Calling async function recursively stops (randomly?) after 126 calls

[SharonBrizinov]

Bug Description

I was playing with async function in Hermes and found something strange - when calling an async function recursively it will execute 126 times while in fact it should run forever, until exhausting the call stack.

Version

Hermes version: v0.12.0 and latest main branch commit 36195b3
OS version (if any): Ubutnu v22.04

Steps To Reproduce

Run the following code snippet

async function foo(i) {
 print(i);
 foo(i+1);
}
foo(0);

Current output

0
1
2
...
...
...
125
126

The Expected Behavior

run until exhausting the call stack at approx ~80654 recursive calls

0
1
2
...
...
...
80653
80654
Uncaught RangeError: Maximum call stack size exceeded
    at foo (/tmp/hermes-input.js:3:5)
    at foo (/tmp/hermes-input.js:3:5)
    at foo (/tmp/hermes-input.js:3:5)

[fbmal7]

This is not a random number, it seems you are running into the native stack limit error. You can read more about the details here #135 (comment)

However, at a first glance of this code, I'm not sure why this is recursing on the native stack.

[SharonBrizinov]

@fbmal7 thanks for your quick answer. But why is it running on the native stack? I'm not using any native functions here. Also, why am I not getting a RangerError for this case?

[fbmal7]

Not sure yet what's going on, but this does look like a bug.

async function foo(i) {
  print(i);
  foo(i+1);
}
foo(0);
print('after');

The print statement for 'after' is executed after 126 is printed out, which definitely is wrong. Thanks for raising this.

[tmikov]

@fbmal7 if I run the example with JSC, I get:

0
1
2
...
...
10641
10642
after

But again no error.

[avp]

@SharonBrizinov what command did you run to get your "Expected Output"?

If I run the code with d8 test.js, I get:

/Users/avp/test.js:1: RangeError: Maximum call stack size exceeded
async function foo(i) {
                  ^
RangeError: Maximum call stack size exceeded
    at foo (/Users/avp/test.js:1:19)
    at foo (/Users/avp/test.js:3:9)
    at foo (/Users/avp/test.js:3:9)
    at foo (/Users/avp/test.js:3:9)
    at foo (/Users/avp/test.js:3:9)
    at foo (/Users/avp/test.js:3:9)
    at foo (/Users/avp/test.js:3:9)
    at foo (/Users/avp/test.js:3:9)
    at foo (/Users/avp/test.js:3:9)
    at foo (/Users/avp/test.js:3:9)

1 pending unhandled Promise rejection(s) detected.

Indicating that the error causing the nonzero exit code is for unhandled Promise rejection, not actually for stack overflow. Hermes does have a mechanism for setting up unhandled Promise rejection tracking, but it's not enabled by default on the CLI tool (much like JSC, based on tmikov's comment above).

https://github.com/facebook/hermes/blob/main/lib/VM/JSLib/HermesInternal.cpp#L697 explains how to set up Promise rejection tracking: one can use the setPromiseRejectionTrackingHook and enablePromiseRejectionTracker to catch the unhandled Promise rejections.

[SharonBrizinov]

Thanks @avp! so the only question remains is why it's using the native stack for this case?

[avp]

Internally, async functions compile to uses of generators, and the generator next function is an internal Hermes native function.

[SharonBrizinov]

Interesting, thanks @avp

[tmikov]

FWIW, the limit will increase when we deploy native stack checking. The current limit is deliberately very conservative, to avoid the possibility of native stack overflow.