Async stack trace stitching

[andrieshiemstra]

When you call await in async functions, it truncates the stack trace on errors.

For example, this:

async function a() {
    await b();
}

async function b() {
    await 1;
    throw Error("poof");
}

a().catch((ex) => {
    console.error(ex);
});

will result in a stack trace like this:

Error: poof
    at b (test_ex3.js:8:16)

But when we remove the await 1; statement we get the full stack:

Error: poof
    at b (test_ex3.js:8:16)
    at a (test_ex3.js:3:12)
    at <eval> (test_ex3.js:11:2)

Could async stack capture be built into QuickJS? For example, by copying the current stack to JSJobEntry when scheduling and then restoring the stack when executing pending jobs. Perhaps this could be enabled only when a DEBUG_PRESERVE_ASYNC_STACKS flag is set.

For reference:

Spidermonkey only does it when debugger tools are open: https://bugzilla.mozilla.org/show_bug.cgi?id=1601179
V8 has a more elaborate solution http://bit.ly/v8-zero-cost-async-stack-traces