## What changed
- Guard executor-backed MCP stdio sends with a single-permit semaphore so a
second JSON-RPC message cannot start writing while the first write is pending.
- Add a regression test that drives two concurrent sends and verifies the
process receives each newline-delimited message in order without overlap.
GitOrigin-RevId: 30c2e51827ecf57b5eb23d9ad1e876734d6e784e
## Why
Remote MCP stdio buffers executor-provided stdout until a JSON-RPC line
completes and stderr until a diagnostic line completes. A malicious or
broken executor can keep sending chunks without a newline, making
orchestrator memory grow without bound.
Executor event boundaries are arbitrary, so a valid message before an
oversized line must not be lost merely because both arrived in one
event.
## What changed
- Limit one stdout JSON-RPC line to 8 MiB.
- Limit one stderr diagnostic line to 1 MiB.
- Check each logical line before copying its bytes into the retained
buffer.
- Preserve complete stdout messages that precede an oversized line and
deliver them before closing the stream.
- Close the MCP transport on overflow; existing transport cleanup
terminates the executor-managed process.
- Preserve multiple lines per chunk, CRLF framing, and final
unterminated messages at EOF.
- Box the pending stdio transport so the added framing state does not
inflate every `ClientState` value.
- Add regression coverage for cross-chunk overflow, bounded lines in a
larger chunk, and a valid prefix before an oversized line.
## Scope
This bounds pre-newline buffering for executor-backed MCP stdio only. It
does not change local stdio framing or address memory amplification
while parsing a complete JSON message.
## Summary
This PR adds `memchr` for some low-hanging performance improvements
(namely, in MCP stdio, Ollama streaming, and full message-history
newline counts).
Codex produced the following release benchmarks:
| Operation | Before | After | Speedup |
| --- | ---: | ---: | ---: |
| MCP 1 MiB chunked line | 2.172 s | 3.984 ms | 545x |
| Ollama 1 MiB chunked line | 1.673 s | 2.790 ms | 600x |
| Count newlines in 10 MiB history | 132.83 ms | 20.05 ms | 6.6x |
With a "real" MCP setup (`ExecutorStdioServerLauncher` started a Python
MCP server, completed `initialize`, requested `tools/list`, and
deserialized a 1 MiB tool description over newline-delimited stdio),
it's about 16x faster end-to-end:
| Branch | 50 calls | Per call |
| --- | ---: | ---: |
| `main` | 862.53 ms | 17.25 ms |
| this branch | 53.89 ms | 1.08 ms |
`memchr` is already in our dependency tree and extremely widely used for
this kind of optimized scanning.
Adam Perry @ OpenAI