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GeezerVison is showing github.com — Preserve ctypes py_object provenance across casts and pointer writes by 1ndahous3 · Pull Request #9391 · RustPython/RustPython

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1ndahous3 commented Oct 26, 2026 •

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Extends the ctypes ownership handling from #9251 to pointer writes and byref casts.

Summary

Resolve py_object values written through pointers from the original storage and sibling views, including independent from_buffer acquisitions and aggregate writes that overlap a smaller view.
Use weak links to the roots retaining written values, including writes through temporary pointer fields, without extending their lifetime. Amortize cleanup across new registrations and keep the destination owner alive through conversion, writing and ownership publication. Publish actual write ranges after user conversion so nested writes cannot replace the outer operation's range.
Reuse validated weak ownership paths for direct pointers, casts and aggregate pointer fields. Cache each field on its retention root, including pointer roots, matching byref casts and private byref arguments copied into fields. Validate the exact private slot on reuse so a surviving sibling owner cannot validate a replaced field's path.
Avoid repeated ownership-graph searches for writes into nested storage. Follow buffer-exporter links for borrowed views, including views of pointer contents and writes through pointers targeting those views.
Keep both current contents and retained cast storage able to resolve later writes, including differently sized views of the same buffer and retargeting through intermediate pointers or aggregate fields.
Lazily index direct retention entries for reads through registered writers, avoiding repeated scans of all retained values after pointer-field writes. The index stores O(N) Rust metadata and validates cached keys against current values without retaining extra Python references.
Retain byref arguments privately when casting them, retaining the originating ctypes object for provenance lookup without changing the public _objects value. Link borrowed sources weakly to their writers even when the write falls outside the source view, so independent buffer views created before or after the write can resolve the value.
Preserve existing owners when assigning None. Keep from_buffer acquisitions alive through scalar _objects replacement, use one range-index implementation for independent aliases and actual pointer writes, with weak per-interpreter registrations removed when their owners are cleared or destroyed. Actual-write entries belong to the retention root, cover copied pointers after the original source view dies, and retain only the latest range per ownership slot. Prefer recorded writes and overlapping aliases on reads. On a miss, search current registered owners so earlier addresses remain readable after pointer retargeting, without accumulating address history. Follow native and Python-defined buffer exporters in both read and write provenance.

The private byref retention is stronger than CPython's cast lifetime guarantee. from_buffer retains its original memoryview after scalar assignment; unlike CPython, scalar assignment alone no longer permits resizing a bytearray still borrowed by the view. Reads never reconstruct a Rust reference from an arbitrary native address.

Performance

Write benchmarks compare with 073983299; read and mixed benchmarks compare with 1d9469382. The cast-rooted and byref-rooted field rows compare with a3a072dc3; shared-buffer read rows compare with 842238ce6. The private-byref-field and out-of-view writer rows compare with c65a11a7f. Median of five runs:

Windows 11 x64

Workload	Before	After
4,000 writes through a pointer field	259.1 ms	5.28 ms
Warm writes through 1,000 separate pointer fields	211.1 ms	1.66 ms
Read 4,000 backing-array elements after pointer-field writes	406.4 ms	2.75 ms
4,000 pointer-field writes followed by backing-array reads	439.6 ms	10.01 ms
Warm writes through 800 cast-rooted pointer fields	318.71 ms	1.58 ms
Warm writes through 800 byref-rooted pointer fields	237.68 ms	1.78 ms
Read 800 independent views of one shared buffer	203.86 ms	1.27 ms
Warm writes through 800 fields containing separate byref casts	224.02 ms	1.60 ms
Read 800 slots written outside one byref source view	178.68 ms	0.58 ms

Debian 12 amd64

Workload	Before	After
4,000 writes through a pointer field	264.1 ms	4.92 ms
Warm writes through 1,000 separate pointer fields	194.1 ms	1.65 ms
Read 4,000 backing-array elements after pointer-field writes	369.0 ms	2.62 ms
4,000 pointer-field writes followed by backing-array reads	378.3 ms	9.73 ms
Warm writes through 800 cast-rooted pointer fields	298.40 ms	1.60 ms
Warm writes through 800 byref-rooted pointer fields	226.40 ms	1.98 ms
Read 800 independent views of one shared buffer	201.25 ms	1.21 ms
Warm writes through 800 fields containing separate byref casts	218.89 ms	1.53 ms
Read 800 slots written outside one byref source view	183.18 ms	0.56 ms

Compared with a3a072dc3, registering a from_buffer view adds about 0.5–0.6 µs to construction on these systems.

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

Summary by CodeRabbit

Bug Fixes
- Improved ctypes.py_object assignments through arrays, pointers, and shared views so updates are reflected consistently.
- Improved object lifetime handling when pointer contents are replaced, retaining the replacement and releasing the previous value appropriately.
- Fixed assigning None so it does not prematurely release a buffer export.
- Improved casts from byref() arguments so the source object remains available while the cast pointer exists.
Tests
- Added regression coverage for shared views, pointer updates, object replacement, buffer exports, and restoring an in-range index after pointer contents change.

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-27T08:36:45.249150Z	`37c84fb`	New commits

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crates/vm/src/stdlib/_ctypes/base.rs
extra_tests/snippets/stdlib_ctypes.py

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Walkthrough

The change adds Python-object writer tracking across ctypes storage views. Selected assignments register destination views, casts retain CArgObject sources, and regression tests cover related writes, object replacement, ownership, and buffer exports.

Changes: ctypes py_object writer tracking

Layer / File(s)	Summary
Writer metadata and object index `crates/vm/src/stdlib/_ctypes/base.rs`	`PrivateRefs` gains writer-path caches and an object index. Retention operations update the index, and exact integer keys support indexed lookups.
Writer provenance and assignment registration `crates/vm/src/stdlib/_ctypes/base.rs`, `crates/vm/src/stdlib/_ctypes/function.rs`, `crates/vm/src/stdlib/_ctypes/array.rs`, `crates/vm/src/stdlib/_ctypes/pointer.rs`, `crates/vm/src/stdlib/_ctypes/simple.rs`	Provenance traversal discovers and validates writer paths across ctypes storage and related owners. Selected field, array, pointer, and simple-value assignments register views. Casts retain `CArgObject` sources.
Writer and view regression coverage `extra_tests/snippets/stdlib_ctypes.py`	Tests cover writes through related and overlapping views, object replacement, cast ownership, temporary pointer fields, and buffer-export retention.

Priority: Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Assignment as ctypes assignment
  participant Provenance as writer provenance
  participant Views as related storage views
  participant Retention as Python-object retention
  Assignment->>Provenance: register destination storage
  Provenance->>Views: discover and validate overlapping writers
  Assignment->>Retention: prepare and write assigned value
  Retention->>Views: update retained object references

Suggested reviewers: youknowone

Merge Risk: Minimal · previous review a3a07

No confirmed issue remains that requires correction before merging.

Pre-merge checks: 4

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: preserving ctypes py_object provenance across casts and pointer writes.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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🧹 Nitpick comments: 1

extra_tests/snippets/stdlib_ctypes.py: 1

981-995: Trivial · Functional Correctness · Quick win

Add a RustPython-only release-after-cast assertion.

The current assertions keep _value alive through _pointer and _sibling, so they would pass even if the private cast-owner retention were removed. Add an isolated lifetime check. CPython releases this source after the cast, so guard the new assertion only.

Suggested fix

     assert _value.value is _pointer[0] is _sibling[0]
 
+if _sys.implementation.name == "rustpython":
+    _cast_value = ctypes.py_object(None)
+    _cast_ref = weakref.ref(_cast_value)
+    _cast_arg = ctypes.byref(_cast_value)
+    _cast_pointer = ctypes.cast(
+        _cast_arg, ctypes.POINTER(ctypes.py_object)
+    )
+    del _cast_arg, _cast_value
+    gc.collect()
+    assert _cast_ref() is not None
+
 # A warmed writer must follow contents changes without retaining old storage.

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Review comment at @extra_tests/snippets/stdlib_ctypes.py around lines 981 - 995:
Add a RustPython-only lifetime check after the pointer-view assertions: create a
separate ctypes source, retain a weak reference, cast it, then delete the source
and cast argument and verify the source remains alive. Keep this check isolated
from the existing _value, _pointer, and _sibling assertions, which retain the
source independently.

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Nitpick comments:
Review comments at @extra_tests/snippets/stdlib_ctypes.py:
- Around line 981-995: Add a RustPython-only lifetime check after the
pointer-view assertions: create a separate ctypes source, retain a weak
reference, cast it, then delete the source and cast argument and verify the
source remains alive. Keep this check isolated from the existing _value,
_pointer, and _sibling assertions, which retain the source independently.

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crates/vm/src/stdlib/_ctypes/array.rs
crates/vm/src/stdlib/_ctypes/base.rs
crates/vm/src/stdlib/_ctypes/function.rs
crates/vm/src/stdlib/_ctypes/pointer.rs
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youknowone approved these changes

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1ndahous3 added 2 commits

October 26, 2026 15:32


          Preserve ctypes py_object provenance across casts and pointer writes

154d1c5

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          Validate ctypes writer paths across casts and retargeting

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1ndahous3 force-pushed the ctypes_pyobject_owners branch from 6b2ccac to 2078378 Compare

October 26, 2026 12:45

coderabbitai Bot mentioned this pull request

Oct 26, 2026

Share ctypes backing-owner snapshots between aggregate copies #9392

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Actionable comments posted: 1

🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @crates/vm/src/stdlib/_ctypes/base.rs:
- Around line 1531-1543: Restrict the fallback walk in the provenance loop using
`provenance_children` to ctypes ownership edges, such as `PyCData`,
`CArgObject`, `PyMemoryView`, and writer links; do not recursively descend into
arbitrary `PyDict` or `PyTuple` values stored in `py_object` slots. Preserve
`storage_root` checks and writer registration for reachable ownership targets.

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crates/vm/src/stdlib/_ctypes/array.rs
crates/vm/src/stdlib/_ctypes/base.rs
crates/vm/src/stdlib/_ctypes/function.rs
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1ndahous3 commented Oct 26, 2026

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@coderabbitai The isolated cast-lifetime check is included in 2078378. It drops the source and byref argument, confirms retention by the cast pointer, then confirms release after deleting that pointer. Only this stronger RustPython lifetime assertion is guarded: CPython 3.15 releases the source earlier. Checked on Windows and Debian.

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@1ndahous3 Thanks for adding the isolated check in 2078378cf. The sequence you describe covers both source retention while the cast pointer exists and source release after deleting that pointer.

Guarding only the stronger RustPython lifetime assertion is appropriate given your reported CPython 3.15 behavior. Thanks for checking on Windows and Debian.

🧠 Learnings used

Learnt from: 1ndahous3
Repo: RustPython/RustPython PR: 9229
File: crates/vm/src/stdlib/_ctypes/base.rs:887-920
Timestamp: 2026-10-19T21:22:19.002Z
Learning: In RustPython's ctypes implementation, shared `_objects` dictionaries are intentional CPython compatibility behavior. In `crates/vm/src/stdlib/_ctypes/base.rs`, do not require blanket copy-on-write in `PyCData::keep_ref`, `PyCData::store_in_object`, or recursive dictionary snapshots in `PyCData::keep_cdata` solely to prevent nested-source callback replacement from releasing a copied callback's owner. The author reports that CPython 3.15 exhibits the same ownership hazard on Windows and Linux. Stronger ownership snapshots require a separate lifetime/design decision.

Learnt from: CR
Repo: RustPython/RustPython PR: 0
File: AGENTS.md:0-0
Timestamp: 2026-10-20T02:42:54.271Z
Learning: Source excerpt:
# AI Agent Instructions for RustPython

## AI Agent Rules

- Follow RustPython's [AI Policy](https://github.com/RustPython/.github/blob/main/AI_POLICY.md) for every AI-assisted contribution.

Learnt from: 1ndahous3
Repo: RustPython/RustPython PR: 9186
File: crates/vm/src/stdlib/_ctypes/base.rs:1048-1057
Timestamp: 2026-10-18T09:29:14.644Z
Learning: In RustPython's crates/vm/src/stdlib/_ctypes/base.rs, non-character ctypes array fields must share the containing object's storage so addressof(), FFI calls, and buffer writes access the owner’s memory. Restoring owned field copies breaks this requirement. Retaining the owner does not pin its allocation, and locks on separate views do not synchronize shared storage. CPython compatibility does not establish Rust reference validity; storage-lifetime and aliasing protection require separate analysis.

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          Preserve ctypes writer roots and avoid nested storage scans

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          Register ctypes exporters and cache pointer-field writers

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youknowone enabled auto-merge (squash)

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          Register overlapping ctypes views and index retained objects

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          Track shared ctypes buffer owners and cache cast fields

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codspeed Bot commented Oct 26, 2026 •

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Merging this PR will not alter performance

✅ 62 untouched benchmarks
⏩ 4 skipped benchmarks¹

_{Comparing 1ndahous3:ctypes_pyobject_owners (c65a11a) with main (6934845)}

Footnotes

4 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

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          Index ctypes buffer aliases and register pointer exporters

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          Publish disjoint ctypes byref writes to borrowed sources

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          Index ctypes write provenance and validate private field paths

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          Resolve retained ctypes values after pointer retargeting

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          Merge main into ctypes_pyobject_owners

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This PR makes more ctypes.py_object ownership and aliasing scenarios work and adds indexed lookups for many of those paths. Some fallback searches remain linear per read, so repeated reads can still become quadratic, including the retargeted-pointer case identified in review. That case failed with ValueError before this PR; the remaining cost is a limitation of the newly supported behavior.

Covering all slow paths needs a broader review of the ownership-index model, rather than more special-case fast paths. A follow-up should index live ownership independently of the latest write address, while preserving object lifetimes, GC and finalizer reentry, and keeping metadata bounded. This is recorded as a separate improvement; this PR does not claim to eliminate all quadratic ownership searches.

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youknowone merged commit df0d1b7 into RustPython:main

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Integrate remaining Python 3.15 final runtime and libraries #8954

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Reuse fallible ctypes scalar conversion #9654

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Windows 11 x64

Debian 12 amd64

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Merging this PR will not alter performance

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