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chore: For WASI builds only, use fatalError in all .wait() calls. Recommend using .get() instead. #3421

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chore: For WASI builds only, use fatalError in all .wait() calls. Recommend using .get() instead. #3421

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20 changes: 20 additions & 0 deletions Sources/NIOCore/EventLoopFuture.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -1073,6 +1073,26 @@ extension EventLoopFuture {
@preconcurrency
@inlinable
public func wait(file: StaticString = #file, line: UInt = #line) throws -> Value where Value: Sendable {
#if os(WASI)
// NOTE: As of July 22, 2025 `wait()` calling wait() is not supported on WASI platforms.
//
// This may change down the road if and when true multi-threading evolves. But right now
// calling wait here results in the following runtime crash:
//
// ```
// SomeExecutable.wasm:0x123456 Uncaught (in promise) RuntimeError: Atomics.wait cannot be called in this context
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Does this also happen with multi-threaded Swift SDKs for WASI? If not, we should only make this conditional on multi-threading availability.

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It's not an SDK limitation, but the browser's limitation. They don't allow exeucting memory.atomic.wait32 instruction on the main thread regardless of the SDK variants. On multi-threading context, the opt-in busy-wait mode in wasi-libc (WebAssembly/wasi-libc#562) might make sense under some conditions, though.

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Is the limitation only on the main thread? If so, we could plausibly detect specifically that condition on WASI.

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Correct, this is a limitation only applied to the main thread.

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Ok, my recommendation then would be that we try to only diagnose this on the main thread. Can that be detected in WASI?

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Yeah, it would be really nice to be able to restrict this to just the main thread. Unfortunately from what I'm seeing I don't think that Thread.isMain compiles in Swift for WebAssembly. @MaxDesiatov are you aware of any other solutions that might work well here?

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@kateinoigakukun kateinoigakukun Oct 25, 2025 •
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Hmm, I don't think we have a good way other than recording the main thread handle in a ctor and comparing with it later.

static pthread_t main_thread;

__attribute__((constructor))
void init_main_thread() {
    main_thread = pthread_self();
}

bool is_main_thread() {
    return pthread_equal(pthread_self(), main_thread);
}

But I think we can unlock Thread.isMainThread in foundation using this approach.

// ```
//
// Using the following fatal error here gives wasm runtime users a much more clear error message
// to identify the issue.
//
// If you're running into this error on WASI, refactoring to `get()` instead of `wait()` will
// likely solve the issue.
fatalError(
"NIO's wait() function should not be called on WASI platforms. It will freeze or crash. Use get() instead."
)
#endif // os(WASI)

try self._blockingWaitForFutureCompletion(file: file, line: line)
}

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