[C++][Acero] Apparent deadlock in Table.join_asof

[erikhansenwong]

Describe the bug, including details regarding any error messages, version, and platform.

On some calls to Table.join_asof my python process becomes unresponsive and is using zero cpu. It appears to be a thread deadlock or something similar. I have created an example that causes the deadlock with high probability.

Here are the details of my setup:

• Python 3.12.7
• pyarrow==19.0.1
• numpy==2.2.4
• pandas==2.2.3
• Ubuntu 22.04.5
• CPU: 13th Gen Intel(R) Core(TM) i9-13980HX

I was also able to produce the deadlock on a colleague's Mac laptop with Apple silicon using this example, so I assume it won't make a big difference what hardware it runs on.

On my laptop this always gets deadlocked before the 300th iteration

import numpy as np
import pandas as pd
import pyarrow as pa

n_left = 100
n_right = 200_000
left_start = pd.Timestamp("2025-04-07T07:45:55", tz="UTC")
right_start = pd.Timestamp("2025-04-07T00:00:00", tz="UTC")
time_end = pd.Timestamp("2025-04-07T12:05:59", tz="UTC")

tolerance_nanos = 60 * 1_000_000_000
np.random.seed(0)


def get_timestamps(start, end, n):
 seconds = (end - start).total_seconds()
 td = np.random.uniform(0, 1, n)
 td *= np.random.choice([0, 1], n)
 td *= seconds / td.sum()
 td = td.cumsum()
 return start + pd.to_timedelta(td, "seconds")


left_schema = pa.schema([pa.field("timestamp", pa.timestamp("ns", "UTC"))])
right_schema = pa.schema(
 [
 pa.field("timestamp", pa.timestamp("ns", "UTC")),
 pa.field("value", pa.float64()),
 ]
)

left = pa.table(
 {"timestamp": get_timestamps(left_start, time_end, n_left)},
 schema=left_schema,
)
right = pa.table(
 {
 "timestamp": get_timestamps(right_start, time_end, n_right),
 "value": np.random.normal(100, 5, n_right),
 },
 schema=right_schema,
)

for i in range(1000):
 print(f"{i:>5} | {pd.Timestamp.now()}")
 left.join_asof(
 right,
 on="timestamp",
 by=[],
 tolerance=tolerance_nanos,
 )

Component(s)

Python

[amoeba]

Hi @erikhansenwong, thanks for the excellent reproducer. I can confirm this deadlocks for me with a similar environment.

I notice this looks similar to a previous deadlock that was supposedly fixed, #37796. If I reduce n_right in your example code to something like 200, I no longer get the deadlock. Maybe the issue wasn't entirely fixed. cc @bkietz @westonpace

[erikhansenwong]

Hi @erikhansenwong, thanks for the excellent reproducer. I can confirm this deadlocks for me with a similar environment.

I notice this looks similar to a previous deadlock that was supposedly fixed, #37796. If I reduce n_right in your example code to something like 200, I no longer get the deadlock. Maybe the issue wasn't entirely fixed. cc @bkietz @westonpace

There is another thing I noticed with my example that might potentially provide a clue to anyone trying to track down the root cause - if I set left_start equal to right_start then I did not experience any deadlocks. I was only seeing deadlocks when I set left_start to a time significantly later than right_start.

[zanmato1984]

I can reproduce the deadlock in a C++ case at a fair chance (thanks @erikhansenwong for the tip of increasing the chance, it works!).

I'm now trying to locate the cause.

[zanmato1984]

Issue resolved by pull request 46300
#46300

[erikhansenwong]

Thank you @zanmato1984 🙏