asyncLoad: fix empty error message

[Kojoley]

Before:

[UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). The promise rejected with the reason "undefined".] {
  code: 'ERR_UNHANDLED_REJECTION'
}

After:

[Error: ENOENT: no such file or directory, open 'A:\path\to\foo.wasm'] {
  errno: -4058,
  code: 'ENOENT',
  syscall: 'open',
  path: 'A:\\path\\to\\foo.wasm'
}

[sbc100]

Thanks!

[kripken]

I looked at the first few users of this and didn't see any that use the parameter - where did you see it used? Or was this for an out-of-tree usage?

[sbc100]

@Kojoley could you elaborate on what API you were using when you saw the difference above? How where you using it exactly?

[Kojoley]

mkdir link_test
echo ; > link_test.cpp
emcc -fPIC -sSIDE_MODULE -Wall -g -c "link_test.cpp" -o "link_test.o"
emcc -fPIC -sSIDE_MODULE "link_test.o" -o "link_test/foo.wasm"

echo int main() { return 0; } > main.cpp
emcc -fPIC -sMAIN_MODULE -Wall -g -c "main.cpp" -o "main.o"
emcc -fPIC -sMAIN_MODULE "main.o" "link_test/foo.wasm" -o "link_test.js"

node link_test.js

[kripken]

Do you know where in that output it uses the parameter? Or is it hard to figure out? (I'd hope you can add console.log(new Error().stack) right where the parameter is received, to show the stack trace.)

[Kojoley]

I'm not the person that can comfortably debug that for you, sorry. I don't write Javascript nor have a clue about emscripten internals. It took me a while already to get to this point.

[sbc100]

Thanks for the repro case. I think this change is good but I will do a little more investigation here before landing so I (we) fully understand what is going on here.

[kripken]

Perhaps it makes sense to keep the text here? The context could be nice. I'm not sure if there's a way to append more text to an existing Error object, though...

[Kojoley]

Look at the OP/commit message, readAsync already has the accurate error information, this does not add any additional information, quite an opposite, it has less information.

[kripken]

Interesting. So the text "Loading data file .. failed" was swallowed somewhere, I guess? Anyhow, sounds good that this improves things.