Preserve typed gRPC code-mode session admission errors (#60496)

## Why

Session-opening failures were returned as strings, preventing callers from inspecting the terminal gRPC admission status directly.

## What changed

- Add `GrpcCodeModeSessionProvider::open_session` returning `GrpcSessionOpenError`, distinguishing admission status codes, admission timeouts, and other setup failures.
- Keep string errors for the existing `CodeModeSessionProvider` trait API.
- Re-export `ProcessOwnedGrpcCodeModeSessionProvider` from `codex-core`.

## Testing

Extend admission tests to assert terminal status codes, cover exhausted `ResourceExhausted` retries, and verify that `DeadlineExceeded` is not retried.

GitOrigin-RevId: 9d7f2e733e12b8bb7def368fa5fc5a09ea2a374e

codex-rs/code-mode/src/grpc_session/deadline.rs

@@ -1,4 +1,5 @@
 use std::fmt::Display;
+use std::fmt::Formatter;
 use std::future::Future;
 use std::time::Duration;
 
@@ -7,6 +8,35 @@ use super::SessionInner;
 const TRANSPORT_TIMEOUT: Duration = Duration::from_secs(60);
 const MAX_ERROR_BYTES: usize = 512;
 
+/// Terminal failure while establishing a new gRPC code-mode session.
+#[derive(Debug, PartialEq, Eq)]
+pub enum GrpcSessionOpenError {
+    /// Final status from OpenSession or the first read of its lease, with the bounded diagnostic.
+    AdmissionStatus { code: tonic::Code, message: String },
+    /// The client deadline elapsed while waiting for admission, including retries.
+    AdmissionTimeout(String),
+    /// Setup failed outside admission, for example during transport setup or tool subscription.
+    Other(String),
+}
+
+impl Display for GrpcSessionOpenError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::AdmissionStatus { message, .. }
+            | Self::AdmissionTimeout(message)
+            | Self::Other(message) => f.write_str(message),
+        }
+    }
+}
+
+impl std::error::Error for GrpcSessionOpenError {}
+
+impl<T: Into<String>> From<T> for GrpcSessionOpenError {
+    fn from(message: T) -> Self {
+        Self::Other(message.into())
+    }
+}
+
 pub(super) async fn startup<T, E: Display>(
     operation: &str,
     request: impl Future<Output = Result<T, E>>,
@@ -85,7 +115,7 @@ pub(super) fn failure(operation: &str, error: impl Display) -> String {
     message
 }
 
-async fn enforce<T, E>(
+pub(super) async fn enforce<T, E>(
     operation: &str,
     runtime_timeout: Duration,
     request: impl Future<Output = Result<T, E>>,
@@ -100,7 +130,7 @@ async fn enforce<T, E>(
     }
 }
 
-enum RequestError<E> {
+pub(super) enum RequestError<E> {
     Failed(E),
     TimedOut(String),
 }

codex-rs/code-mode/src/grpc_session/mod.rs

@@ -27,6 +27,7 @@ use codex_code_mode_protocol::grpc;
 use codex_code_mode_protocol::grpc::code_mode_host_client::CodeModeHostClient;
 use codex_http_client::HttpClientFactory;
 use codex_http_client::OutboundProxyPolicy;
+use futures::TryFutureExt;
 use rand::Rng;
 use tokio::sync::Semaphore;
 use tokio::sync::watch;
@@ -35,6 +36,8 @@ use tokio_util::task::TaskTracker;
 use tonic::transport::Channel;
 use tracing::Instrument;
 
+pub use self::deadline::GrpcSessionOpenError;
+use self::deadline::RequestError;
 use self::operations::WaitSlot;
 use self::state::ClosedCell;
 use self::state::SessionState;
@@ -134,12 +137,22 @@ impl GrpcCodeModeSessionProvider {
         }
     }
 
+    /// Opens a new session and preserves the terminal admission status for the caller.
+    pub async fn open_session(
+        &self,
+        options: CodeModeSessionOptions,
+    ) -> Result<Arc<dyn CodeModeSession>, GrpcSessionOpenError> {
+        let session = Arc::new(reconnect::ReconnectableSession::new(self.clone(), options));
+        session.initialize().await?;
+        Ok(session)
+    }
+
     #[tracing::instrument(name = "code_mode.grpc.open_binding", level = "info", skip_all)]
     async fn open_binding(
         &self,
         options: &CodeModeSessionOptions,
         pinned_mode: Option<CodeModeOutputTruncation>,
-    ) -> Result<Arc<GrpcCodeModeSession>, String> {
+    ) -> Result<Arc<GrpcCodeModeSession>, GrpcSessionOpenError> {
         let client = deadline::startup("transport connection", self.transport.client()).await?;
         let requested_mode = pinned_mode.unwrap_or(options.output_truncation);
         let limits = &options.cell_execution_limits;
@@ -155,7 +168,8 @@ impl GrpcCodeModeSessionProvider {
             || limits.max_heap_size_bytes.is_some())
         .then_some(limits);
         let open_session_span = tracing::info_span!("code_mode.grpc.open_session");
-        let (lease, first, route) = deadline::startup("session opening", async {
+        let operation = "session opening";
+        let (lease, first, route) = deadline::enforce(operation, Duration::ZERO, async {
             let mut retries = 0;
             loop {
                 let mut request = tonic::Request::new(grpc::OpenSessionRequest {
@@ -211,9 +225,16 @@ impl GrpcCodeModeSessionProvider {
             }
         })
         .instrument(open_session_span.clone())
-        .await?;
+        .await
+        .map_err(|error| match error {
+            RequestError::Failed(status) => GrpcSessionOpenError::AdmissionStatus {
+                code: status.code(),
+                message: deadline::failure(operation, status),
+            },
+            RequestError::TimedOut(reason) => GrpcSessionOpenError::AdmissionTimeout(reason),
+        })?;
         let Some(grpc::session_event::Event::Opened(opened)) = first.event else {
-            return Err("gRPC code-mode session lease omitted its opening event".to_string());
+            return Err("gRPC code-mode session lease omitted its opening event".into());
         };
         validate_identifier(&opened.session_id, "session ID")?;
         let host_mode =
@@ -230,13 +251,13 @@ impl GrpcCodeModeSessionProvider {
         {
             return Err(format!(
                 "gRPC code-mode host selected {output_truncation:?} output truncation, requested {requested_mode:?}"
-            ));
+            ).into());
         }
         if pinned_mode.is_some_and(|previous| previous != output_truncation) {
             // Dropping the opening lease releases this incompatible session before subscribing.
             return Err(
                 "gRPC code-mode host changed the session output truncation mode on reconnect"
-                    .to_string(),
+                    .into(),
             );
         }
 
@@ -277,7 +298,7 @@ impl GrpcCodeModeSessionProvider {
                 Ok(response) => response,
                 Err(error) => {
                     let _ = wait_for_watch(inner.request_shutdown()).await;
-                    return Err(error);
+                    return Err(error.into());
                 }
             };
         inner.spawn_tool_subscription(response.into_inner());
@@ -313,11 +334,10 @@ impl CodeModeSessionProvider for GrpcCodeModeSessionProvider {
         &'a self,
         options: CodeModeSessionOptions,
     ) -> CodeModeSessionProviderFuture<'a> {
-        Box::pin(async move {
-            let session = Arc::new(reconnect::ReconnectableSession::new(self.clone(), options));
-            session.initialize().await?;
-            Ok(session as _)
-        })
+        Box::pin(
+            self.open_session(options)
+                .map_err(|error| error.to_string()),
+        )
     }
 }
 

codex-rs/code-mode/src/grpc_session/reconnect.rs

@@ -20,6 +20,7 @@ use tokio_util::sync::CancellationToken;
 
 use super::GrpcCodeModeSession;
 use super::GrpcCodeModeSessionProvider;
+use super::GrpcSessionOpenError;
 use super::SHUTDOWN_ERROR;
 use super::generation;
 use super::generation::GenerationDelegate;
@@ -66,7 +67,7 @@ impl ReconnectableSession {
         }
     }
 
-    pub(super) async fn initialize(&self) -> Result<(), String> {
+    pub(super) async fn initialize(&self) -> Result<(), GrpcSessionOpenError> {
         self.inner.get_or_open_binding().await.map(|_| ())
     }
 }
@@ -151,7 +152,10 @@ impl Drop for ReconnectableSession {
 
 impl ReconnectInner {
     async fn binding_for_request(&self, generation: Option<u64>) -> Result<SessionBinding, String> {
-        let binding = self.get_or_open_binding().await?;
+        let binding = self
+            .get_or_open_binding()
+            .await
+            .map_err(|error| error.to_string())?;
         if generation != Some(binding.generation) {
             return Err(concat!(
                 "The exec runtime was replaced. Stored values and running cells were lost. ",
@@ -162,9 +166,9 @@ impl ReconnectInner {
         Ok(binding)
     }
 
-    async fn get_or_open_binding(&self) -> Result<SessionBinding, String> {
+    async fn get_or_open_binding(&self) -> Result<SessionBinding, GrpcSessionOpenError> {
         if self.shutdown_requested.is_cancelled() {
-            return Err(SHUTDOWN_ERROR.to_string());
+            return Err(SHUTDOWN_ERROR.into());
         }
         if let Some(binding) = self.live_binding() {
             return Ok(binding);
@@ -173,13 +177,13 @@ impl ReconnectInner {
         let _opening_permit = tokio::select! {
             biased;
             _ = self.shutdown_requested.cancelled() => {
-                return Err(SHUTDOWN_ERROR.to_string());
+                return Err(SHUTDOWN_ERROR.into());
             }
             permit = self.opening_permit.acquire() => permit
                 .map_err(|_| "gRPC code-mode session opening coordinator closed".to_string())?,
         };
         if self.shutdown_requested.is_cancelled() {
-            return Err(SHUTDOWN_ERROR.to_string());
+            return Err(SHUTDOWN_ERROR.into());
         }
         if let Some(binding) = self.live_binding() {
             return Ok(binding);
@@ -196,7 +200,7 @@ impl ReconnectInner {
         let session = tokio::select! {
             biased;
             _ = self.shutdown_requested.cancelled() => {
-                return Err(SHUTDOWN_ERROR.to_string());
+                return Err(SHUTDOWN_ERROR.into());
             }
             session = self.provider.open_binding(&self.options, self.output_truncation.get().copied()) => session?,
         };
@@ -217,7 +221,7 @@ impl ReconnectInner {
         };
         if !published {
             let _ = wait_for_watch(binding.session.inner.request_shutdown()).await;
-            return Err(SHUTDOWN_ERROR.to_string());
+            return Err(SHUTDOWN_ERROR.into());
         }
         Ok(binding)
     }

codex-rs/code-mode/src/lib.rs

@@ -3,6 +3,7 @@ mod remote_session;
 
 pub use codex_code_mode_protocol::*;
 pub use grpc_session::GrpcCodeModeSessionProvider;
+pub use grpc_session::GrpcSessionOpenError;
 pub use remote_session::DisabledCodeModeSessionProvider;
 pub use remote_session::ProcessOwnedCodeModeSession;
 pub use remote_session::ProcessOwnedCodeModeSessionProvider;