swift-subprocess

[Alexander-Ignition]

Description

This PR integrates the official swift-subprocess package into XSTooling.

To maintain flexibility, this dependency is introduced conditionally behind a new feature trait: --traits EnableSubprocess.

Context & Motivation

On Linux environments, the existing implementation suffers from a critical known bug in Swift Corelibs Foundation related to FileHandle.readabilityHandler. It frequently fails to trigger on EOF or completely hangs during asynchronous process I/O tracking.

For reference, see the official tracker issues:

• swift-corelibs-foundation#3275 — readabilityHandler on pipe that is Process output hangs on Linux.
• swift-corelibs-foundation#5197 — Async Process IO hangs 100% of the time on Linux.

The modern swift-subprocess library leverages Swift Concurrency natively (async/await) and completely bypasses these legacy low-level Foundation limitations on Linux.

Platform-specific Requirements:

• Linux: Enabling the EnableSubprocess trait is mandatory to ensure reliable, non-blocking process execution and streaming.
• macOS: Enabling the trait is optional, as macOS uses Darwin's native Foundation where readabilityHandler works as expected.