feat(ses): fix undefined AsyncGenerator prototype on Hermes

[leotm]

closes: #XXXX
refs: #1891

Description

Context: Running SES on React Native on Android (Hermes)

The AsyncGenerator prototype const AsyncGeneratorPrototype = AsyncGenerator.prototype is undefined on Hermes
but we could get round this by redefining the AsyncIteratorPrototype instead
we could also keep the original definition, but fallingback to this one

This is throwing an error calling addIntrinsics(getAnonymousIntrinsics())

at const AsyncIteratorPrototype= getPrototypeOf(AsyncGeneratorPrototype);
// TypeError: Cannot convert undefined value to object, js engine: hermes

Fixed in a repro here: leotm/RN07117SES@cb44efd
(ignoring the redefinition of AsyncFunctionInstance for now, which is coming as separate pr part2)

TODO: fix failing test

Alternatives considered

• redefining the AsyncGeneratorPrototype if possible
• a separate SES shim for Hermes with AsyncGeneratorPrototype and relevant parts removed

Security Considerations

Scaling Considerations

Documentation Considerations

Testing Considerations

Compatibility Considerations

Upgrade Considerations

• Includes *BREAKING*: in the commit message with migration instructions for any breaking change.
• Updates NEWS.md for user-facing changes.

[leotm]

TODO: fix failing test

  Uncaught exception in test/test-static-module-record.js

  TypeError: Unexpected intrinsic intrinsics.%AsyncGeneratorPrototype%.__proto__ at %AsyncIteratorPrototype%

  TypeError: Unexpected intrinsic intrinsics.%AsyncGeneratorPrototype%.__proto__ at %AsyncIteratorPrototype%
    at visitPrototype (file:///Users/leo/Documents/GitHub/endo/packages/ses/src/permits-intrinsics.js:160:11)
    at visitProperties (file:///Users/leo/Documents/GitHub/endo/packages/ses/src/permits-intrinsics.js:272:5)
    at isAllowedPropertyValue (file:///Users/leo/Documents/GitHub/endo/packages/ses/src/permits-intrinsics.js:170:7)
    at isAllowedProperty (file:///Users/leo/Documents/GitHub/endo/packages/ses/src/permits-intrinsics.js:233:14)
    at visitProperties (file:///Users/leo/Documents/GitHub/endo/packages/ses/src/permits-intrinsics.js:283:26)
    at whitelistIntrinsics (file:///Users/leo/Documents/GitHub/endo/packages/ses/src/permits-intrinsics.js:323:5)
    at repairIntrinsics (file:///Users/leo/Documents/GitHub/endo/packages/ses/src/lockdown.js:348:3)
    at globalThis.lockdown (file:///Users/leo/Documents/GitHub/endo/packages/ses/src/lockdown-shim.js:16:28)
    at file:///Users/leo/Documents/GitHub/endo/packages/static-module-record/test/lockdown.js:3:1
    at ModuleJob.run (node:internal/modules/esm/module_job:194:25)

  ✘ test/test-static-module-record.js exited with a non-zero exit code: 1

caused by importing lockdown (required otherwise Compartment is undefined)

endo/packages/static-module-record/test/test-static-module-record.js

Line 7 in dc486dc

import './lockdown.js';

after our redefinition of the AsyncIteratorPrototype

endo/packages/ses/src/get-anonymous-intrinsics.js

Line 125 in dc486dc

'%AsyncIteratorPrototype%': AsyncIteratorPrototype,

throwing at visitPrototype

endo/packages/ses/src/permits-intrinsics.js

Line 160 in dc486dc

throw TypeError(`Unexpected intrinsic ${path}.__proto__ at ${protoName}`);

unable to clean the new [[prototype]] we've introduced

Unexpected intrinsic intrinsics.%AsyncGeneratorPrototype%.__proto__ at %AsyncIteratorPrototype%

the error is confusing because intrinsics.%AsyncGeneratorPrototype%.__proto__ seems expected, which outputs [object Object] in other engines using
(async function* () {}.constructor).prototype.prototype.__proto__

and we can't simply skip throwing in this case, otherwise subsequent related test breaks
Uncaught exception in test/test-anticipate-async-iterator-helpers-shimmed.js
✘ test/test-anticipate-async-iterator-helpers-shimmed.js

so perhaps redefining AsyncGeneratorPrototype instead may yield a better result if possible
or an alternative redefinition of AsyncIteratorPrototype

or a hermes-flavour shim that excludes AsyncGeneratorPrototype and relevant parts entirely

[mhofman]

Wouldn't it be sufficient to do

Suggested change

	const AsyncIteratorPrototype = getPrototypeOf(asyncIterator);
	const AsyncIteratorPrototype = getPrototypeOf(AsyncGeneratorFunctionInstance());

[naugtur]

That's likely. Worth testing if it falls into the category of what Hermes supports, but looks equivalent to what we've tried here.

[leotm]

the suggestion works too ^ but still causes the test to fail, i've looked into it here #2220 (comment) if any thoughts

[mhofman]

I think I'm getting confused by what the prototype chains of iterators/generators are in Hermes, and how they differ from the spec. The permits list really is a representation of the intrinsics graph a standards compliant engine is supposed to setup. Any way to summarize the difference?

[leotm]

I think I'm getting confused by what the prototype chains of iterators/generators are in Hermes, and how they differ from the spec. The permits list really is a representation of the intrinsics graph a standards compliant engine is supposed to setup. Any way to summarize the difference?

makes sense ^ i misread the SES permit TypeError: Unexpected intrinsic intrinsics.%AsyncGeneratorPrototype%.__proto__ at %AsyncIteratorPrototype%
thinking it's a false positive since

// unexpected %AsyncGeneratorPrototype%.__proto__
const AsyncGeneratorFunction = async function* () {}.constructor;
const AsyncGenerator = AsyncGeneratorFunction.prototype;
const AsyncGeneratorPrototype = AsyncGenerator.prototype;
const AsyncGeneratorPrototype__proto__ = AsyncGeneratorPrototype.__proto__
// expected [object Object] in engines supporting async generators

the Hermes async (arrow)fn/generator/iterator is bit confusing w vague/old features list, but w the testsuite confirmed

async functions supported
async arrow functions unsupported transformed by babel
async generators/iterators unsupported (will be added to Static Hermes)
it looks like babel differs from the spec and was hinted at before
so @babel/plugin-transform-async-to-generator may need an update
then bumping in react-native-babel-preset and metro-react-native-babel-preset if backported

some errors in the ses shim are surpressed so raised this here

• Importing files suppress async generators are unsupported errors facebook/hermes#1389

also Metro errors on RN 71.15 when bundling async functions/generators are unsupported but not in later versions

so think should close this pr and chase up with babel then update here

• Ecosystem Compatibility (2024-02-05 onward) #2037