[Flight] Ignore async stack frames when determining if a Promise was created from user space #33739
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… frames on a Promise
sebmarkbage
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Jul 9, 2025
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| export async function sdkMethod(input, init) { | |||
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This function is a bit weird because it's using an async function without actually awaiting anywhere. You might think of optimizing it to not be an async function and just return the Promise.
Unfortunately, then it doesn't get any name at all and no stack for the I/O (just the await). Because now there's no Promise with a good stack around it.
Ideally we'd probably use the .then call as the best available stack for the I/O but not sure how that makes sense since it's not the cause spawning the I/O. Maybe it's really the Promise that it is awaiting that's the best possible Promise in that case. I.e. the first fetch. But I'm not going to address that now.
eps1lon
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Jul 9, 2025
This was referenced Jul 9, 2025
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We use the stack of a Promise as the start of the I/O instead of the actual I/O since that can symbolize the start of the operation even if the actual I/O is batched, deduped or pooled. It can also group multiple I/O operations into one.
We want the deepest possible Promise since otherwise it would just be the Component's Promise.
However, we don't really need deeper than the boundary between first party and third party. We can't just take the outer most that has third party things on the stack though because third party can have callbacks into first party and then we want the inner one. So we take the inner most Promise that depends on I/O that has a first party stack on it.
The realization is that for the purposes of determining whether we have a first party stack we need to ignore async stack frames. They can appear on the stack when we resume third party code inside a resumption frame of a first party stack.