Add GC infrastructure: tracking bits, tp_clear by youknowone · Pull Request #6977 · RustPython/RustPython

youknowone · 2026-02-03T08:59:40Z

GC bit operations (_PyObject_GC_TRACK/UNTRACK equivalent):

Add set_gc_bit() helper for atomic GC bit manipulation
Add set_gc_tracked() / clear_gc_tracked() methods
Update is_gc_tracked() to use GcBits::TRACKED flag
Call set_gc_tracked() in track_object()
Call clear_gc_tracked() for static types (they are immortal)

tp_clear infrastructure (for breaking reference cycles):

Add try_clear_obj() function to call payload's try_clear
Add clear field to PyObjVTable
Add clear() method to PyType's Traverse impl

Summary by CodeRabbit

Bug Fixes
- Enhanced garbage collection system to more effectively detect and break circular references, improving memory management and reducing potential memory leaks.
- Improved object deallocation process to ensure proper cleanup of child references before memory is freed.
- Strengthened object tracking mechanisms to better identify objects requiring garbage collection.

coderabbitai · 2026-02-03T08:59:48Z

📝 Walkthrough

Walkthrough

This PR introduces garbage collection cycle detection and breaking mechanisms for RustPython's VM. It adds a clear traversal hook to extract child references from objects, implements GC bit tracking operations on PyObject, integrates automatic object tracking during creation, and modifies deallocation to run tp_clear before freeing memory.

Changes

Cohort / File(s)	Summary
GC Traversal Clear Hook `crates/vm/src/builtins/type.rs`, `crates/vm/src/object/traverse_object.rs`	Introduces clear callback infrastructure to PyObjVTable with optional `clear` field; PyType now implements a traversal hook to collect referenced objects during GC clearing.
GC Bit Management & Deallocation `crates/vm/src/object/core.rs`	Adds atomic GC bit operations (`set_gc_bit`, `set_gc_tracked`, `clear_gc_tracked`); implements `try_clear_obj` to extract child references; modifies `default_dealloc` to invoke tp_clear before memory release and extract collected edges.
Object Tracking Integration `crates/vm/src/gc_state.rs`, `crates/vm/src/object/core.rs`	Calls `set_gc_tracked()` during object tracking in `gc_state::track_object`; extends `PyRef<T>::new_ref` to automatically track objects in GC based on traversal capability, instance dict presence, and heap-type status; explicitly untracks weakref_type after initialization.

Sequence Diagram

sequenceDiagram
    participant Code as Client Code
    participant PyRef as PyRef Creation
    participant PyObj as PyObject
    participant GCState as GC State
    participant Dealloc as Deallocation
    
    Code->>PyRef: new_ref(payload, type, dict)
    PyRef->>PyObj: Create with payload
    PyRef->>GCState: Check traversal capability
    alt Has Traverse + Dict/HeapType
        PyRef->>GCState: track_object()
        GCState->>PyObj: set_gc_tracked()
        PyObj->>PyObj: Mark GC bit
    end
    
    Note over GCState: GC Cycle Detection Triggered
    GCState->>PyObj: Invoke clear callback
    PyObj->>PyObj: Call tp_clear (via try_clear_obj)
    PyObj-->>GCState: Return child references
    GCState->>GCState: Process references for cycles
    
    Code->>Dealloc: Object deallocation
    Dealloc->>PyObj: default_dealloc()
    Dealloc->>PyObj: tp_clear() if not cleared
    Dealloc->>PyObj: Extract collected edges
    Dealloc->>Dealloc: Free memory

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Possibly related PRs

gc module internal structure and API #6910: Directly modifies the same GC subsystem including gc_state::track_object, PyObject GC bits, and vtable traversal/clear hooks.
Improve object traversal for heap types and mro #6976: Alters PyType's traversal behavior to complement the new clear hook mechanism introduced here.
impl gc finialized #6689: Modifies PyObject GC-bit management and related operations that are extended in this PR.

Suggested reviewers

ShaharNaveh

Poem

🐰 A rabbit's ode to cycles broke,
With clear hooks and GC strokes,
Bits are tracked with care so fine,
Memory freed in perfect line,
No more cycles to provoke! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main changes: adding GC infrastructure including tracking bits and tp_clear functionality across multiple files.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

📝 Generate docstrings

🧪 Generate unit tests (beta)

Create PR with unit tests
Post copyable unit tests in a comment

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

GC bit operations (_PyObject_GC_TRACK/UNTRACK equivalent): - Add set_gc_bit() helper for atomic GC bit manipulation - Add set_gc_tracked() / clear_gc_tracked() methods - Update is_gc_tracked() to use GcBits::TRACKED flag - Call set_gc_tracked() in track_object() - Call clear_gc_tracked() for static types (they are immortal) tp_clear infrastructure (for breaking reference cycles): - Add try_clear_obj() function to call payload's try_clear - Add clear field to PyObjVTable - Add clear() method to PyType's Traverse impl

coderabbitai

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@crates/vm/src/builtins/type.rs`:
- Around line 424-427: When marking static types immortal, in addition to
calling new_type.as_object().clear_gc_tracked() you must also remove the object
from the VM's GC tracking sets in GcState so it no longer appears in
gc.get_objects or generation counts; update the same block (the one handling
static types) to call the same removal logic used for weakref_type (i.e., remove
the object from the tracking sets / GcState tracking collections) immediately
after clear_gc_tracked() so is_gc_tracked() and the GC sets stay consistent.

coderabbitai · 2026-02-03T10:39:27Z

crates/vm/src/builtins/type.rs

+        // Static types are not tracked by GC.
+        // They are immortal and never participate in collectable cycles.
+        new_type.as_object().clear_gc_tracked();
+


⚠️ Potential issue | 🟠 Major

Untrack static types from GC state as well.

Clearing only the TRACKED bit leaves the type in GcState sets, so gc.get_objects/generation counts can still include it and diverge from is_gc_tracked(). Please also remove it from the tracking sets (similar to weakref_type).

💡 Suggested fix

- new_type.as_object().clear_gc_tracked(); + let obj = new_type.as_object(); + unsafe { crate::gc_state::gc_state().untrack_object(NonNull::from(obj)); } + obj.clear_gc_tracked();

📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change

// Static types are not tracked by GC.

// They are immortal and never participate in collectable cycles.

new_type.as_object().clear_gc_tracked();

// Static types are not tracked by GC.

// They are immortal and never participate in collectable cycles.

let obj = new_type.as_object();

unsafe { crate::gc_state::gc_state().untrack_object(NonNull::from(obj)); }

obj.clear_gc_tracked();

🤖 Prompt for AI Agents

In `@crates/vm/src/builtins/type.rs` around lines 424 - 427, When marking static types immortal, in addition to calling new_type.as_object().clear_gc_tracked() you must also remove the object from the VM's GC tracking sets in GcState so it no longer appears in gc.get_objects or generation counts; update the same block (the one handling static types) to call the same removal logic used for weakref_type (i.e., remove the object from the tracking sets / GcState tracking collections) immediately after clear_gc_tracked() so is_gc_tracked() and the GC sets stay consistent.

youknowone force-pushed the gc-infra branch from f4ea079 to f86093d Compare February 3, 2026 09:53

youknowone marked this pull request as ready for review February 3, 2026 10:30

coderabbitai bot reviewed Feb 3, 2026

View reviewed changes

youknowone merged commit f71fe9b into RustPython:main Feb 3, 2026
13 checks passed

youknowone deleted the gc-infra branch February 3, 2026 11:00

coderabbitai bot mentioned this pull request Feb 4, 2026

Add GC infrastructure: object tracking, tp_clear, and helper methods #6994

Merged

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Add GC infrastructure: tracking bits, tp_clear#6977

Add GC infrastructure: tracking bits, tp_clear#6977
youknowone merged 1 commit intoRustPython:mainfrom
youknowone:gc-infra

youknowone commented Feb 3, 2026 •

edited by coderabbitai bot

Loading

Uh oh!

coderabbitai bot commented Feb 3, 2026 •

edited

Loading

Walkthrough

Changes

Sequence Diagram

Estimated code review effort

Possibly related PRs

Suggested reviewers

Poem

Uh oh!

coderabbitai bot left a comment

Uh oh!

coderabbitai bot Feb 3, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

1 participant

Conversation

youknowone commented Feb 3, 2026 • edited by coderabbitai bot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Summary by CodeRabbit

Uh oh!

coderabbitai bot commented Feb 3, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Walkthrough

Changes

Sequence Diagram

Estimated code review effort

Possibly related PRs

Suggested reviewers

Poem

Uh oh!

coderabbitai bot left a comment

Choose a reason for hiding this comment

Uh oh!

coderabbitai bot Feb 3, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

1 participant

youknowone commented Feb 3, 2026 •

edited by coderabbitai bot

Loading

coderabbitai bot commented Feb 3, 2026 •

edited

Loading