CDP Runtime.evaluate does not drain async tasks

[EmielM]

Bug Description

I'm trying to run some automated testing scripts through the Hermes CDP interface using Runtime.evaluate.

First off, the awaitPromises flag does not seem to be supported at all. There are some work-arounds to this, so that's fine.

A bigger issue seems to be that after Runtime.evaluate, planned Promise tasks are not executed. What I see is usually after a second or 5-10, the queue is triggered and drained again. (I also tried to workaround this by running HermesInternal.drainJobs after every command, but this is unexposed without hermes-internal-test-methods flag see; I didn't manage to enable that flag in an otherwise vanilla RN project either.)

Tried to create a simple case to replicate, but the overall setup is pretty involving.

[dannysu]

hi @EmielM,

The current Hermes CDP implementation is scoped to enable the set of features needed by the new RN debugger coming in 0.76. We specifically only meant for it to support the exact version of that RN DevTools.

Unfortunately awaitPromises isn't supported yet. We do want to close the gap but don't have timeline for that.

There are some work-arounds to this

In terms of the workarounds you talked about, could you please elaborate what that is?

planned Promise tasks are not executed

What RN version are you using?

The draining of microtask queue may be the responsibility of the code on the RN side and not something Hermes the engine controls. What are these promise tasks you're talking about? How are you creating them and what's the code you're using that's noticing them resolving after 5-10 seconds.

Having code speaks most clearly. Thanks.

[EmielM]

Apologies, I c/should have been a bit more detailed, indeed. Thanks for your patience.

I attach a chrome inspector via CDP, then evaluate (new Promise((resolve) => resolve('hey'))).then((x) => console.log(x));.

Here's a screenshot that illustrates the (timing) issue:

My hypothesis is that the task queue is simply not drained in the Hermes CDP client, and after a while some other message goes over the RN bridge or so and that triggers the pending queue to flush.

Logging the backend CDP interaction:

> {"id":552,"method":"Runtime.evaluate","params":{"expression":"(new Promise((resolve) =\u003e resolve('hey'))).then((x) =\u003e console.log(x
));","objectGroup":"console","includeCommandLineAPI":true,"silent":false,"returnByValue":false,"generatePreview":true,"userGesture":true,"awaitPromise":false,"replMode":true,"allowUnsafeEvalBlockedByCSP":false,"contextId":1}}
< {"id":552,"result":{"result":{"className":"Object","description":"Object","objectId":"453","preview":{"description":"Object","overflow":false,"properties":[{"name":"_h","type":"number","value":"0"},{"name":"_i","type":"number","value":"0"},{"name":"_j","subtype":"null","type":"object","value":"null"},{"name":"_k","subtype":"null","type":"object","value":"null"}],"type":"object"},"type":"object"}}}                                                                    

One little update: I did find out that HermesInternal.useEngineQueue() is false in my setup.

Will attempt to recreate in latest RN/Hermes, but that will take some time in my setup unfortunately.

[tmikov]

One little update: I did find out that HermesInternal.useEngineQueue() is false in my set

@EmielM Right. That is only enabled with the New Architecture. But either way, Hermes doesn't control the event loop and the draining of the queue. I think we should transfer this issue to React Native.

[dannysu]

@EmielM please note that we also don't support just any Chrome DevTools, so if you're just opening up Chrome browser and using it then things are not 100% compatible. We only support the specific fork of DevTools we're calling React DevTools.

The promise queue draining behavior looks to be RN related, so I'll transfer this issue over.

[react-native-bot]

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[react-native-bot]

⚠️	Missing Reproducible Example
ℹ️	We could not detect a reproducible example in your issue report. Please provide either: If your bug is UI related: a Snack If your bug is build/update related: use our Reproducer Template

[EmielM]

@tmikov Alright (although that's a bit surprising to me as the CDPAgent implementation seems to live in the Hermes codebase)

@dannysu Fair, but this doesn't work (as expected) in the RN inspector either: as I understand it's a pretty straight forward fork from the devtools project, and also sends Runtime.evaluate to RN/hermes. Screenshot:

@react-native-bot Yes!

[hoxyq]

Thanks for reporting this!

A bigger issue seems to be that after Runtime.evaluate, planned Promise tasks are not executed.

We are aware of this issue and planning to fix this on React Native side. I will share more details once we confirm the timeline for it.