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ASYNCIFY_IGNORE_INDIRECT Javascript callback #10941

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JesseFarebro opened this issue Apr 16, 2020 · 2 comments
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ASYNCIFY_IGNORE_INDIRECT Javascript callback #10941

JesseFarebro opened this issue Apr 16, 2020 · 2 comments

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@JesseFarebro
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JesseFarebro commented Apr 16, 2020 •
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I've noticed an issue when using Javascript callbacks and ASYNCIFY_IGNORE_INDIRECT. If wakeUp is called in a nested callback it will raise function signature mismatch with -O3 and unreachable otherwise. With -O3 the function call still works as expected, emscripten is woken up and parameters are passed correctly, it just raises the runtime error.
Is this the desired behaviour? From what I understand the indirect call only refers to a C++ indirect call?

Here is a minimal reproduction:

#include <emscripten.h>

EM_JS(void, callback, (), {
  Asyncify.handleSleep((wakeUp) => {
    Module.callback(wakeUp);
  })
})

void loop() {
  callback();
}

int main() {
  emscripten_set_main_loop(loop, 0, 0);
}
emcc main.cpp -O3 \ 
        -s ASYNCIFY=1 \
        -s ASYNCIFY_IGNORE_INDIRECT \
        -s ASYNCIFY_IMPORTS=['callback'] \
        -s MODULARIZE
<html>
  <head>
    <script type="text/javascript" src="a.out.js"></script>
    <script type="text/javascript">
      Module({
        // First callback, raises an error when wakeUp is called
        callback: (wakeUp) => {
          setTimeout(() => {
            console.log("First callback")
            wakeUp();
          }, 0);
        }
      }).then((module) => {
        console.log(module)
        window.MODULE = module
      })

      // Second callback, works without raising an error
      setTimeout(() => {
        window.MODULE.callback = (wakeUp) => {
          console.log("Second callback")
          wakeUp();
        }
      }, 1000)
    </script>
  </head>
  <body>
  </body>
</html>

This following common pattern also doesn't work, i.e., chaining wakeUp on a promise returned from the callback.

EM_JS(void, callback, (), {
  return Asyncify.handleSleep((wakeUp) => {
    Module.callback().then(wakeUp);
  })
})

Thanks for taking a look!
@kripken
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kripken commented Apr 21, 2020

The full stack trace, when building with function names (-g or --profiling) is

a.out.js:2090 exception thrown: RuntimeError: unreachable,RuntimeError: unreachable
    at dynCall_v (wasm-function[27]:0x6820)
    at ret.<computed> (http://localhost:8000/a.out.js:2893:35)
    at Object.Module.dynCall_v (http://localhost:8000/a.out.js:3151:37)
    at browserIterationFunc (http://localhost:8000/a.out.js:2759:30)
    at Object.runIter (http://localhost:8000/a.out.js:2085:13)
    at Browser_mainLoop_runner (http://localhost:8000/a.out.js:2815:26)

That dynCall_v is doing an indirect call, which is why ASYNCIFY_IGNORE_INDIRECT breaks things. It's because of how the main loop works: it gets a function pointer, then does an indirect call to that. So you can't ignore indirect calls in this case.

@JesseFarebro
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Thanks for looking into this @kripken, that makes sense. I was overthinking my main loop, making the proper design decisions was the proper way to go about my issue.

@stelcheck stelcheck mentioned this issue Jul 30, 2020
Open
@andrewevstyukhin andrewevstyukhin mentioned this issue Oct 21, 2021
Closed
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@kripken @JesseFarebro