rluvaton commented on code in PR #23530: URL: https://github.com/apache/datafusion/pull/23530#discussion_r3570884432
########## datafusion/execution/src/async_stream.rs: ########## @@ -0,0 +1,622 @@ +// Licensed to the Apache Software Foundation (ASF) under one +// or more contributor license agreements. See the NOTICE file +// distributed with this work for additional information +// regarding copyright ownership. The ASF licenses this file +// to you under the Apache License, Version 2.0 (the +// "License"); you may not use this file except in compliance +// with the License. You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, +// software distributed under the License is distributed on an +// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +// KIND, either express or implied. See the License for the +// specific language governing permissions and limitations +// under the License. + +use futures::Stream; +use futures::future::FusedFuture; +use futures::stream::FusedStream; +use pin_project_lite::pin_project; +use std::ops::DerefMut; +use std::pin::Pin; +use std::sync::{Arc}; +use std::task::{Context, Poll}; +use parking_lot::Mutex; + +/// A handle for emitting values from an [`async_stream`] generator. +/// +/// The generator closure receives an `Emitter<T>` as its argument. +pub struct Emitter<T> { + slot: Arc<Mutex<Option<T>>>, +} + +/// A handle for emitting values from an [`async_try_stream`] generator. +/// +/// The generator closure receives a `TryEmitter<T, E>` as its argument. +pub struct TryEmitter<T, E> { + slot: Arc<Mutex<Option<Result<T, E>>>>, +} + +struct Receiver<T> { + slot: Arc<Mutex<Option<T>>>, +} + +impl<T> Emitter<T> { + /// Returns a `Future` that emits `value` as the next stream item. + /// + /// The returned future **must be awaited immediately**. On its first poll it + /// yields `Poll::Pending`, handing control back to the stream consumer so it + /// can observe the emitted value. On the next poll (triggered by the + /// consumer calling `poll_next` again) it completes with `Poll::Ready(())`, + /// resuming the generator. + /// + /// # Panics + /// + /// Panics if `emit` is called a second time before the previous future has + /// been awaited, because doing so would silently overwrite the unconsumed + /// value. + pub fn emit(&mut self, value: T) -> impl FusedFuture<Output = ()> { + let mut guard = self.slot.lock(); + match guard.deref_mut() { + Some(_) => panic!("Misuse: await was not called after calling emit"), + slot => *slot = Some(value), + } + + Emit { done: false } + } +} + +impl<T, E> TryEmitter<T, E> { + /// Emits `Ok(value)` as the next stream item and suspends the generator. + /// + /// Behaves identically to [`Emitter::emit`]: the returned future must be + /// awaited immediately and yields `Poll::Pending` on its first poll to + /// transfer control to the stream consumer. + /// + /// # Panics + /// + /// Panics if called before the previous emit future has been awaited. + pub fn emit(&mut self, value: T) -> impl FusedFuture<Output = ()> { + let mut guard = self.slot.lock(); + match guard.deref_mut() { + Some(_) => panic!("Misuse: await was not called after calling emit"), + slot => *slot = Some(Ok::<T, E>(value)), + } + + Emit { done: false } + } +} + +struct Emit { + done: bool, +} + +impl FusedFuture for Emit { + fn is_terminated(&self) -> bool { + self.done + } +} + +impl Future for Emit { + type Output = (); + + fn poll(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<()> { + if !self.done { + self.done = true; + // Poll::Pending causes the generator to yield, returning control back to the + // calling Stream + Poll::Pending + } else { + Poll::Ready(()) + } + } +} + +pin_project! { + struct AsyncStream<T, U> { + rx: Receiver<T>, + done: bool, + #[pin] + generator: U, + } +} + +impl<T, U> AsyncStream<T, U> { + fn new(rx: Receiver<T>, generator: U) -> AsyncStream<T, U> { + AsyncStream { + rx, + done: false, + generator, + } + } +} + +impl<T, U> FusedStream for AsyncStream<T, U> +where + U: Future<Output = ()>, +{ + fn is_terminated(&self) -> bool { + self.done + } +} + +impl<T, U> Stream for AsyncStream<T, U> +where + U: Future<Output = ()>, +{ + type Item = T; + + fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> { + let this = self.project(); + + if *this.done { + return Poll::Ready(None); + } + + // The `Option::take` call below ensures the next time poll is called the slot is + // already set to None + debug_assert!(this.rx.slot.lock().is_none()); + let res = this.generator.poll(cx); + *this.done = res.is_ready(); + + match this.rx.slot.lock().take() { + // Generator filled slot -> return next stream item + Some(v) => Poll::Ready(Some(v)), + // Generator did not fill slot and completed -> return None to indicate end of stream + None if *this.done => Poll::Ready(None), + // Generator did not fill slot and not completed -> return Pending since some Future + // other than Emit returned Pending. + None => Poll::Pending, + } + } + + fn size_hint(&self) -> (usize, Option<usize>) { + if self.done { (0, Some(0)) } else { (0, None) } + } +} + +fn tx_rx<T>() -> (Emitter<T>, Receiver<T>) { + let slot = Arc::new(Mutex::new(None)); + ( + Emitter { + slot: Arc::clone(&slot), + }, + Receiver { slot }, + ) +} + +/// Creates a [`Stream`] from an async generator function. +/// +/// The `generator` closure receives an [`Emitter<T>`] and runs as an async +/// block. Each `emitter.emit(value).await` call suspends the generator and +/// produces the next item in the stream. The stream ends when the generator +/// future resolves. +/// +/// # Example +/// +/// ``` +/// use datafusion_execution::async_stream; +/// use futures::StreamExt; +/// +/// # #[tokio::main(flavor = "current_thread")] +/// # async fn main() { +/// let stream = async_stream(|mut emitter| async move { +/// for i in 0_i32..3 { +/// emitter.emit(i).await; +/// } +/// }); +/// +/// let values: Vec<i32> = stream.collect().await; +/// assert_eq!(values, vec![0, 1, 2]); +/// # } +/// ``` +pub fn async_stream<T, F: Future<Output = ()>>( + generator: impl FnOnce(Emitter<T>) -> F, +) -> impl FusedStream<Item = T> { + let (emitter, receiver) = tx_rx(); + AsyncStream::new(receiver, generator(emitter)) +} Review Comment: Can you move this function as well as the `try_async_stream` function to the top of the file so the reader will see the entry point immediately? -- This is an automated message from the Apache Git Service. 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