Carlos Cuevas 2d67166cae fix: add write lock to prevent concurrent transport writes (#391) ... ## TL;DR Adds a write lock to `SubprocessCLITransport` to prevent concurrent writes from parallel subagents. --- ## Overview When multiple subagents run in parallel and invoke MCP tools, the CLI sends concurrent `control_request` messages. Each handler tries to write a response back to the subprocess stdin at the same time. Trio's `TextSendStream` isn't thread-safe for concurrent access, so this causes `BusyResourceError`. This PR adds an `anyio.Lock` around all write operations (`write()`, `end_input()`, and the stdin-closing part of `close()`). The lock serializes concurrent writes so they happen one at a time. The `_ready` flag is now set inside the lock during `close()` to prevent a TOCTOU race where `write()` checks `_ready`, then `close()` sets it and closes the stream before `write()` actually sends data. --- ## Call Flow ```mermaid flowchart TD A["write()<br/>subprocess_cli.py:505"] --> B["acquire _write_lock<br/>subprocess_cli.py:507"] B --> C["check _ready & stream<br/>subprocess_cli.py:509"] C --> D["_stdin_stream.send()<br/>subprocess_cli.py:523"] E["close()<br/>subprocess_cli.py:458"] --> F["acquire _write_lock<br/>subprocess_cli.py:478"] F --> G["set _ready = False<br/>subprocess_cli.py:479"] G --> H["close _stdin_stream<br/>subprocess_cli.py:481"] I["end_input()<br/>subprocess_cli.py:531"] --> J["acquire _write_lock<br/>subprocess_cli.py:533"] J --> K["close _stdin_stream<br/>subprocess_cli.py:535"] ```		2025-12-04 14:27:01 -08:00
..
conftest.py
test_changelog.py
test_client.py
test_errors.py
test_integration.py
test_message_parser.py
test_sdk_mcp_integration.py
test_streaming_client.py
test_subprocess_buffering.py
test_tool_callbacks.py
test_transport.py	fix: add write lock to prevent concurrent transport writes (#391)	2025-12-04 14:27:01 -08:00
test_types.py