crepererum commented on code in PR #14028:
URL: https://github.com/apache/datafusion/pull/14028#discussion_r1906929864
##########
datafusion/core/src/datasource/physical_plan/file_stream.rs:
##########
@@ -478,7 +478,12 @@ impl<F: FileOpener> FileStream<F> {
reader,
)),
partition_values,
- }
+ };
+ // Return control to the runtime
when we're ready to open the next file
+ // to prevent uncancellable
queries in scenarios with many large files.
+ // This functions similarly to a
`tokio::task::yield_now()`.
+ cx.waker().wake_by_ref();
+ return Poll::Pending;
Review Comment:
> > I think this may not be correct. The `wake_by_ref` should be called
AFTER returning `Pending`. I think you may be lucky that this works, but it's
undefined behavior. If I understand the `yield_now` impl. correctly, it tries
to delay the "wake" call.
>
> I was thinking similarly, but those have changed my mind:
[rust-lang/rust@`1f81f90`/library/core/src/task/wake.rs#L423-L426](https://github.com/rust-lang/rust/blob/1f81f906893d166d05fb4839f169983f2b564cc7/library/core/src/task/wake.rs#L423-L426)
>
> and this example:
[rust-lang/rust@`1f81f90`/library/core/src/task/wake.rs#L703-L704](https://github.com/rust-lang/rust/blob/1f81f906893d166d05fb4839f169983f2b564cc7/library/core/src/task/wake.rs#L703-L704)
good point, thanks
> Do we have any example of this in the codebase (returning pending not
because of an IO)?
We do, see #5299 .
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