[Flight] Insert an extra await node for awaiting on the promise returned by then callback #33713
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sebmarkbage
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Jul 6, 2025
| let firstFrame = 0; | ||
| while ( | ||
| fullStack.length > firstFrame && | ||
| fullStack[firstFrame][0] === 'Promise.then' |
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If you call raw .then() then that gets a stack frame that's not in user space which we need to ignore. The first frame after user space should be considered for the heuristic.
sebmarkbage
commented
Jul 6, 2025
| ], | ||
| "start": 0, | ||
| "value": { | ||
| "value": undefined, |
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This is a little counter intuitive but the I/O that was awaited didn't have a value since that's the .then() call on delay() which resolves to undefined.
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Comparing: 4aad5e4...18af1b8 Critical size changesIncludes critical production bundles, as well as any change greater than 2%:
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eps1lon
approved these changes
Jul 6, 2025
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When a
.then()callback returns another Promise, there's effectively another "await" on that Promise that happens in the internals but that was not modeled. In effect the Promise returned by.then()is blocked on both the original Promise AND the promise returned by the callback.This models that by cloning the original node and treat that as the await on the original Promise. Then we use the existing Node to await the new Promise but its "previous" points to the clone. That way we have a forked node that awaits both.