Fix relative MCP server spawning on macOS - #42117
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## Why Rust falls back to `fork` when a macOS command combines a relative executable path with a working directory, due to a historical `posix_spawnp` issue. This can make local MCP server startup less reliable. ## What changed - Spawn relative macOS MCP executables directly with `posix_spawn`, preserving the configured path, `argv[0]`, working directory, environment, stdio, and process group. - Keep the existing launcher for `PATH` lookup and executable text files without a shebang. - Use one local child wrapper for legacy and 2026-07-28 protocol framing, with cancellation-safe waiting and cleanup that kills and reaps dropped children. ## Testing Add macOS regression coverage for relative scripts, argument and descriptor preservation, launch errors, cancelled waits, and cleanup after runtime shutdown. GitOrigin-RevId: 1cbc6b90aed6f67e0a38f551964d80496ebc4471
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Fix relative MCP server spawning on macOS
Why
Rust falls back to
forkwhen a macOS command combines a relative executablepath with a working directory, due to a historical
posix_spawnpissue. Thiscan make local MCP server startup less reliable.
What changed
posix_spawn, preservingthe configured path,
argv[0], working directory, environment, stdio, andprocess group.
PATHlookup and executable text files withouta shebang.
cancellation-safe waiting and cleanup that kills and reaps dropped children.
Testing
Add macOS regression coverage for relative scripts, argument and descriptor
preservation, launch errors, cancelled waits, and cleanup after runtime shutdown.