unistd.h declares threads support even when not enabled

[anuraaga]

I noticed that unistd.h seems to declare support for threads on all wasi-libc builds

https://github.com/WebAssembly/wasi-libc/blob/main/libc-top-half/musl/include/unistd.h#L339

This is despite pthread.h not being available. As far as I can tell, it's not possible to compile a library that has branches based on whether threads are available or not, for example to use a real or no-op mutex.

It seems like guarding on __wasilibc_unmodified_upstream would reflect the current state of wasi-libc. But I know there is the ongoing THREAD_MODEL=posix work as well. It's not obvious to me how to tweak this include file to reflect the default state of wasi-libc while also set the flags when in pthread mode.

Is it possible to fix this so the variables aren't set in the default build?

[sunfishcode]

One option here would be to use ifdef _REENTRANT to do this. I wonder if we'll ever want a mode where threads are supported and _REENTRANT is not defined.

Another option would be to move forward with the new target triple idea that's been tossed around. wasm32-wasi-pthread or so, and then we could get a predefined target macro like __wasi_pthread__ or so.

[sbc100]

Use _REENTRANT (at least for now) seems pretty reasonable to me. Clang currently defines _REENTRANT whenever posix threads is enabled, so it seems like pretty precise correlation (at least today):

https://github.com/llvm/llvm-project/blob/6697140ba1d5ab03ad8102b139f5c543f1d8e616/clang/lib/Basic/Targets/OSTargets.cpp#L47-L48