app-server: move transport into dedicated crate (#20545)

## Why

`codex-app-server` currently owns both request-processing code and
transport implementation details. Splitting the transport layer into its
own crate makes that boundary explicit, reduces the amount of
transport-specific dependency surface carried by `codex-app-server`, and
gives future transport work a narrower place to evolve.

## What changed

- Added `codex-app-server-transport` and moved the existing transport
tree into it, including stdio, unix socket, websocket, remote-control
transport, and websocket auth.
- Moved shared transport-facing message types into the new crate so both
the transport implementation and `codex-app-server` use the same
definitions.
- Kept processor-facing connection state and outbound routing in
`codex-app-server`, with the routing tests moved next to that local
wrapper.
- Updated workspace metadata, Bazel crate metadata, and
`codex-app-server` dependencies for the new crate boundary.

## Validation

- `cargo metadata --locked --no-deps`
- `git diff --check`
- Attempted `cargo test -p codex-app-server-transport`, `cargo test -p
codex-app-server`, `just fix -p codex-app-server-transport`, and `just
fix -p codex-app-server`; all were blocked before compilation by the
existing `packageproxy` resolution failure for locked `rustls-webpki =
0.103.13`.
- Attempted Bazel build / lockfile validation; those were blocked by
external fetch failures against BuildBuddy / GitHub while resolving
`v8`.

codex-rs/app-server/src/transport.rs

@@ -0,0 +1,232 @@
+use crate::message_processor::ConnectionSessionState;
+use crate::outgoing_message::OutgoingEnvelope;
+use codex_app_server_protocol::ExperimentalApi;
+use codex_app_server_protocol::ServerRequest;
+use std::collections::HashMap;
+use std::collections::HashSet;
+use std::sync::Arc;
+use std::sync::RwLock;
+use std::sync::atomic::AtomicBool;
+use std::sync::atomic::Ordering;
+use tokio::sync::mpsc;
+use tokio_util::sync::CancellationToken;
+use tracing::warn;
+
+pub use codex_app_server_transport::AppServerTransport;
+pub(crate) use codex_app_server_transport::CHANNEL_CAPACITY;
+pub(crate) use codex_app_server_transport::ConnectionId;
+pub(crate) use codex_app_server_transport::ConnectionOrigin;
+pub(crate) use codex_app_server_transport::OutgoingMessage;
+pub(crate) use codex_app_server_transport::QueuedOutgoingMessage;
+pub(crate) use codex_app_server_transport::RemoteControlHandle;
+pub(crate) use codex_app_server_transport::TransportEvent;
+pub use codex_app_server_transport::app_server_control_socket_path;
+pub use codex_app_server_transport::auth;
+pub(crate) use codex_app_server_transport::start_control_socket_acceptor;
+pub(crate) use codex_app_server_transport::start_remote_control;
+pub(crate) use codex_app_server_transport::start_stdio_connection;
+pub(crate) use codex_app_server_transport::start_websocket_acceptor;
+
+pub(crate) struct ConnectionState {
+    pub(crate) outbound_initialized: Arc<AtomicBool>,
+    pub(crate) outbound_experimental_api_enabled: Arc<AtomicBool>,
+    pub(crate) outbound_opted_out_notification_methods: Arc<RwLock<HashSet<String>>>,
+    pub(crate) session: Arc<ConnectionSessionState>,
+}
+
+impl ConnectionState {
+    pub(crate) fn new(
+        origin: ConnectionOrigin,
+        outbound_initialized: Arc<AtomicBool>,
+        outbound_experimental_api_enabled: Arc<AtomicBool>,
+        outbound_opted_out_notification_methods: Arc<RwLock<HashSet<String>>>,
+    ) -> Self {
+        Self {
+            outbound_initialized,
+            outbound_experimental_api_enabled,
+            outbound_opted_out_notification_methods,
+            session: Arc::new(ConnectionSessionState::new(origin)),
+        }
+    }
+}
+
+pub(crate) struct OutboundConnectionState {
+    pub(crate) initialized: Arc<AtomicBool>,
+    pub(crate) experimental_api_enabled: Arc<AtomicBool>,
+    pub(crate) opted_out_notification_methods: Arc<RwLock<HashSet<String>>>,
+    pub(crate) writer: mpsc::Sender<QueuedOutgoingMessage>,
+    disconnect_sender: Option<CancellationToken>,
+}
+
+impl OutboundConnectionState {
+    pub(crate) fn new(
+        writer: mpsc::Sender<QueuedOutgoingMessage>,
+        initialized: Arc<AtomicBool>,
+        experimental_api_enabled: Arc<AtomicBool>,
+        opted_out_notification_methods: Arc<RwLock<HashSet<String>>>,
+        disconnect_sender: Option<CancellationToken>,
+    ) -> Self {
+        Self {
+            initialized,
+            experimental_api_enabled,
+            opted_out_notification_methods,
+            writer,
+            disconnect_sender,
+        }
+    }
+
+    fn can_disconnect(&self) -> bool {
+        self.disconnect_sender.is_some()
+    }
+
+    pub(crate) fn request_disconnect(&self) {
+        if let Some(disconnect_sender) = &self.disconnect_sender {
+            disconnect_sender.cancel();
+        }
+    }
+}
+
+fn should_skip_notification_for_connection(
+    connection_state: &OutboundConnectionState,
+    message: &OutgoingMessage,
+) -> bool {
+    let Ok(opted_out_notification_methods) = connection_state.opted_out_notification_methods.read()
+    else {
+        warn!("failed to read outbound opted-out notifications");
+        return false;
+    };
+    match message {
+        OutgoingMessage::AppServerNotification(notification) => {
+            if notification.experimental_reason().is_some()
+                && !connection_state
+                    .experimental_api_enabled
+                    .load(Ordering::Acquire)
+            {
+                return true;
+            }
+            let method = notification.to_string();
+            opted_out_notification_methods.contains(method.as_str())
+        }
+        _ => false,
+    }
+}
+
+fn disconnect_connection(
+    connections: &mut HashMap<ConnectionId, OutboundConnectionState>,
+    connection_id: ConnectionId,
+) -> bool {
+    if let Some(connection_state) = connections.remove(&connection_id) {
+        connection_state.request_disconnect();
+        return true;
+    }
+    false
+}
+
+async fn send_message_to_connection(
+    connections: &mut HashMap<ConnectionId, OutboundConnectionState>,
+    connection_id: ConnectionId,
+    message: OutgoingMessage,
+    write_complete_tx: Option<tokio::sync::oneshot::Sender<()>>,
+) -> bool {
+    let Some(connection_state) = connections.get(&connection_id) else {
+        warn!("dropping message for disconnected connection: {connection_id:?}");
+        return false;
+    };
+    let message = filter_outgoing_message_for_connection(connection_state, message);
+    if should_skip_notification_for_connection(connection_state, &message) {
+        return false;
+    }
+
+    let writer = connection_state.writer.clone();
+    let queued_message = QueuedOutgoingMessage {
+        message,
+        write_complete_tx,
+    };
+    if connection_state.can_disconnect() {
+        match writer.try_send(queued_message) {
+            Ok(()) => false,
+            Err(mpsc::error::TrySendError::Full(_)) => {
+                warn!(
+                    "disconnecting slow connection after outbound queue filled: {connection_id:?}"
+                );
+                disconnect_connection(connections, connection_id)
+            }
+            Err(mpsc::error::TrySendError::Closed(_)) => {
+                disconnect_connection(connections, connection_id)
+            }
+        }
+    } else if writer.send(queued_message).await.is_err() {
+        disconnect_connection(connections, connection_id)
+    } else {
+        false
+    }
+}
+
+fn filter_outgoing_message_for_connection(
+    connection_state: &OutboundConnectionState,
+    message: OutgoingMessage,
+) -> OutgoingMessage {
+    let experimental_api_enabled = connection_state
+        .experimental_api_enabled
+        .load(Ordering::Acquire);
+    match message {
+        OutgoingMessage::Request(ServerRequest::CommandExecutionRequestApproval {
+            request_id,
+            mut params,
+        }) => {
+            if !experimental_api_enabled {
+                params.strip_experimental_fields();
+            }
+            OutgoingMessage::Request(ServerRequest::CommandExecutionRequestApproval {
+                request_id,
+                params,
+            })
+        }
+        _ => message,
+    }
+}
+
+pub(crate) async fn route_outgoing_envelope(
+    connections: &mut HashMap<ConnectionId, OutboundConnectionState>,
+    envelope: OutgoingEnvelope,
+) {
+    match envelope {
+        OutgoingEnvelope::ToConnection {
+            connection_id,
+            message,
+            write_complete_tx,
+        } => {
+            let _ =
+                send_message_to_connection(connections, connection_id, message, write_complete_tx)
+                    .await;
+        }
+        OutgoingEnvelope::Broadcast { message } => {
+            let target_connections: Vec<ConnectionId> = connections
+                .iter()
+                .filter_map(|(connection_id, connection_state)| {
+                    if connection_state.initialized.load(Ordering::Acquire)
+                        && !should_skip_notification_for_connection(connection_state, &message)
+                    {
+                        Some(*connection_id)
+                    } else {
+                        None
+                    }
+                })
+                .collect();
+
+            for connection_id in target_connections {
+                let _ = send_message_to_connection(
+                    connections,
+                    connection_id,
+                    message.clone(),
+                    /*write_complete_tx*/ None,
+                )
+                .await;
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+#[path = "transport_tests.rs"]
+mod tests;