Ensure that Lineariser doesn't buffer unboundedly

[wence-]

Description

The previous approach to buffered refill in the Lineariser could result in unbounded buffering of tasks. If the next sequence number tasks was slow, we would still arbitrarily refill from fast producers, resulting in excessive memory pressure and removing the backpressure the finite capacity channels are intended to provide.

To fix this, give each producer a size-1 semaphore that it must acquire before being allowed to kick off a memory-using task. This semaphore is only released once the matching message has been forwarded into the downstream channel.

This doesn't change the reordering properties of the lineariser, but does make it more important to assign message sequence ids in round-robin fashion to the producer tasks to ensure performance is good.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[coderabbitai]

📝 Summary

Summary by CodeRabbit

• Bug Fixes

  • Improved streaming data processing by applying backpressure per producer, helping prevent excessive buffering.
  • Preserved ordered output while waiting for delayed data, improving consistency in streaming results.

• Reliability

  • Improved coordination between streaming scans and downstream processing to support more predictable execution under uneven data arrival.

Walkthrough

Lineariser now limits each producer to one in-flight message, preserves ordered delivery, and releases capacity after forwarding. DataFrame and streaming scan producers use the new acquisition API. An integration test verifies producer backpressure and ordered output.

Changes

Lineariser backpressure

Layer / File(s)	Summary
Per-producer capacity and ordered delivery python/cudf_polars/cudf_polars/streaming/actor_graph/io.py	Lineariser adds per-producer semaphores, tracks producer IDs, and releases capacity after forwarding messages.
Scan producer integration python/cudf_polars/cudf_polars/streaming/actor_graph/io.py	DataFrame and streaming scan producers acquire their input channels before processing assigned work. Task wiring passes producer IDs.
Backpressure integration test python/cudf_polars/tests/streaming/test_tracing.py	The test verifies blocking during a sequence gap and confirms ordered delivery of six messages.

Priority: ⬇️ Low — Defer this change because it narrowly limits Lineariser buffering and adds an ordered-backpressure test without supplied evidence of broader product impact.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🟡 Moderate · up to c3f1e

The new producer backpressure can deadlock a streaming query when duplicate sequence numbers overwrite a buffered message, leaving one producer unable to submit further work. Reject duplicate or already-forwarded sequence numbers before merge.

Suggested reviewers: rjzamora

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 38.46% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 13 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely states the main change: preventing unbounded buffering in Lineariser.
Description check	✅ Passed	The description directly explains the unbounded buffering problem, the semaphore-based fix, backpressure behavior, and test coverage.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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Actionable comments posted: 1

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Inline comments:
In `@python/cudf_polars/cudf_polars/streaming/actor_graph/io.py`:
- Line 132: Update the sequence-number handling around buffer and next_seq so
globally unique sequence numbers are required: reject any number below next_seq
or already present in buffer instead of overwriting the existing message, while
preserving valid zero-based ordering. Add a regression test covering duplicate
sequence numbers sent by separate producers and verify both are rejected without
losing semaphore ownership.

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📥 Commits

Reviewing files that changed from the base of the PR and between c27d94e and c3f1e92.

📒 Files selected for processing (2)

• python/cudf_polars/cudf_polars/streaming/actor_graph/io.py
• python/cudf_polars/tests/streaming/test_tracing.py

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[pentschev]

LGTM. Thanks @wence- .