Add GC infrastructure: tracking bits, tp_clear #6977

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

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🧪 Generate unit tests (beta)

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

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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

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