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[Flight] Clone subsequent I/O nodes if it's resolved more than once - #35003
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Oct 28, 2025
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IO tasks can execute more than once. E.g. a connection may fire each time a new message or chunk comes in or a setInterval every time it executes.
We used to treat these all as one I/O node and just updated the end time as we go. Most of the time this was fine because typically you would have a Promise instance whose end time is really the one that gets used as the I/O anyway.
However, in a pattern like this it could be problematic:
Because the I/O's end time is before the render started so it should be excluded from being considered I/O as part of the render. It happened outside of render. But because the
Promise.resolve()is inside render its end time is after the render start so the promise is considered part of the render. This is usually not a problem because the end time of the I/O is still before the start of the render so even though the Promise is valid it has no I/O source so it's properly excluded.However, if the I/O's end time updates before we observe this then the I/O can be considered part of the render. E.g. if this was a setInterval it would be clearly wrong. But it turns out that even setTimeout can sometimes execute more than once in the async_hooks because each run of "process.nextTick" and microtasks respectively are ran in their own before/after. When a micro task executes after this main body it'll update the end time which can then turn the whole I/O as being inside the render.
To solve this properly I create a new I/O node each time before() is invoked so that each one gets to observe a different end time. This has a potential CPU and memory allocation cost when there's a lot of them like in a quick stream.