[WASI] Compilation failure on Swift 6.1+ due to missing pthread symbols in WASI toolchains

[tayloraswift]

Summary

In swift-system 1.8.0, compiling for WebAssembly target (wasm32-unknown-wasi) on Swift 6.1+ fails with scope resolution errors for POSIX pthread APIs (pthread_setspecific, pthread_getspecific, pthread_key_create).

Details

In Sources/System/Internals/Exports.swift, TLS storage for WASI is gated on:

#elseif os(WASI) && (swift(<6.1) || !_runtime(_multithreaded))

When compiling with Swift 6.1+ on a standard WASI SDK (wasm32-unknown-wasi):

1. swift(<6.1) evaluates to false.
2. The compiler falls into the #else branch which assumes POSIX pthread_key_t, pthread_key_create, pthread_setspecific, and pthread_getspecific exist.
3. Standard WASI libc does not expose POSIX pthread functions in scope for non-threaded WASI target toolchains.

Compilation Errors

Sources/System/Internals/Exports.swift:221:14: error: cannot find 'pthread_key_create' in scope
Sources/System/Internals/Exports.swift:233:14: error: cannot find 'pthread_setspecific' in scope
Sources/System/Internals/Exports.swift:242:10: error: cannot find 'pthread_getspecific' in scope

Expected Behavior

swift-system should continue using the single-threaded WASI TLS fallback when POSIX pthread APIs are not available in scope for WASI targets.