WebAssembly does not support runtime checking of isolated conformances

[DougGregor]

Description

Isolated conformance checking at runtime is currently disabled in WebAssembly, as indicated by this disabled runtime test. The underlying problem is that we WebAssembly doesn't have __attribute__((constructor)) support on the C side, so the concurrency setup doesn't get executed. We'll need to find some way to run that setup code for WebAssembly, e.g., by injecting a call to the setup function from the compiler.

Reproduction

Re-enable the test linked above for WebAssembly.

Expected behavior

Test should be enabled and run,

Environment

Swift main, 6.2

Additional information

No response

[MaxDesiatov]

wasm-ld supports WASM_INIT_FUNCS custom linking metadata sections and in linking modes that we're interested in these functions should be called from entrypoint functions per https://github.com/WebAssembly/tool-conventions/blob/3822c2b4365ac2849c85ea078d7679ed896f2fd2/BasicModuleABI.md?plain=1#L12

So it seems like it's on Clang to support __attribute__((constructor)) if it doesn't do so yet. IIUC the underlying facilities for that are present.

[MaxDesiatov]

cc @kateinoigakukun

[kateinoigakukun]

Yes, wasm-ld definitely has ctor support. I suspect the ctor definition is not retained in the final image because it's defined in a separate object file Setup.cpp.o and there is no other symbol in the object.
So I guess the root problem here is not specific to Wasm but generally problematic when linking libswift_Concurrency statically. (but I haven't locally checked the issue yet)

[achinwo]

Hey guys I appreciate you looking into this issue! I'm searching for a workaround while this is resolved.

If I'm understanding correctly, does this mean all I'd need to do is avoid isolated actor conformances like below?

public struct ShapeView: **@MainActor** PrimitiveView, Layout {}

[kateinoigakukun]

Not sure if the workaround is valid, but I'll take a look on this next week.

[DougGregor]

If I'm understanding correctly, does this mean all I'd need to do is avoid isolated actor conformances like below?

With my temporary workaround, it's fine to use isolated conformances like you showed, but dynamic casts to them (e.g., as? PrimitiveView) might succeed at runtime when you aren't on the main actor. If your code is single-threaded, it doesn't matter.