repartition.rs

// 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.

//! The repartition operator maps N input partitions to M output partitions based on a
//! partitioning scheme.

use std::pin::Pin;
use std::sync::Arc;
use std::task::{Context, Poll};
use std::time::Instant;
use std::{any::Any, vec};

use crate::error::{DataFusionError, Result};
use crate::physical_plan::{DisplayFormatType, ExecutionPlan, Partitioning, SQLMetric};
use arrow::record_batch::RecordBatch;
use arrow::{array::Array, error::Result as ArrowResult};
use arrow::{compute::take, datatypes::SchemaRef};
use tokio_stream::wrappers::UnboundedReceiverStream;

use super::{hash_join::create_hashes, RecordBatchStream, SendableRecordBatchStream};
use async_trait::async_trait;

use futures::stream::Stream;
use futures::StreamExt;
use hashbrown::HashMap;
use tokio::sync::{
    mpsc::{self, UnboundedReceiver, UnboundedSender},
    Mutex,
};
use tokio::task::JoinHandle;

type MaybeBatch = Option<ArrowResult<RecordBatch>>;

/// The repartition operator maps N input partitions to M output partitions based on a
/// partitioning scheme. No guarantees are made about the order of the resulting partitions.
#[derive(Debug)]
pub struct RepartitionExec {
    /// Input execution plan
    input: Arc<dyn ExecutionPlan>,
    /// Partitioning scheme to use
    partitioning: Partitioning,
    /// Channels for sending batches from input partitions to output partitions.
    /// Key is the partition number
    channels: Arc<
        Mutex<
            HashMap<usize, (UnboundedSender<MaybeBatch>, UnboundedReceiver<MaybeBatch>)>,
        >,
    >,

    /// Execution metrics
    metrics: RepartitionMetrics,
}

#[derive(Debug, Clone)]
struct RepartitionMetrics {
    /// Time in nanos to execute child operator and fetch batches
    fetch_nanos: Arc<SQLMetric>,
    /// Time in nanos to perform repartitioning
    repart_nanos: Arc<SQLMetric>,
    /// Time in nanos for sending resulting batches to channels
    send_nanos: Arc<SQLMetric>,
}

impl RepartitionMetrics {
    fn new() -> Self {
        Self {
            fetch_nanos: SQLMetric::time_nanos(),
            repart_nanos: SQLMetric::time_nanos(),
            send_nanos: SQLMetric::time_nanos(),
        }
    }
    /// Convert into the external metrics form
    fn to_hashmap(&self) -> HashMap<String, SQLMetric> {
        let mut metrics = HashMap::new();
        metrics.insert("fetchTime".to_owned(), self.fetch_nanos.as_ref().clone());
        metrics.insert(
            "repartitionTime".to_owned(),
            self.repart_nanos.as_ref().clone(),
        );
        metrics.insert("sendTime".to_owned(), self.send_nanos.as_ref().clone());
        metrics
    }
}

impl RepartitionExec {
    /// Input execution plan
    pub fn input(&self) -> &Arc<dyn ExecutionPlan> {
        &self.input
    }

    /// Partitioning scheme to use
    pub fn partitioning(&self) -> &Partitioning {
        &self.partitioning
    }
}

#[async_trait]
impl ExecutionPlan for RepartitionExec {
    /// Return a reference to Any that can be used for downcasting
    fn as_any(&self) -> &dyn Any {
        self
    }

    /// Get the schema for this execution plan
    fn schema(&self) -> SchemaRef {
        self.input.schema()
    }

    fn children(&self) -> Vec<Arc<dyn ExecutionPlan>> {
        vec![self.input.clone()]
    }

    fn with_new_children(
        &self,
        children: Vec<Arc<dyn ExecutionPlan>>,
    ) -> Result<Arc<dyn ExecutionPlan>> {
        match children.len() {
            1 => Ok(Arc::new(RepartitionExec::try_new(
                children[0].clone(),
                self.partitioning.clone(),
            )?)),
            _ => Err(DataFusionError::Internal(
                "RepartitionExec wrong number of children".to_string(),
            )),
        }
    }

    fn output_partitioning(&self) -> Partitioning {
        self.partitioning.clone()
    }

    async fn execute(&self, partition: usize) -> Result<SendableRecordBatchStream> {
        // lock mutexes
        let mut channels = self.channels.lock().await;

        let num_input_partitions = self.input.output_partitioning().partition_count();
        let num_output_partitions = self.partitioning.partition_count();

        // if this is the first partition to be invoked then we need to set up initial state
        if channels.is_empty() {
            // create one channel per *output* partition
            for partition in 0..num_output_partitions {
                // Note that this operator uses unbounded channels to avoid deadlocks because
                // the output partitions can be read in any order and this could cause input
                // partitions to be blocked when sending data to output UnboundedReceivers that are not
                // being read yet. This may cause high memory usage if the next operator is
                // reading output partitions in order rather than concurrently. One workaround
                // for this would be to add spill-to-disk capabilities.
                let (sender, receiver) =
                    mpsc::unbounded_channel::<Option<ArrowResult<RecordBatch>>>();
                channels.insert(partition, (sender, receiver));
            }
            // Use fixed random state
            let random = ahash::RandomState::with_seeds(0, 0, 0, 0);

            // launch one async task per *input* partition
            for i in 0..num_input_partitions {
                let txs: HashMap<_, _> = channels
                    .iter()
                    .map(|(partition, (tx, _rx))| (*partition, tx.clone()))
                    .collect();

                let input_task: JoinHandle<Result<()>> =
                    tokio::spawn(Self::pull_from_input(
                        random.clone(),
                        self.input.clone(),
                        i,
                        txs.clone(),
                        self.partitioning.clone(),
                        self.metrics.clone(),
                    ));

                // In a separate task, wait for each input to be done
                // (and pass along any errors, including panic!s)
                tokio::spawn(Self::wait_for_task(input_task, txs));
            }
        }

        // now return stream for the specified *output* partition which will
        // read from the channel
        Ok(Box::pin(RepartitionStream {
            num_input_partitions,
            num_input_partitions_processed: 0,
            schema: self.input.schema(),
            input: UnboundedReceiverStream::new(channels.remove(&partition).unwrap().1),
        }))
    }

    fn metrics(&self) -> HashMap<String, SQLMetric> {
        self.metrics.to_hashmap()
    }

    fn fmt_as(
        &self,
        t: DisplayFormatType,
        f: &mut std::fmt::Formatter,
    ) -> std::fmt::Result {
        match t {
            DisplayFormatType::Default => {
                write!(f, "RepartitionExec: partitioning={:?}", self.partitioning)
            }
        }
    }
}

impl RepartitionExec {
    /// Create a new RepartitionExec
    pub fn try_new(
        input: Arc<dyn ExecutionPlan>,
        partitioning: Partitioning,
    ) -> Result<Self> {
        Ok(RepartitionExec {
            input,
            partitioning,
            channels: Arc::new(Mutex::new(HashMap::new())),
            metrics: RepartitionMetrics::new(),
        })
    }

    /// Pulls data from the specified input plan, feeding it to the
    /// output partitions based on the desired partitioning
    ///
    /// i is the input partition index
    ///
    /// txs hold the output sending channels for each output partition
    async fn pull_from_input(
        random_state: ahash::RandomState,
        input: Arc<dyn ExecutionPlan>,
        i: usize,
        mut txs: HashMap<usize, UnboundedSender<Option<ArrowResult<RecordBatch>>>>,
        partitioning: Partitioning,
        metrics: RepartitionMetrics,
    ) -> Result<()> {
        let num_output_partitions = txs.len();

        // execute the child operator
        let now = Instant::now();
        let mut stream = input.execute(i).await?;
        metrics.fetch_nanos.add_elapsed(now);

        let mut counter = 0;
        let hashes_buf = &mut vec![];

        // While there are still outputs to send to, keep
        // pulling inputs
        while !txs.is_empty() {
            // fetch the next batch
            let now = Instant::now();
            let result = stream.next().await;
            metrics.fetch_nanos.add_elapsed(now);

            // Input is done
            if result.is_none() {
                break;
            }
            let result: ArrowResult<RecordBatch> = result.unwrap();

            match &partitioning {
                Partitioning::RoundRobinBatch(_) => {
                    let now = Instant::now();
                    let output_partition = counter % num_output_partitions;
                    // if there is still a receiver, send to it
                    if let Some(tx) = txs.get_mut(&output_partition) {
                        if tx.send(Some(result)).is_err() {
                            // If the other end has hung up, it was an early shutdown (e.g. LIMIT)
                            txs.remove(&output_partition);
                        }
                    }
                    metrics.send_nanos.add_elapsed(now);
                }
                Partitioning::Hash(exprs, _) => {
                    let now = Instant::now();
                    let input_batch = result?;
                    let arrays = exprs
                        .iter()
                        .map(|expr| {
                            Ok(expr
                                .evaluate(&input_batch)?
                                .into_array(input_batch.num_rows()))
                        })
                        .collect::<Result<Vec<_>>>()?;
                    hashes_buf.clear();
                    hashes_buf.resize(arrays[0].len(), 0);
                    // Hash arrays and compute buckets based on number of partitions
                    let hashes = create_hashes(&arrays, &random_state, hashes_buf)?;
                    let mut indices = vec![vec![]; num_output_partitions];
                    for (index, hash) in hashes.iter().enumerate() {
                        indices[(*hash % num_output_partitions as u64) as usize]
                            .push(index as u64)
                    }
                    metrics.repart_nanos.add_elapsed(now);
                    for (num_output_partition, partition_indices) in
                        indices.into_iter().enumerate()
                    {
                        let now = Instant::now();
                        let indices = partition_indices.into();
                        // Produce batches based on indices
                        let columns = input_batch
                            .columns()
                            .iter()
                            .map(|c| {
                                take(c.as_ref(), &indices, None).map_err(|e| {
                                    DataFusionError::Execution(e.to_string())
                                })
                            })
                            .collect::<Result<Vec<Arc<dyn Array>>>>()?;
                        let output_batch =
                            RecordBatch::try_new(input_batch.schema(), columns);
                        metrics.repart_nanos.add_elapsed(now);
                        let now = Instant::now();
                        // if there is still a receiver, send to it
                        if let Some(tx) = txs.get_mut(&num_output_partition) {
                            if tx.send(Some(output_batch)).is_err() {
                                // If the other end has hung up, it was an early shutdown (e.g. LIMIT)
                                txs.remove(&num_output_partition);
                            }
                        }
                        metrics.send_nanos.add_elapsed(now);
                    }
                }
                other => {
                    // this should be unreachable as long as the validation logic
                    // in the constructor is kept up-to-date
                    return Err(DataFusionError::NotImplemented(format!(
                        "Unsupported repartitioning scheme {:?}",
                        other
                    )));
                }
            }
            counter += 1;
        }

        Ok(())
    }

    /// Waits for `input_task` which is consuming one of the inputs to
    /// complete. Upon each successful completion, sends a `None` to
    /// each of the output tx channels to signal one of the inputs is
    /// complete. Upon error, propagates the errors to all output tx
    /// channels.
    async fn wait_for_task(
        input_task: JoinHandle<Result<()>>,
        txs: HashMap<usize, UnboundedSender<Option<ArrowResult<RecordBatch>>>>,
    ) {
        // wait for completion, and propagate error
        // note we ignore errors on send (.ok) as that means the receiver has already shutdown.
        match input_task.await {
            // Error in joining task
            Err(e) => {
                for (_, tx) in txs {
                    let err = DataFusionError::Execution(format!("Join Error: {}", e));
                    let err = Err(err.into_arrow_external_error());
                    tx.send(Some(err)).ok();
                }
            }
            // Error from running input task
            Ok(Err(e)) => {
                for (_, tx) in txs {
                    // wrap it because need to send error to all output partitions
                    let err = DataFusionError::Execution(e.to_string());
                    let err = Err(err.into_arrow_external_error());
                    tx.send(Some(err)).ok();
                }
            }
            // Input task completed successfully
            Ok(Ok(())) => {
                // notify each output partition that this input partition has no more data
                for (_, tx) in txs {
                    tx.send(None).ok();
                }
            }
        }
    }
}

struct RepartitionStream {
    /// Number of input partitions that will be sending batches to this output channel
    num_input_partitions: usize,
    /// Number of input partitions that have finished sending batches to this output channel
    num_input_partitions_processed: usize,
    /// Schema
    schema: SchemaRef,
    /// channel containing the repartitioned batches
    input: UnboundedReceiverStream<Option<ArrowResult<RecordBatch>>>,
}

impl Stream for RepartitionStream {
    type Item = ArrowResult<RecordBatch>;

    fn poll_next(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Self::Item>> {
        match self.input.poll_next_unpin(cx) {
            Poll::Ready(Some(Some(v))) => Poll::Ready(Some(v)),
            Poll::Ready(Some(None)) => {
                self.num_input_partitions_processed += 1;
                if self.num_input_partitions == self.num_input_partitions_processed {
                    // all input partitions have finished sending batches
                    Poll::Ready(None)
                } else {
                    // other partitions still have data to send
                    self.poll_next(cx)
                }
            }
            Poll::Ready(None) => Poll::Ready(None),
            Poll::Pending => Poll::Pending,
        }
    }
}

impl RecordBatchStream for RepartitionStream {
    /// Get the schema
    fn schema(&self) -> SchemaRef {
        self.schema.clone()
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{
        assert_batches_sorted_eq,
        physical_plan::{expressions::col, memory::MemoryExec},
        test::exec::{BarrierExec, ErrorExec, MockExec},
    };
    use arrow::datatypes::{DataType, Field, Schema};
    use arrow::record_batch::RecordBatch;
    use arrow::{
        array::{ArrayRef, StringArray, UInt32Array},
        error::ArrowError,
    };

    #[tokio::test]
    async fn one_to_many_round_robin() -> Result<()> {
        // define input partitions
        let schema = test_schema();
        let partition = create_vec_batches(&schema, 50);
        let partitions = vec![partition];

        // repartition from 1 input to 4 output
        let output_partitions =
            repartition(&schema, partitions, Partitioning::RoundRobinBatch(4)).await?;

        assert_eq!(4, output_partitions.len());
        assert_eq!(13, output_partitions[0].len());
        assert_eq!(13, output_partitions[1].len());
        assert_eq!(12, output_partitions[2].len());
        assert_eq!(12, output_partitions[3].len());

        Ok(())
    }

    #[tokio::test]
    async fn many_to_one_round_robin() -> Result<()> {
        // define input partitions
        let schema = test_schema();
        let partition = create_vec_batches(&schema, 50);
        let partitions = vec![partition.clone(), partition.clone(), partition.clone()];

        // repartition from 3 input to 1 output
        let output_partitions =
            repartition(&schema, partitions, Partitioning::RoundRobinBatch(1)).await?;

        assert_eq!(1, output_partitions.len());
        assert_eq!(150, output_partitions[0].len());

        Ok(())
    }

    #[tokio::test]
    async fn many_to_many_round_robin() -> Result<()> {
        // define input partitions
        let schema = test_schema();
        let partition = create_vec_batches(&schema, 50);
        let partitions = vec![partition.clone(), partition.clone(), partition.clone()];

        // repartition from 3 input to 5 output
        let output_partitions =
            repartition(&schema, partitions, Partitioning::RoundRobinBatch(5)).await?;

        assert_eq!(5, output_partitions.len());
        assert_eq!(30, output_partitions[0].len());
        assert_eq!(30, output_partitions[1].len());
        assert_eq!(30, output_partitions[2].len());
        assert_eq!(30, output_partitions[3].len());
        assert_eq!(30, output_partitions[4].len());

        Ok(())
    }

    #[tokio::test]
    async fn many_to_many_hash_partition() -> Result<()> {
        // define input partitions
        let schema = test_schema();
        let partition = create_vec_batches(&schema, 50);
        let partitions = vec![partition.clone(), partition.clone(), partition.clone()];

        let output_partitions = repartition(
            &schema,
            partitions,
            Partitioning::Hash(vec![col("c0", &schema)?], 8),
        )
        .await?;

        let total_rows: usize = output_partitions.iter().map(|x| x.len()).sum();

        assert_eq!(8, output_partitions.len());
        assert_eq!(total_rows, 8 * 50 * 3);

        Ok(())
    }

    fn test_schema() -> Arc<Schema> {
        Arc::new(Schema::new(vec![Field::new("c0", DataType::UInt32, false)]))
    }

    fn create_vec_batches(schema: &Arc<Schema>, n: usize) -> Vec<RecordBatch> {
        let batch = create_batch(schema);
        let mut vec = Vec::with_capacity(n);
        for _ in 0..n {
            vec.push(batch.clone());
        }
        vec
    }

    fn create_batch(schema: &Arc<Schema>) -> RecordBatch {
        RecordBatch::try_new(
            schema.clone(),
            vec![Arc::new(UInt32Array::from(vec![1, 2, 3, 4, 5, 6, 7, 8]))],
        )
        .unwrap()
    }

    async fn repartition(
        schema: &SchemaRef,
        input_partitions: Vec<Vec<RecordBatch>>,
        partitioning: Partitioning,
    ) -> Result<Vec<Vec<RecordBatch>>> {
        // create physical plan
        let exec = MemoryExec::try_new(&input_partitions, schema.clone(), None)?;
        let exec = RepartitionExec::try_new(Arc::new(exec), partitioning)?;

        // execute and collect results
        let mut output_partitions = vec![];
        for i in 0..exec.partitioning.partition_count() {
            // execute this *output* partition and collect all batches
            let mut stream = exec.execute(i).await?;
            let mut batches = vec![];
            while let Some(result) = stream.next().await {
                batches.push(result?);
            }
            output_partitions.push(batches);
        }
        Ok(output_partitions)
    }

    #[tokio::test]
    async fn many_to_many_round_robin_within_tokio_task() -> Result<()> {
        let join_handle: JoinHandle<Result<Vec<Vec<RecordBatch>>>> =
            tokio::spawn(async move {
                // define input partitions
                let schema = test_schema();
                let partition = create_vec_batches(&schema, 50);
                let partitions =
                    vec![partition.clone(), partition.clone(), partition.clone()];

                // repartition from 3 input to 5 output
                repartition(&schema, partitions, Partitioning::RoundRobinBatch(5)).await
            });

        let output_partitions = join_handle
            .await
            .map_err(|e| DataFusionError::Internal(e.to_string()))??;

        assert_eq!(5, output_partitions.len());
        assert_eq!(30, output_partitions[0].len());
        assert_eq!(30, output_partitions[1].len());
        assert_eq!(30, output_partitions[2].len());
        assert_eq!(30, output_partitions[3].len());
        assert_eq!(30, output_partitions[4].len());

        Ok(())
    }

    #[tokio::test]
    async fn unsupported_partitioning() {
        // have to send at least one batch through to provoke error
        let batch = RecordBatch::try_from_iter(vec![(
            "my_awesome_field",
            Arc::new(StringArray::from(vec!["foo", "bar"])) as ArrayRef,
        )])
        .unwrap();

        let schema = batch.schema();
        let input = MockExec::new(vec![Ok(batch)], schema);
        // This generates an error (partitioning type not supported)
        // but only after the plan is executed. The error should be
        // returned and no results produced
        let partitioning = Partitioning::UnknownPartitioning(1);
        let exec = RepartitionExec::try_new(Arc::new(input), partitioning).unwrap();
        let output_stream = exec.execute(0).await.unwrap();

        // Expect that an error is returned
        let result_string = crate::physical_plan::common::collect(output_stream)
            .await
            .unwrap_err()
            .to_string();
        assert!(
            result_string
                .contains("Unsupported repartitioning scheme UnknownPartitioning(1)"),
            "actual: {}",
            result_string
        );
    }

    #[tokio::test]
    async fn error_for_input_exec() {
        // This generates an error on a call to execute. The error
        // should be returned and no results produced.

        let input = ErrorExec::new();
        let partitioning = Partitioning::RoundRobinBatch(1);
        let exec = RepartitionExec::try_new(Arc::new(input), partitioning).unwrap();

        // Note: this should pass (the stream can be created) but the
        // error when the input is executed should get passed back
        let output_stream = exec.execute(0).await.unwrap();

        // Expect that an error is returned
        let result_string = crate::physical_plan::common::collect(output_stream)
            .await
            .unwrap_err()
            .to_string();
        assert!(
            result_string.contains("ErrorExec, unsurprisingly, errored in partition 0"),
            "actual: {}",
            result_string
        );
    }

    #[tokio::test]
    async fn repartition_with_error_in_stream() {
        let batch = RecordBatch::try_from_iter(vec![(
            "my_awesome_field",
            Arc::new(StringArray::from(vec!["foo", "bar"])) as ArrayRef,
        )])
        .unwrap();

        // input stream returns one good batch and then one error. The
        // error should be returned.
        let err = Err(ArrowError::ComputeError("bad data error".to_string()));

        let schema = batch.schema();
        let input = MockExec::new(vec![Ok(batch), err], schema);
        let partitioning = Partitioning::RoundRobinBatch(1);
        let exec = RepartitionExec::try_new(Arc::new(input), partitioning).unwrap();

        // Note: this should pass (the stream can be created) but the
        // error when the input is executed should get passed back
        let output_stream = exec.execute(0).await.unwrap();

        // Expect that an error is returned
        let result_string = crate::physical_plan::common::collect(output_stream)
            .await
            .unwrap_err()
            .to_string();
        assert!(
            result_string.contains("bad data error"),
            "actual: {}",
            result_string
        );
    }

    #[tokio::test]
    async fn repartition_with_delayed_stream() {
        let batch1 = RecordBatch::try_from_iter(vec![(
            "my_awesome_field",
            Arc::new(StringArray::from(vec!["foo", "bar"])) as ArrayRef,
        )])
        .unwrap();

        let batch2 = RecordBatch::try_from_iter(vec![(
            "my_awesome_field",
            Arc::new(StringArray::from(vec!["frob", "baz"])) as ArrayRef,
        )])
        .unwrap();

        // The mock exec doesn't return immediately (instead it
        // requires the input to wait at least once)
        let schema = batch1.schema();
        let expected_batches = vec![batch1.clone(), batch2.clone()];
        let input = MockExec::new(vec![Ok(batch1), Ok(batch2)], schema);
        let partitioning = Partitioning::RoundRobinBatch(1);

        let exec = RepartitionExec::try_new(Arc::new(input), partitioning).unwrap();

        let expected = vec![
            "+------------------+",
            "| my_awesome_field |",
            "+------------------+",
            "| foo              |",
            "| bar              |",
            "| frob             |",
            "| baz              |",
            "+------------------+",
        ];

        assert_batches_sorted_eq!(&expected, &expected_batches);

        let output_stream = exec.execute(0).await.unwrap();
        let batches = crate::physical_plan::common::collect(output_stream)
            .await
            .unwrap();

        assert_batches_sorted_eq!(&expected, &batches);
    }

    #[tokio::test]
    #[ignore = "fails in CubeStore"]
    async fn repartition_with_dropping_output_stream() {
        #[derive(Debug)]
        struct Case<'a> {
            partitioning: Partitioning,
            expected: Vec<&'a str>,
        }

        let cases = vec![
            Case {
                partitioning: Partitioning::RoundRobinBatch(2),
                expected: vec![
                    "+------------------+",
                    "| my_awesome_field |",
                    "+------------------+",
                    "| baz              |",
                    "| frob             |",
                    "| gaz              |",
                    "| grob             |",
                    "+------------------+",
                ],
            },
            Case {
                partitioning: Partitioning::Hash(
                    vec![Arc::new(crate::physical_plan::expressions::Column::new(
                        "my_awesome_field",
                        0,
                    ))],
                    2,
                ),
                expected: vec![
                    "+------------------+",
                    "| my_awesome_field |",
                    "+------------------+",
                    "| frob             |",
                    "+------------------+",
                ],
            },
        ];

        for case in cases {
            println!("Running case {:?}", case.partitioning);

            // The barrier exec waits to be pinged
            // requires the input to wait at least once)
            let input = Arc::new(make_barrier_exec());

            // partition into two output streams
            let exec =
                RepartitionExec::try_new(input.clone(), case.partitioning).unwrap();

            let output_stream0 = exec.execute(0).await.unwrap();
            let output_stream1 = exec.execute(1).await.unwrap();

            // now, purposely drop output stream 0
            // *before* any outputs are produced
            std::mem::drop(output_stream0);

            // Now, start sending input
            input.wait().await;

            // output stream 1 should *not* error and have one of the input batches
            let batches = crate::physical_plan::common::collect(output_stream1)
                .await
                .unwrap();

            assert_batches_sorted_eq!(&case.expected, &batches);
        }
    }

    /// Create a BarrierExec that returns two partitions of two batches each
    fn make_barrier_exec() -> BarrierExec {
        let batch1 = RecordBatch::try_from_iter(vec![(
            "my_awesome_field",
            Arc::new(StringArray::from(vec!["foo", "bar"])) as ArrayRef,
        )])
        .unwrap();

        let batch2 = RecordBatch::try_from_iter(vec![(
            "my_awesome_field",
            Arc::new(StringArray::from(vec!["frob", "baz"])) as ArrayRef,
        )])
        .unwrap();

        let batch3 = RecordBatch::try_from_iter(vec![(
            "my_awesome_field",
            Arc::new(StringArray::from(vec!["goo", "gar"])) as ArrayRef,
        )])
        .unwrap();

        let batch4 = RecordBatch::try_from_iter(vec![(
            "my_awesome_field",
            Arc::new(StringArray::from(vec!["grob", "gaz"])) as ArrayRef,
        )])
        .unwrap();

        // The barrier exec waits to be pinged
        // requires the input to wait at least once)
        let schema = batch1.schema();
        BarrierExec::new(vec![vec![batch1, batch2], vec![batch3, batch4]], schema)
    }
}