Preserve list mutation progress and release removed items outside locks by 1ndahous3 · Pull Request #8894 · RustPython/RustPython · GitHub

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Summary

Process generic inputs incrementally in list.extend, += and initialization, preserving accepted elements when iteration raises. Reinitialization clears the list before reading its input.
Release replaced and deleted elements after the complete mutation and outside list locks, preventing deadlocks when finalizers access the same list. Preserve replacement lifetimes during reentrant cleanup.
Use the iterator's length hint for slice assignment and preserve native self-extension and self-assignment for list subclasses.

Follow-up to #8840.

Performance

Windows 11 x64

Operation, 4,096 elements	Before	After	Speedup
Copy a list and extend it with itself	77.16 µs	56.92 µs	1.36×
Clear and extend a list from a tuple	29.06 µs	26.60 µs	1.09×

Generic iterable extension no longer materializes an additional full input buffer.

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Written with Codex (GPT-6), reviewed by a human before submission.

Summary by CodeRabbit

Bug Fixes
- List extension and slice assignment now process iterable items incrementally, preserving completed updates if iteration raises an error.
- Self-extension uses the list’s existing contents, without calling a list subclass’s custom iterator or length methods.
- Slice assignment handles inaccurate length hints, and failed extended-slice assignments preserve the original list.
- List mutations, clearing, and in-place multiplication handle removed items consistently, including when object finalizers run.
- In-place multiplication with a nonpositive count clears the list.

Assisted-by: Codex:GPT-6

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No actionable comments were generated in the recent review. 🎉

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Reviewing files that changed from the base of the PR and between b787bfa and ee165cd.

📒 Files selected for processing (2)

crates/vm/src/builtins/list.rs
extra_tests/snippets/builtin_list.py

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📝 Walkthrough

Walkthrough

List extension and initialization now consume iterables incrementally. Item and slice assignment and deletion use dedicated mutation helpers. The changes also update list clearing and nonpositive in-place repetition. Tests cover iterator errors, capacity behavior, and object finalizer order.

Changes

List operations

Layer / File(s)	Summary
Iterable ingestion and initialization crates/vm/src/builtins/list.rs, extra_tests/snippets/builtin_list.py	extend handles self-extension and selected native container sources, and consumes other iterables incrementally using a length hint. extract_cloned collects cloned references, and initialization clears then extends the list. Tests cover partial updates, capacity handling, and iterator finalization.
Item and slice mutation lifecycle crates/vm/src/builtins/list.rs, extra_tests/snippets/builtin_list.py	Dedicated helpers handle item and slice assignment and deletion, including allocation and extended-slice length checks. Removed references are dropped after releasing the write lock. Tests cover mutation errors and finalizer order.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Suggested reviewers: youknowone

Merge Risk: ⚪ Minimal · up to ee165

No actionable issue was established in the reviewed list changes; the PR is mergeable after normal checks.

Security Architecture Review

Security architecture risk: 🔵 Low · up to ee165

The changed list behavior merits design review, but the reviewed paths do not establish a new security boundary bypass or material expansion of exposure. Some failure behavior changes intentionally, so callers that require all-or-nothing updates may need to account for it.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

inferred — The demonstrated effect is on the contents and cleanup lifecycle of lists receiving Python-level mutations; the inspected paths do not establish a new privileged sink or cross-system authority transition.

Trust Boundaries and Controls

observed — Python-provided iterables can execute code during iterator and length-hint evaluation. These operations precede destination locking in the changed generic path; insertion and capacity changes occur under the storage lock.

Resilience and Maintainability Implications

observed — The tests exercise finalizer reentry after removal and confirm that slice-input errors leave the destination unchanged while releasing collected replacement items.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 6.98% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 43 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely summarizes the main changes: preserving list mutation progress and releasing removed items outside list locks.
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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Assisted-by: Codex:GPT-6

Actionable comments posted: 1

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Inline comments:
Review comments at @crates/vm/src/builtins/list.rs:
- Around line 465-481: In the generic iterable-extension path, retain the loop’s
success or error result, then use the raw `elements` lock to shrink the vector
when its length is below capacity before returning that result. This trims
excess capacity on both completion and iteration failure without incrementing
`mutation_counter`.

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crates/vm/src/builtins/list.rs

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👍

Preserve list mutation progress and release removed items outside locks

ed886c5

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Merge main into list mutation fixes

b787bfa

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coderabbitai Bot reviewed Sep 29, 2026

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Comment thread crates/vm/src/builtins/list.rs

Trim excessive list extension reservations

ee165cd

Assisted-by: Codex:GPT-6

youknowone approved these changes Sep 30, 2026

View reviewed changes

youknowone merged commit e8919b9 into RustPython:main Sep 30, 2026
20 checks passed

This was referenced Oct 4, 2026

Specialized list item assignment deadlocks when the replaced element's finalizer reads the list #8955

Closed

Fix store subscr list finalizer deadlock #8956

Merged

Preserve list mutation progress and release removed items outside locks by 1ndahous3 · Pull Request #8894 · RustPython/RustPython · GitHub

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Preserve list mutation progress and release removed items outside locks - #8894

Preserve list mutation progress and release removed items outside locks#8894
youknowone merged 3 commits into
RustPython:mainfrom
1ndahous3:list_mutation_callbacks

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Preserve list mutation progress and release removed items outside locks by 1ndahous3 · Pull Request #8894 · RustPython/RustPython · GitHub

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Preserve list mutation progress and release removed items outside locks - #8894

Preserve list mutation progress and release removed items outside locks#8894youknowone merged 3 commits intoRustPython:mainRustPython/RustPython:mainfrom 1ndahous3:list_mutation_callbacks1ndahous3/RustPython:list_mutation_callbacksCopy head branch name to clipboard

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Preserve list mutation progress and release removed items outside locks#8894
youknowone merged 3 commits into
RustPython:mainfrom
1ndahous3:list_mutation_callbacks

1ndahous3 commented Sep 28, 2026 •
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