebTransport endpoint must contain a host"));
        }
        return Ok(Self { endpoint: parsed, certificate_hash, transport: crate::WebTransportConfig::default() });
    }

    /// Returns a copy with explicit reliable-path limits and deadlines.
    ///
    /// The browser path applies the message-size limit and operation deadlines to connect, primary-stream open and send.
    #[must_use]
    pub fn with_transport_config(mut self, transport: crate::WebTransportConfig) -> Self {
        self.transport = transport;
        return self;
    }

    /// Returns the validated WebTransport endpoint URL.
    #[must_use]
    pub fn endpoint(&self) -> &str {
        return self.endpoint.as_str();
    }

    /// Returns the pinned SHA-256 server-certificate fingerprint.
    #[must_use]
    pub fn certificate_hash(&self) -> &WebTransportCertificateHash {
        return &self.certificate_hash;
    }

    /// Returns the reliable-path limits and deadlines.
    #[must_use]
    pub fn transport_config(&self) -> crate::WebTransportConfig {
        return self.transport;
    }
}

/// Established browser WebTransport session before the primary application stream is selected.
pub struct WebTransportSession {
    inner: web_transport_wasm::Session,
    transport: crate::WebTransportConfig,
}

impl WebTransportSession {
    fn new(inner: web_transport_wasm::Session, transport: crate::WebTransportConfig) -> Self {
        return Self { inner, transport };
    }

    /// Opens the single primary bidirectional stream and adapts it to the transport-neutral realtime contract.
    pub async fn open_primary_connection(self) -> Result<WebTransportConnection, game_realtime_transport_lib::TransportError> {
        let timeout = self.transport.primary_stream_timeout();
        let timeout_millis = match browser_timeout_millis(timeout, "primary_stream_timeout") {
            Ok(value) => value,
            Err(error) => return Err(error),
        };
        let opened = await_with_timeout(self.inner.open_bi(), timeout_millis).await;
        let (sender, receiver) = match opened {
            Some(Ok(value)) => value,
            Some(Err(error)) => {
                let mapped = transport_error(game_realtime_transport_lib::TransportErrorKind::Protocol, error.to_string());
                tracing::warn!(target: TRACING_TARGET, detail = mapped.detail(), "browser WebTransport primary bidirectional stream open failed");
                return Err(mapped);
            },
            None => {
                let mapped = timeout_error("browser WebTransport primary bidirectional stream open", timeout);
                tracing::warn!(target: TRACING_TARGET, timeout_ms = timeout.as_millis(), "browser WebTransport primary bidirectional stream open timed out");
                return Err(mapped);
            },
        };
        tracing::debug!(target: TRACING_TARGET, endpoint = self.inner.url().as_str(), "browser WebTransport primary bidirectional stream opened");
        return Ok(WebTransportConnection::new(self.inner, sender, receiver, self.transport));
    }

    /// Returns the endpoint URL backing this browser WebTransport session.
    #[must_use]
    pub fn endpoint(&self) -> &str {
        return self.inner.url().as_str();
    }

    /// Returns the maximum payload size accepted by the browser WebTransport datagram path.
    #[must_use]
    pub fn max_datagram_size(&self) -> usize {
        return self.inner.max_datagram_size();
    }

    /// Sends one browser WebTransport datagram without reliability or ordering guarantees.
    ///
    /// This capability is intentionally not part of `RealtimeConnection`.
    pub async fn send_datagram(&self, payload: &[u8]) -> Result<(), game_realtime_transport_lib::TransportError> {
        let max_datagram_size = self.max_datagram_size();
        if payload.len() > max_datagram_size {
            return Err(datagram_too_large(payload.len(), max_datagram_size));
        }
        return match self.inner.send_datagram(payload.to_vec().into()).await {
            Ok(()) => Ok(()),
            Err(error) => Err(map_write_error(error)),
        };
    }

    /// Receives one browser WebTransport datagram without reliability or ordering guarantees.
    ///
    /// The caller is responsible for applying any operation deadline required by its use case.
    pub async fn receive_datagram(&self) -> Result<Vec<u8>, game_realtime_transport_lib::TransportError> {
        return match self.inner.recv_datagram().await {
            Ok(payload) => Ok(payload.to_vec()),
            Err(error) => Err(map_read_error(error)),
        };
    }
}

/// Established browser WebTransport connection carrying the single reliable primary stream.
pub struct WebTransportConnection {
    receiver: web_transport_wasm::RecvStream,
    sender: web_transport_wasm::SendStream,
    session: web_transport_wasm::Session,
    transport: crate::WebTransportConfig,
}

impl WebTransportConnection {
    fn new(
        session: web_transport_wasm::Session,
        sender: web_transport_wasm::SendStream,
        receiver: web_transport_wasm::RecvStream,
        transport: crate::WebTransportConfig,
    ) -> Self {
        return Self { receiver, sender, session, transport };
    }
}

impl game_realtime_transport_lib::RealtimeConnection for WebTransportConnection {
    type Receiver = crate::WebTransportReceiver;
    type Sender = crate::WebTransportSender;

    fn split(self) -> (Self::Sender, Self::Receiver) {
        let receiver_session = self.session.clone();
        return (
            crate::WebTransportSender {
                inner: self.sender,
                _session: self.session,
                max_message_size: self.transport.max_message_size(),
                send_timeout: self.transport.send_timeout(),
                terminal: false,
            },
            crate::WebTransportReceiver {
                inner: self.receiver,
                _session: receiver_session,
                max_message_size: self.transport.max_message_size(),
                header: [0_u8; PRIMARY_FRAME_HEADER_SIZE],
                header_read: 0,
                payload: Vec::new(),
                payload_read: 0,
                clean_closed: false,
                terminal: false,
            },
        );
    }
}

/// Receive half of the browser primary reliable WebTransport stream.
pub struct WebTransportReceiver {
    inner: web_transport_wasm::RecvStream,
    _session: web_transport_wasm::Session,
    max_message_size: usize,
    header: [u8; PRIMARY_FRAME_HEADER_SIZE],
    header_read: usize,
    payload: Vec<u8>,
    payload_read: usize,
    clean_closed: bool,
    terminal: bool,
}

impl WebTransportReceiver {
    /// Abruptly stops the reliable receive direction with one WebTransport application error code.
    pub fn abort(&mut self, code: u32) -> Result<(), game_realtime_transport_lib::TransportError> {
        if self.clean_closed || self.terminal {
            return Err(transport_error(game_realtime_transport_lib::TransportErrorKind::Closed, "browser WebTransport receiver is already terminal"));
        }
        self.inner.stop(code);
        self.terminal = true;
        tracing::debug!(target: TRACING_TARGET, code = code, "browser WebTransport primary receive stream aborted");
        return Ok(());
    }

    async fn receive_frame(&mut self) -> Result<game_realtime_transport_lib::TransportReceive, game_realtime_transport_lib::TransportError> {
        if self.clean_closed {
            return Ok(game_realtime_transport_lib::TransportReceive::Closed);
        }
        if self.terminal {
            return Err(transport_error(
                game_realtime_transport_lib::TransportErrorKind::Aborted,
                "browser WebTransport receiver is unavailable after a terminal stream failure or abort",
            ));
        }
        loop {
            if self.header_read < PRIMARY_FRAME_HEADER_SIZE {
                let remaining = PRIMARY_FRAME_HEADER_SIZE - self.header_read;
                let read = self.inner.read(remaining).await;
                match read {
                    Ok(Some(chunk)) => {
                        if chunk.is_empty() {
                            return self.fail_protocol("browser WebTransport primary stream returned an empty read in the middle of a frame header");
                        }
                        let end = self.header_read + chunk.len();
                        self.header[self.header_read..end].copy_from_slice(chunk.as_ref());
                        self.header_read = end;
                        continue;
                    },
                    Ok(None) => {
                        if self.header_read == 0 {
                            self.clean_closed = true;
                            tracing::debug!(target: TRACING_TARGET, "remote browser WebTransport primary stream closed cleanly");
                            return Ok(game_realtime_transport_lib::TransportReceive::Closed);
                        }
                        return self.fail_protocol("browser WebTransport primary stream closed in the middle of a frame header");
                    },
                    Err(error) => return self.fail_read(error),
                }
            }
            if self.payload.is_empty() && self.payload_read == 0 {
                let payload_len = u32::from_be_bytes(self.header) as usize;
                if payload_len > self.max_message_size {
                    let error = message_too_large(payload_len, self.max_message_size);
                    self.stop_after_failure(FRAME_TOO_LARGE_ERROR_CODE);
                    return Err(error);
                }
                if payload_len == 0 {
                    self.reset_frame_state();
                    tracing::trace!(target: TRACING_TARGET, payload_len = 0, "framed browser WebTransport payload received");
                    return Ok(game_realtime_transport_lib::TransportReceive::Message(game_realtime_transport_lib::TransportMessage::new(Vec::new())));
                }
                self.payload = vec![0_u8; payload_len];
            }
            if self.payload_read < self.payload.len() {
                let remaining = self.payload.len() - self.payload_read;
                let read = self.inner.read(remaining).await;
                match read {
                    Ok(Some(chunk)) => {
                        if chunk.is_empty() {
                            return self.fail_protocol("browser WebTransport primary stream returned an empty read in the middle of a frame payload");
                        }
                        let end = self.payload_read + chunk.len();
                        self.payload[self.payload_read..end].copy_from_slice(chunk.as_ref());
                        self.payload_read = end;
                        if self.payload_read < self.payload.len() {
                            continue;
                        }
                    },
                    Ok(None) => return self.fail_protocol("browser WebTransport primary stream closed in the middle of a frame payload"),
                    Err(error) => return self.fail_read(error),
                }
            }
            let payload = core::mem::take(&mut self.payload);
            self.reset_frame_state();
            tracing::trace!(target: TRACING_TARGET, payload_len = payload.len(), "framed browser WebTransport payload received");
            return Ok(game_realtime_transport_lib::TransportReceive::Message(game_realtime_transport_lib::TransportMessage::new(payload)));
        }
    }

    fn fail_protocol(&mut self, detail: &str) -> Result<game_realtime_transport_lib::TransportReceive, game_realtime_transport_lib::TransportError> {
        let error = protocol_error(detail);
        self.stop_after_failure(FRAME_PROTOCOL_ERROR_CODE);
        return Err(error);
    }

    fn fail_read(
        &mut self,
        error: web_transport_wasm::Error,
    ) -> Result<game_realtime_transport_lib::TransportReceive, game_realtime_transport_lib::TransportError> {
        self.terminal = true;
        let mapped = map_read_error(error);
        tracing::warn!(target: TRACING_TARGET, kind = %mapped.kind(), detail = mapped.detail(), "browser WebTransport primary stream receive failed");
        return Err(mapped);
    }

    fn reset_frame_state(&mut self) {
        self.header = [0_u8; PRIMARY_FRAME_HEADER_SIZE];
        self.header_read = 0;
        self.payload.clear();
        self.payload_read = 0;
    }

    fn stop_after_failure(&mut self, code: u32) {
        self.inner.stop(code);
        self.terminal = true;
    }
}

impl Drop for WebTransportReceiver {
    fn drop(&mut self) {
        if !self.clean_closed && !self.terminal {
            self.inner.stop(STREAM_CANCELLED_ERROR_CODE);
            self.terminal = true;
        }
    }
}

impl game_realtime_transport_lib::RealtimeReceiver for WebTransportReceiver {
    type ReceiveFuture<'a>
        = std::pin::Pin<
        Box<dyn core::future::Future<Output = Result<game_realtime_transport_lib::TransportReceive, game_realtime_transport_lib::TransportError>> + 'a>,
    >
    where
        Self: 'a;

    fn receive(&mut self) -> Self::ReceiveFuture<'_> {
        return Box::pin(async move { return self.receive_frame().await });
    }
}

/// Send half of the browser primary reliable WebTransport stream.
pub struct WebTransportSender {
    inner: web_transport_wasm::SendStream,
    _session: web_transport_wasm::Session,
    max_message_size: usize,
    send_timeout: std::time::Duration,
    terminal: bool,
}

impl WebTransportSender {
    /// Abruptly resets the reliable send direction with one WebTransport application error code.
    pub fn abort(&mut self, code: u32) -> Result<(), game_realtime_transport_lib::TransportError> {
        if self.terminal {
            return Err(transport_error(game_realtime_transport_lib::TransportErrorKind::Closed, "browser WebTransport sender is already terminal"));
        }
        self.inner.reset(code);
        self.terminal = true;
        tracing::debug!(target: TRACING_TARGET, code = code, "browser WebTransport primary send stream aborted");
        return Ok(());
    }
}

impl Drop for WebTransportSender {
    fn drop(&mut self) {
        if !self.terminal {
            self.inner.reset(STREAM_CANCELLED_ERROR_CODE);
            self.terminal = true;
        }
    }
}

impl game_realtime_transport_lib::RealtimeSender for WebTransportSender {
    type CloseFuture<'a>
        = std::pin::Pin<Box<dyn core::future::Future<Output = Result<(), game_realtime_transport_lib::TransportError>> + 'a>>
    where
        Self: 'a;
    type SendFuture<'a>
        = std::pin::Pin<Box<dyn core::future::Future<Output = Result<(), game_realtime_transport_lib::TransportError>> + 'a>>
    where
        Self: 'a;

    fn close(&mut self) -> Self::CloseFuture<'_> {
        return Box::pin(async move {
            if self.terminal {
                return Err(transport_error(game_realtime_transport_lib::TransportErrorKind::Closed, "browser WebTransport sender is already terminal"));
            }
            self.terminal = true;
            return match self.inner.finish() {
                Ok(()) => {
                    tracing::debug!(target: TRACING_TARGET, "local browser WebTransport primary stream close initiated");
                    Ok(())
                },
                Err(error) => {
                    let mapped = map_write_error(error);
                    tracing::warn!(target: TRACING_TARGET, detail = mapped.detail(), "browser WebTransport primary stream close failed");
                    Err(mapped)
                },
            };
        });
    }

    fn send(&mut self, message: game_realtime_transport_lib::TransportMessage) -> Self::SendFuture<'_> {
        return Box::pin(async move {
            if self.terminal {
                return Err(transport_error(
                    game_realtime_transport_lib::TransportErrorKind::Aborted,
                    "browser WebTransport sender is unavailable after close, abort, cancellation or terminal send failure",
                ));
            }
            let payload_len = message.len();
            let frame_header = match frame_header(payload_len, self.max_message_size) {
                Ok(value) => value,
                Err(error) => return Err(error),
            };
            let send_timeout = self.send_timeout;
            let timeout_millis = match browser_timeout_millis(send_timeout, "send_timeout") {
                Ok(value) => value,
                Err(error) => return Err(error),
            };
            let mut guard = SendOperationGuard::new(&mut self.inner, &mut self.terminal);
            let operation = guard.write_frame(&frame_header, message.as_bytes());
            let result = await_with_timeout(operation, timeout_millis).await;
            return match result {
                Some(Ok(())) => {
                    guard.complete();
                    tracing::trace!(target: TRACING_TARGET, payload_len = payload_len, "framed browser WebTransport payload sent");
                    Ok(())
                },
                Some(Err(error)) => {
                    guard.abort(SEND_FAILURE_ERROR_CODE);
                    let mapped = map_write_error(error);
                    tracing::warn!(target: TRACING_TARGET, payload_len = payload_len, kind = %mapped.kind(), detail = mapped.detail(), "browser WebTransport framed send failed");
                    Err(mapped)
                },
                None => {
                    guard.abort(SEND_TIMEOUT_ERROR_CODE);
                    let mapped = timeout_error("browser WebTransport framed send", send_timeout);
                    tracing::warn!(target: TRACING_TARGET, payload_len = payload_len, timeout_ms = send_timeout.as_millis(), "browser WebTransport framed send timed out under flow control/backpressure");
                    Err(mapped)
                },
            };
        });
    }
}

struct SendOperationGuard<'a> {
    inner: &'a mut web_transport_wasm::SendStream,
    terminal: &'a mut bool,
    armed: bool,
}

impl<'a> SendOperationGuard<'a> {
    fn new(inner: &'a mut web_transport_wasm::SendStream, terminal: &'a mut bool) -> Self {
        return Self { inner, terminal, armed: true };
    }

    fn abort(&mut self, code: u32) {
        if self.armed {
            self.inner.reset(code);
            *self.terminal = true;
            self.armed = false;
        }
    }

    fn complete(&mut self) {
        self.armed = false;
    }

    async fn write_frame(&mut self, frame_header: &[u8; PRIMARY_FRAME_HEADER_SIZE], payload: &[u8]) -> Result<(), web_transport_wasm::Error> {
        match self.inner.write(frame_header).await {
            Ok(()) => {},
            Err(error) => return Err(error),
        }
        if !payload.is_empty() {
            match self.inner.write(payload).await {
                Ok(()) => {},
                Err(error) => return Err(error),
            }
        }
        return Ok(());
    }
}

impl Drop for SendOperationGuard<'_> {
    fn drop(&mut self) {
        self.abort(STREAM_CANCELLED_ERROR_CODE);
    }
}

/// Establishes one browser WebTransport session using an exact SHA-256 certificate pin.
pub async fn connect(config: &WebTransportClientConfig) -> Result<WebTransportSession, game_realtime_transport_lib::TransportError> {
    if let Err(error) = config.transport.validate() {
        return Err(error);
    }
    if let Err(error) = validate_browser_deadlines(config.transport) {
        return Err(error);
    }
    let client = web_transport_wasm::ClientBuilder::new()
        .with_pooling(false)
        .with_unreliable(true)
        .with_server_certificate_hashes(vec![config.certificate_hash.as_bytes().to_vec()]);
    let timeout = config.transport.connect_timeout();
    let timeout_millis = match browser_timeout_millis(timeout, "connect_timeout") {
        Ok(value) => value,
        Err(error) => return Err(error),
    };
    let connected = await_with_timeout(client.connect(config.endpoint.clone()), timeout_millis).await;
    let session = match connected {
        Some(Ok(value)) => value,
        Some(Err(error)) => {
            let mapped = transport_error(game_realtime_transport_lib::TransportErrorKind::Connect, error.to_string());
            tracing::warn!(
                target: TRACING_TARGET,
                endpoint = config.endpoint.as_str(),
                detail = mapped.detail(),
                "browser WebTransport client connection failed"
            );
            return Err(mapped);
        },
        None => {
            let mapped = timeout_error("browser WebTransport client connect", timeout);
            tracing::warn!(target: TRACING_TARGET, endpoint = config.endpoint.as_str(), timeout_ms = timeout.as_millis(), "browser WebTransport client connection timed out");
            return Err(mapped);
        },
    };
    tracing::info!(target: TRACING_TARGET, endpoint = config.endpoint.as_str(), "browser WebTransport client connected");
    return Ok(WebTransportSession::new(session, config.transport));
}

async fn await_with_timeout<F>(operation: F, timeout_millis: u32) -> Option<F::Output>
where
    F: core::future::Future,
{
    let operation = Box::pin(operation);
    let timeout = Box::pin(gloo_timers::future::TimeoutFuture::new(timeout_millis));
    return match futures_util::future::select(operation, timeout).await {
        futures_util::future::Either::Left((output, _)) => Some(output),
        futures_util::future::Either::Right(((), _)) => None,
    };
}

fn browser_timeout_millis(duration: std::time::Duration, name: &str) -> Result<u32, game_realtime_transport_lib::TransportError> {
    return match u32::try_from(duration.as_millis()) {
        Ok(value) if value > 0 => Ok(value),
        Ok(_) => Err(invalid_configuration(format!("{name} must resolve to at least one browser timer millisecond"))),
        Err(_) => Err(invalid_configuration(format!("{name} exceeds the browser timer range of u32 milliseconds"))),
    };
}

fn timeout_error(operation: &str, timeout: std::time::Duration) -> game_realtime_transport_lib::TransportError {
    return transport_error(
        game_realtime_transport_lib::TransportErrorKind::Timeout,
        format!("{operation} exceeded configured deadline of {} ms", timeout.as_millis()),
    );
}

fn validate_browser_deadlines(config: crate::WebTransportConfig) -> Result<(), game_realtime_transport_lib::TransportError> {
    if let Err(error) = browser_timeout_millis(config.connect_timeout(), "connect_timeout") {
        return Err(error);
    }
    if let Err(error) = browser_timeout_millis(config.primary_stream_timeout(), "primary_stream_timeout") {
        return Err(error);
    }
    if let Err(error) = browser_timeout_millis(config.send_timeout(), "send_timeout") {
        return Err(error);
    }
    return Ok(());
}

fn frame_header(payload_len: usize, max_message_size: usize) -> Result<[u8; PRIMARY_FRAME_HEADER_SIZE], game_realtime_transport_lib::TransportError> {
    if payload_len > max_message_size {
        return Err(message_too_large(payload_len, max_message_size));
    }
    let payload_len = match u32::try_from(payload_len) {
        Ok(value) => value,
        Err(_) => return Err(message_too_large(payload_len, max_message_size)),
    };
    return Ok(payload_len.to_be_bytes());
}

fn invalid_configuration(detail: impl Into<String>) -> game_realtime_transport_lib::TransportError {
    return transport_error(game_realtime_transport_lib::TransportErrorKind::InvalidConfiguration, detail);
}

fn map_read_error(error: web_transport_wasm::Error) -> game_realtime_transport_lib::TransportError {
    let kind = match &error {
        web_transport_wasm::Error::Closed => game_realtime_transport_lib::TransportErrorKind::Closed,
        web_transport_wasm::Error::Session { .. } => game_realtime_transport_lib::TransportErrorKind::Closed,
        web_transport_wasm::Error::Stream(_) => game_realtime_transport_lib::TransportErrorKind::Aborted,
        web_transport_wasm::Error::Unknown(_) => game_realtime_transport_lib::TransportErrorKind::Io,
    };
    return transport_error(kind, error.to_string());
}

fn map_write_error(error: web_transport_wasm::Error) -> game_realtime_transport_lib::TransportError {
    let kind = match &error {
        web_transport_wasm::Error::Closed => game_realtime_transport_lib::TransportErrorKind::Closed,
        web_transport_wasm::Error::Session { .. } => game_realtime_transport_lib::TransportErrorKind::Closed,
        web_transport_wasm::Error::Stream(_) => game_realtime_transport_lib::TransportErrorKind::Aborted,
        web_transport_wasm::Error::Unknown(_) => game_realtime_transport_lib::TransportErrorKind::Io,
    };
    return transport_error(kind, error.to_string());
}

fn datagram_too_large(payload_len: usize, max_datagram_size: usize) -> game_realtime_transport_lib::TransportError {
    return transport_error(
        game_realtime_transport_lib::TransportErrorKind::MessageTooLarge,
        format!("WebTransport datagram payload size {payload_len} exceeds current session maximum {max_datagram_size} bytes"),
    );
}

fn message_too_large(payload_len: usize, max_message_size: usize) -> game_realtime_transport_lib::TransportError {
    return transport_error(
        game_realtime_transport_lib::TransportErrorKind::MessageTooLarge,
        format!("WebTransport framed payload length {payload_len} exceeds configured maximum {max_message_size}"),
    );
}

fn protocol_error(detail: impl Into<String>) -> game_realtime_transport_lib::TransportError {
    return transport_error(game_realtime_transport_lib::TransportErrorKind::Protocol, detail);
}

fn transport_error(kind: game_realtime_transport_lib::TransportErrorKind, detail: impl Into<String>) -> game_realtime_transport_lib::TransportError {
    return game_realtime_transport_lib::TransportError::new(kind, detail);
}