Linux support

[bguidolim]

Summary

Make mcs build and run on Linux. The codebase is already well-structured with Environment and ShellRunner as natural abstraction points, so most changes are conditional compilation guards and constant swaps.

Effort Estimate

Moderate — ~15-20 focused changes, no architectural rewrite needed.

Required Changes

Build Configuration

• Remove or conditionalize .macOS(.v13) in Package.swift (line 7-8) — currently blocks compilation entirely

Conditional Compilation Guards

• Darwin.read() → Glibc.read() in CLIOutput.swift:523 — use #if canImport(Darwin) / #if canImport(Glibc)
• termios VMIN/VTIME indices in CLIOutput.swift:236-237 — macOS uses tuple indices 16/17, Linux uses 6/5. Replace magic numbers with platform-conditional named constants
• CryptoKit → add #if canImport(CryptoKit) with import Crypto fallback (from apple/swift-crypto) in:
  • FileHasher.swift:1
  • SettingsHasher.swift:1
  • SectionValidator.swift:1
  • PackTrustManager.swift:1
• OSAllocatedUnfairLock in ScriptRunner.swift:174 — Apple-only (os framework). Replace with Swift 6 Mutex (Synchronization module) or NSLock wrapper

Homebrew on Linux

• Add Linuxbrew prefix /home/linuxbrew/.linuxbrew to Homebrew.allPrefixes (Homebrew.swift:6)
• Update Environment.swift:59-63 brew prefix fallback to include Linux default path

Verification

• Foundation Process, FileManager, NSString, ObjCBool — these work on Linux via swift-corelibs-foundation (verified in audit)
• Hardcoded POSIX paths (/usr/bin/env, /usr/bin/which, /bin/bash in Constants.swift:36-42) — exist on all standard Linux distros, no change needed
• POSIX APIs (flock, ioctl+TIOCGWINSZ, isatty, open/close, .posixPermissions) — all available on Linux, no change needed

Files Affected

File	Change
Package.swift	Remove/conditionalize macOS platform restriction
Sources/mcs/Core/CLIOutput.swift	#if canImport(Darwin/Glibc) for read(), fix termios indices
Sources/mcs/Core/FileHasher.swift	CryptoKit conditional import
Sources/mcs/Core/SettingsHasher.swift	CryptoKit conditional import
Sources/mcs/Doctor/SectionValidator.swift	CryptoKit conditional import
Sources/mcs/ExternalPack/PackTrustManager.swift	CryptoKit conditional import
Sources/mcs/ExternalPack/ScriptRunner.swift	Replace OSAllocatedUnfairLock
Sources/mcs/Core/Homebrew.swift	Add Linuxbrew prefix
Sources/mcs/Core/Environment.swift	Add Linux brew prefix fallback

Notes

• Distribution remains Homebrew-only (Linuxbrew is officially supported)
• All Foundation bridging types (NSString, ObjCBool, NSNumber, NSHomeDirectory) work via swift-corelibs-foundation
• POSIX APIs (flock, ioctl, termios, isatty) are all available on Linux
• No architectural changes required — just conditional compilation