Recent changes made WASI targets use the Unix threading implementation, but
WASI does not support threading. When the Unix code tries to call
pthread_create, it fails with EAGAIN, causing libraries like rayon to
panic when trying to initialize their global thread pool.
This fix adds an early return in Thread::new() that checks for WASI and
returns UNSUPPORTED_PLATFORM. This approach:
- Continues using most of the unix.rs code path (less invasive)
- Only requires a small cfg check at the start of Thread::new()
- Can be easily removed once wasi-sdk is updated with the proper fix
The real fix is being tracked in `WebAssembly/wasi-libc#716` which will
change the error code returned by pthread_create to properly indicate
unsupported operations. Once that propagates to wasi-sdk, this workaround
can be removed.
Fixes the regression where rayon-based code (e.g., lopdf in PDF handling)
panicked on WASI after nightly-2025-12-10.
Before fix:
pthread_create returns EAGAIN (error code 6)
ThreadPoolBuildError { kind: IOError(Os { code: 6,
kind: WouldBlock, message: "Resource temporarily unavailable" }) }
After fix:
Thread::new returns Err(io::Error::UNSUPPORTED_PLATFORM)
Libraries can gracefully handle the lack of threading support
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