Mutex::try_lock doesn't error in wasm32-emscripten

[curiousdannii]

Mutex::try_lock doesn't produce an Err in wasm32-emscripten.

use std::sync::Mutex;
fn main() {
    let m: Mutex<u32> = Mutex::new(1);
    let a = m.try_lock().unwrap();
    let b = m.try_lock().unwrap();
    println!("a + b = {}", *a + *b)
}

In x64, compiling and running this produces:

thread 'main' panicked at try_lock.rs:6:26:
called `Result::unwrap()` on an `Err` value: "WouldBlock"

When built with --target=wasm32-unknown-emscripten (default non-pthreads) there is no error:

a + b = 2

Discovered by @joelburton in curiousdannii/remglk-rs#11

[Urgau]

Seems like emscripten pthread mutex does nothing if not explicitly linked, emscripten-core/emscripten#23661.

Could you try linking with pthread, RUSTFLAGS="-Clink-arg=-pthread".

[Urgau]

I tested it locally and indeed emcc does not automatically link it's pthread implementation.

~/pthread_mutex_trylock$ emcc main.c -o main_without_pthread.js
~/pthread_mutex_trylock$ node main_without_pthread.js
a + b = 2
~/pthread_mutex_trylock$ emcc main.c -o main_with_pthread.js -pthread
~/pthread_mutex_trylock$ node main_with_pthread.js
panic: failed to lock mutex (b) - reentrancy detected

We may want force linking pthread in the standard library like we do for other targets/OS.

rust/library/std/src/sys/pal/unix/mod.rs

Lines 318 to 319 in 0490dd9

	#[link(name = "pthread")]
	unsafe extern "C" {}

[curiousdannii]

Would that force it to use pthreads? We should still have the option for single threaded output. Note that pthreads in Emscripten requires SharedArrayBuffers, which in turn require various HTTP headers to be set, so it shouldn't be the default IMO.

[Urgau]

No idea, but the current behavior is unsound, it permits creating two mutable references to the same place.

@rustbot label +I-unsound
cc @hoodmane @juntyr (as target maintainer)

[juntyr]

Would that force it to use pthreads? We should still have the option for single threaded output. Note that pthreads in Emscripten requires SharedArrayBuffers, which in turn require various HTTP headers to be set, so it shouldn't be the default IMO.

Correct, emscripten without threads is an important target that we should keep.

[juntyr]

My understanding is that on Emscripten without threads, i.e. without atomics enabled, in https://github.com/rust-lang/rust/blob/0490dd938541ad996c5ad1ec6e274012afe3e1d4/library/std/src/sys/sync/mutex/mod.rs the no-threads variant of mutex is chosen. But the try_lock implementation there looks fine to me:

rust/library/std/src/sys/sync/mutex/no_threads.rs

Lines 27 to 30 in 0490dd9

	#[inline]
	pub fn try_lock(&self) -> bool {
	!self.locked.replace(true)
	}

So what am I missing?

[hanna-kruppe]

The emscripten target has cfg(target_family = "unix"), right? Then the pthreads implementation is preferred.

[juntyr]

Ok, here are the cfgs:

rustc +nightly --print cfg --target wasm32-unknown-emscripten
debug_assertions
emscripten_wasm_eh
fmt_debug="full"
overflow_checks
panic="unwind"
relocation_model="pic"
target_abi=""
target_arch="wasm32"
target_endian="little"
target_env=""
target_family="unix"
target_family="wasm"
target_feature="bulk-memory"
target_feature="crt-static"
target_feature="exception-handling"
target_feature="multivalue"
target_feature="mutable-globals"
target_feature="nontrapping-fptoint"
target_feature="reference-types"
target_feature="sign-ext"
target_has_atomic
target_has_atomic="16"
target_has_atomic="32"
target_has_atomic="64"
target_has_atomic="8"
target_has_atomic="ptr"
target_has_atomic_equal_alignment="16"
target_has_atomic_equal_alignment="32"
target_has_atomic_equal_alignment="64"
target_has_atomic_equal_alignment="8"
target_has_atomic_equal_alignment="ptr"
target_has_atomic_load_store
target_has_atomic_load_store="16"
target_has_atomic_load_store="32"
target_has_atomic_load_store="64"
target_has_atomic_load_store="8"
target_has_atomic_load_store="ptr"
target_has_reliable_f128
target_has_reliable_f16
target_has_reliable_f16_math
target_object_format="wasm"
target_os="emscripten"
target_pointer_width="32"
target_thread_local
target_vendor="unknown"
ub_checks
unix