Fix thread::available_parallelism on WASI targets with threads by surban · Pull Request #153604 · rust-lang/rust

surban · 2026-03-09T10:31:51Z

The refactoring in ba46286 ("std: Use more unix.rs code on WASI targets") moved WASI from its own thread module into the shared unix.rs module. However, it did not carry over the available_parallelism() implementation for WASI with threads, causing it to fall through to the unsupported catch-all. This silently regressed the support originally added in f0b7008.

Fix this by adding WASI to the standard UNIX cfg_select branch.

Depends on #155057 (Update libc to v0.2.184).

rustbot · 2026-03-09T10:31:56Z

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Owners of files modified in this PR: @ChrisDenton, libs
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Pull request overview

This PR restores thread::available_parallelism() support for WASI targets compiled with the atomics target feature. A prior refactoring (ba462864f11) unified WASI into the shared unix.rs thread module but failed to migrate the WASI+atomics implementation of available_parallelism(), causing it to silently fall through to the unsupported catch-all.

Changes:

Adds a dedicated cfg_select arm for all(target_os = "wasi", target_feature = "atomics") in available_parallelism() that calls libc::sysconf with a hardcoded constant _SC_NPROCESSORS_ONLN = 84 (since the libc crate does not export this constant for WASI targets).
Plain wasip1 without threads continues to fall through to the _ arm and return an unsupported error, matching pre-regression behavior.

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library/std/src/sys/thread/unix.rs

joboet · 2026-03-10T15:40:41Z

Given that wasi-libc stubs sysconf(_SC_NPROCESSORS_ONLN) and unconditionally returns 1, I'm not sure whether this is actually an improvement. IMHO, available_parallelism should always fail if the answer cannot be determined, programs can always use .unwrap_or(0) if they desire.

CC @alexcrichton

alexcrichton · 2026-03-10T16:17:14Z

Personally I think that this is a reasonable implementation, although I think it'd be best to lump this in with the implementation for _SC_NPROCESSORS_ONLN above with other platforms. That'll probably require a wasi-specific define of the constant, but @surban would you be up for sending a PR to the libc crate to add this constant there too?

For the correctness of this function for single-threaded wasm 1 is the correct answer, and otherwise the only other target would be for wasm32-wasip1-threads in which case I would consider it a bug in libc to stub it to return one, but it's still most accurate for Rust to just delegate to libc.

surban · 2026-03-18T12:23:19Z

Personally I think that this is a reasonable implementation, although I think it'd be best to lump this in with the implementation for _SC_NPROCESSORS_ONLN above with other platforms. That'll probably require a wasi-specific define of the constant, but @surban would you be up for sending a PR to the libc crate to add this constant there too?

Yes, I have opened PR rust-lang/libc#5023 for all _SC_ constants in wasi-libc.
Checks should pass once the PR is merged and Rust updates its libc version.

For the correctness of this function for single-threaded wasm 1 is the correct answer, and otherwise the only other target would be for wasm32-wasip1-threads in which case I would consider it a bug in libc to stub it to return one, but it's still most accurate for Rust to just delegate to libc.

I have now changed it to always return Ok(1) for single-threaded wasm. Should I keep this or always (single-threaded and multi-threaded wasm) delegate to wasi-libc?

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Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

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library/std/src/sys/thread/unix.rs

+        all(target_os = "wasi", not(target_feature = "atomics")) => {
+            // Single-threaded WASM always has exactly one thread of execution.
+            Ok(NonZero::new(1).unwrap())
+        }


library/std/tests/thread.rs

 #[cfg_attr(
    any(
        target_os = "redox",
        target_os = "l4re",
        target_env = "sgx",
        target_os = "solid_asp3",
        target_os = "teeos",
-        target_os = "wasi"
    ),
    should_panic
 )]


alexcrichton · 2026-03-20T06:25:45Z

Should I keep this or always (single-threaded and multi-threaded wasm) delegate to wasi-libc?

I'd recommend always delegating to wasi-libc, and to avoid blocking on a libc release I think it'd be reasonable to inline the definition of the _SC_* symbol here with a wasi-specific cfg and a comment saying it can be removed when libc is updated.

surban · 2026-04-09T17:20:08Z

@rustbot blocked

… r=Mark-Simulacrum Update libc to v0.2.184 This includes the WASI _SC_* sysconf constants needed for `thread::available_parallelism` on WASI targets (rust-lang#153604).

Rollup merge of #155057 - rust-wasi-web:update-libc-0.2.184, r=Mark-Simulacrum Update libc to v0.2.184 This includes the WASI _SC_* sysconf constants needed for `thread::available_parallelism` on WASI targets (#153604).

surban · 2026-04-12T07:50:38Z

@rustbot ready

The refactoring in ba46286 ("std: Use more unix.rs code on WASI targets") moved WASI from its own thread module into the shared unix.rs module. However, it did not carry over the available_parallelism() implementation for WASI, causing it to fall through to the unsupported catch-all. This silently regressed the support originally added in f0b7008. Fix this by adding WASI to the sysconf-based cfg_select arm alongside other platforms that use libc::sysconf(_SC_NPROCESSORS_ONLN). This delegates to wasi-libc, which currently always returns 1 but opens up the possibility for wasi-libc to report actual processor counts in the future. This requires libc to export _SC_NPROCESSORS_ONLN for WASI targets, which has been added in libc 0.2.184.

rustbot · 2026-04-12T07:53:31Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

surban · 2026-04-12T07:54:47Z

I have included the necessary constants in the updated libc. Hence this now follows the standard UNIX path.

alexcrichton · 2026-04-13T14:19:29Z

@bors: r+

Thanks!

rust-bors · 2026-04-13T14:19:32Z

Unknown command ":". Run @bors help to see available commands.

alexcrichton · 2026-04-13T14:21:04Z

@bors r+

rust-bors · 2026-04-13T14:21:08Z

📌 Commit b5ddfd5 has been approved by alexcrichton

It is now in the queue for this repository.

…lelism-fix, r=alexcrichton Fix thread::available_parallelism on WASI targets with threads The refactoring in ba46286 ("std: Use more unix.rs code on WASI targets") moved WASI from its own thread module into the shared unix.rs module. However, it did not carry over the available_parallelism() implementation for WASI with threads, causing it to fall through to the unsupported catch-all. This silently regressed the support originally added in f0b7008. Fix this by adding WASI to the standard UNIX cfg_select branch. Depends on rust-lang#155057 (Update libc to v0.2.184).

…uwer Rollup of 19 pull requests Successful merges: - #155162 (relnotes for 1.95) - #140763 (Change codegen of LLVM intrinsics to be name-based, and add llvm linkage support for `bf16(xN)` and `i1xN`) - #153604 (Fix thread::available_parallelism on WASI targets with threads) - #154193 (Implement EII for statics) - #154389 (Add more robust handling of nested query cycles) - #154435 (resolve: Some import resolution cleanups) - #155236 (Normalize individual predicate of `InstantiatedPredicates` inside `predicates_for_generics`) - #155243 (cg_ssa: transmute between scalable vectors) - #153941 (tests/debuginfo/basic-stepping.rs: Explain why all lines are not steppable) - #154587 (Add --verbose-run-make-subprocess-output flag to suppress verbose run-make output for passing tests) - #154624 (Make `DerefPure` dyn-incompatible) - #154929 (Add `const Default` impls for `LazyCell` and `LazyLock`) - #154944 (Small refactor of `arena_cache` query values) - #155055 (UI automation) - #155062 (Move tests from `tests/ui/issues/` to appropriate directories) - #155131 (Stabilize feature `uint_bit_width`) - #155147 (Stabilize feature `int_lowest_highest_one`) - #155174 (Improve emission of `UnknownDiagnosticAttribute` lint) - #155194 (Fix manpage version replacement and use verbose version)

Rollup merge of #153604 - rust-wasi-web:wasi-available-parallelism-fix, r=alexcrichton Fix thread::available_parallelism on WASI targets with threads The refactoring in ba46286 ("std: Use more unix.rs code on WASI targets") moved WASI from its own thread module into the shared unix.rs module. However, it did not carry over the available_parallelism() implementation for WASI with threads, causing it to fall through to the unsupported catch-all. This silently regressed the support originally added in f0b7008. Fix this by adding WASI to the standard UNIX cfg_select branch. Depends on #155057 (Update libc to v0.2.184).

…uwer Rollup of 19 pull requests Successful merges: - rust-lang/rust#155162 (relnotes for 1.95) - rust-lang/rust#140763 (Change codegen of LLVM intrinsics to be name-based, and add llvm linkage support for `bf16(xN)` and `i1xN`) - rust-lang/rust#153604 (Fix thread::available_parallelism on WASI targets with threads) - rust-lang/rust#154193 (Implement EII for statics) - rust-lang/rust#154389 (Add more robust handling of nested query cycles) - rust-lang/rust#154435 (resolve: Some import resolution cleanups) - rust-lang/rust#155236 (Normalize individual predicate of `InstantiatedPredicates` inside `predicates_for_generics`) - rust-lang/rust#155243 (cg_ssa: transmute between scalable vectors) - rust-lang/rust#153941 (tests/debuginfo/basic-stepping.rs: Explain why all lines are not steppable) - rust-lang/rust#154587 (Add --verbose-run-make-subprocess-output flag to suppress verbose run-make output for passing tests) - rust-lang/rust#154624 (Make `DerefPure` dyn-incompatible) - rust-lang/rust#154929 (Add `const Default` impls for `LazyCell` and `LazyLock`) - rust-lang/rust#154944 (Small refactor of `arena_cache` query values) - rust-lang/rust#155055 (UI automation) - rust-lang/rust#155062 (Move tests from `tests/ui/issues/` to appropriate directories) - rust-lang/rust#155131 (Stabilize feature `uint_bit_width`) - rust-lang/rust#155147 (Stabilize feature `int_lowest_highest_one`) - rust-lang/rust#155174 (Improve emission of `UnknownDiagnosticAttribute` lint) - rust-lang/rust#155194 (Fix manpage version replacement and use verbose version)

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oech3 mentioned this pull request Mar 10, 2026

nproc: support wasm32-wasip* uutils/coreutils#11287

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surban force-pushed the wasi-available-parallelism-fix branch 2 times, most recently from a41f6b5 to 00f08b0 Compare March 18, 2026 12:19

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surban mentioned this pull request Apr 9, 2026

Update libc to v0.2.184 #155057

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Rollup of 20 pull requests #155245

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Rollup of 19 pull requests #155253

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