Fix DataFrame.OrderBy to perform stable sorting by kubaflo · Pull Request #7586 · dotnet/machinelearning

kubaflo · 2026-03-07T21:57:11Z

Summary

Fixes #6443 — DataFrame.OrderBy() did not perform stable sorting. Rows with equal sort keys were reordered unpredictably instead of preserving their original relative order.

Root Cause

Two issues:

Per-buffer sort used IntrospectiveSort (introsort = quicksort + heapsort fallback), which is inherently unstable. The code even had a comment acknowledging this: "Bug fix: QuickSort is not stable."
The multi-buffer merge phase in PopulateColumnSortIndicesWithHeap wrote results right-to-left for descending order, which reversed the relative order of equal elements.

Fix

Per-buffer sort: `IntrospectiveSort` → `MergeSortIndices`

Added a stable merge sort implementation to DataFrameColumn.cs and replaced IntrospectiveSort calls in both PrimitiveDataFrameColumn.Sort.cs and StringDataFrameColumn.cs.

Merge sort is:

Inherently stable — preserves relative order of equal elements (via <= 0 comparison favoring the left half on ties)
O(n log n) guaranteed — no worst-case degradation unlike quicksort
O(n) extra space — uses ArrayPool<int>.Shared to minimize GC pressure

Descending sort fix

For descending order, callers now create the SortedDictionary with a reversed comparer and sort each buffer in descending order. PopulateColumnSortIndicesWithHeap always writes left-to-right, preserving stability in both directions. The unused ascending parameter was removed from the method signature.

Files Changed

File	Change
`src/Microsoft.Data.Analysis/DataFrameColumn.cs`	Added `MergeSortIndices` (stable merge sort); fixed `PopulateColumnSortIndicesWithHeap` to always write left-to-right; removed unused `ascending` parameter
`src/Microsoft.Data.Analysis/PrimitiveDataFrameColumn.Sort.cs`	Replaced `IntrospectiveSort` → `MergeSortIndices`; use reversed comparer for descending
`src/Microsoft.Data.Analysis/DataFrameColumns/StringDataFrameColumn.cs`	Same replacement as above
`test/Microsoft.Data.Analysis.Tests/DataFrameTests.Sort.cs`	Added 5 stability tests

Tests

5 new tests added:

TestOrderBy_StableSort_PreservesOriginalOrder — Reproduces the exact example from issue DataFrame.OrderBy(string columnName) does not perform stable sorting! #6443 (8 rows, 3 duplicate key groups)
TestOrderByDescending_StableSort_PreservesOriginalOrder — Verifies descending stability
TestOrderBy_StableSort_WithNullsAndDuplicates — Stability with null values mixed in
TestStringColumnSort_StableSort — String column stability
TestOrderBy_StableSort_LargeDataset — 100 rows with 5 duplicate keys, triggers quicksort's unstable code path (partitions > 16 elements). This test fails without the fix (Unstable sort at row 3: Key=0, ID=25 should be > previous ID=90) and passes with it.

All 470 existing + new tests pass, 0 failures.

Multi-Model AI Review (Cross-Pollination)

This fix was developed with AI assistance and then reviewed by two additional AI models to catch issues:

Gemini 3 Pro Review

✅ Confirmed merge sort correctness and stability (<= 0 comparison favors left half on ties)
✅ Confirmed reversed comparer approach is correct for descending
⚠️ Suggested ArrayPool<int>.Shared instead of new int[] to reduce GC pressure → Applied
⚠️ Suggested removing unused ascending parameter → Applied

GPT-5.2 Review

✅ Confirmed per-buffer merge sort is correctly stable
⚠️ Same ArrayPool recommendation → Applied
⚠️ Flagged pre-existing multi-buffer edge case: LIFO list removal in PopulateColumnSortIndicesWithHeap could affect stability when duplicate keys span multiple buffers (>536M rows for int). This is a pre-existing issue not introduced by this change, and is impractical to test. Added a note in the test file.
💡 Suggested alternative: keep introsort but add original-index tiebreaker to comparer. We chose merge sort for cleaner semantics and guaranteed O(n log n).

Both models agreed the fix is correct. All actionable suggestions were applied.

Copilot

Pull request overview

Fixes DataFrame.OrderBy() / OrderByDescending() stability so rows with equal sort keys preserve their original relative order, addressing issue #6443.

Changes:

Added a stable MergeSortIndices implementation and updated heap-merge output ordering to support stable ordering (especially for descending).
Switched per-buffer sorts in primitive and string columns from introsort to stable merge sort; descending now uses a reversed comparer.
Added new unit tests validating stability across duplicates, nulls, and descending order.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
`src/Microsoft.Data.Analysis/DataFrameColumn.cs`	Adds stable merge sort helper and changes heap merge to write left-to-right (with reversed comparer for descending).
`src/Microsoft.Data.Analysis/PrimitiveDataFrameColumn.Sort.cs`	Replaces per-buffer introsort with stable merge sort; uses reversed comparer for descending; updates heap usage accordingly.
`src/Microsoft.Data.Analysis/DataFrameColumns/StringDataFrameColumn.cs`	Same stable sort + reversed comparer changes for string columns.
`test/Microsoft.Data.Analysis.Tests/DataFrameTests.Sort.cs`	Adds stability regression tests for OrderBy/OrderByDescending and null handling.

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src/Microsoft.Data.Analysis/DataFrameColumn.cs

test/Microsoft.Data.Analysis.Tests/DataFrameTests.Sort.cs

Replace the unstable IntrospectiveSort (introsort) with a stable merge sort implementation for per-buffer sorting in both PrimitiveDataFrameColumn and StringDataFrameColumn. Also fix the multi-buffer merge phase to always write left-to-right using a direction-aware comparer, ensuring stability for both ascending and descending sorts. Fixes dotnet#6443 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

kubaflo · 2026-03-07T22:17:01Z

🔍 Multimodal Code Review — PR #7586: Stable Sort for DataFrame.OrderBy

Reviewed independently by 3 AI models: Claude Sonnet 4.6 · GPT-5.2 · Gemini 3 Pro Preview

🚨 Cross-Model Consensus (All 3 Models Agree)

🔴 BUG: Multi-buffer merge still breaks stability for equal keys spanning buffers

File: DataFrameColumn.cs → PopulateColumnSortIndicesWithHeap
Agreed by: Claude Sonnet 4.6 ✅ · GPT-5.2 ✅ · Gemini 3 Pro ✅

The LIFO→FIFO switch + existingList.Add(...) (append-to-end) breaks cross-buffer stability when a buffer yields a duplicate key that gets re-inserted behind another buffer's waiting entry.

Concrete scenario (all 3 models produced equivalent traces):

Buffer0: [K, K, ...]    (original rows 0, 1)
Buffer1: [K, ...]        (original row 1000)

Initial heap[K] = [(buf0, idx0), (buf1, idx0)]   ← correct buffer order
Pop FIFO → output buf0(row0) ✓
Advance buf0 → next value is also K → existingList.Add((buf0, idx1))
List is now: [(buf1, idx0), (buf0, idx1)]         ← WRONG ORDER
Pop FIFO → outputs buf1(row1000) before buf0(row1) ← STABILITY VIOLATED

Impact: Only affects DataFrames with columns spanning >1 internal buffer (~1M+ rows for int, ~536M+ for larger types). All 5 new tests use ≤100 rows and never exercise this path.

Suggested fix (Gemini's is most precise): When re-inserting into existingList with the same key, use Insert(0, ...) instead of Add(...), since the current buffer index is guaranteed ≤ any other buffer index in the list.

🟡 WARNING: `RemoveAt(0)` on `List<T>` is O(n) — quadratic risk

File: DataFrameColumn.cs ~line 494
Agreed by: Claude Sonnet 4.6 ✅ · GPT-5.2 ✅

sortAndBufferIndex = tuplesOfSortAndBufferIndex[0];
tuplesOfSortAndBufferIndex.RemoveAt(0);   // O(n) shift every time

The old RemoveAt(Count-1) was O(1). With many buffers sharing a key, this becomes O(k·totalElements). Single-buffer case (≤1M rows) is fine since list length = 1.

Suggested fix: Use Queue<ValueTuple<int,int>> as the dictionary value type — O(1) enqueue/dequeue with correct FIFO semantics.

🟡 WARNING: Tests don't cover the multi-buffer merge path

File: DataFrameTests.Sort.cs
Agreed by: Claude Sonnet 4.6 ✅ · GPT-5.2 ✅ · Gemini 3 Pro ✅

All 5 tests use ≤100 rows, which fit in a single buffer. The PR's stated root cause #2 is the multi-buffer merge, but no test exercises it. The "LargeDataset" test name is misleading for 100 rows.

Suggested fix: Add a test that forces multiple buffers (or at minimum rename TestOrderBy_StableSort_LargeDataset → TestOrderBy_StableSort_ManyDuplicates).

✅ Confirmed Correct (All 3 Models Agree)

Area	Verdict
Merge sort `<= 0` comparison	✅ Correct for stability (left half wins ties)
`ArrayPool.Shared.Rent/Return`	✅ Correct usage, proper `finally` cleanup
Reversed comparer for descending	✅ Sound approach, `SortedDictionary` enumerates in comparer order
Recursive depth	✅ O(log n) per buffer, no stack overflow risk
Null-index starting position after removing `ascending` param	✅ Correct
Per-buffer sort (single-buffer path)	✅ Fully stable and correct

🔵 Suggestions (Individual Model Findings)

Model	Finding	Severity
Gemini	LOH pressure: `ArrayPool.Shared` doesn't pool arrays >2²⁰ (~1M); 100M-row sorts allocate ~400MB on every call	ℹ️ Note
Claude Sonnet	Code duplication between `Merge` (Span) and `MergeList` (IList) — could unify via interface or delegate	🔵 Suggestion
Gemini	Rename `LargeDataset` test → `ManyDuplicates` (100 rows isn't "large")	🔵 Suggestion

Summary Scorecard

Category	Rating
Correctness (single-buffer)	✅ Solid
Correctness (multi-buffer)	🔴 Bug remains
Test coverage	🟡 Insufficient for multi-buffer
Performance	🟡 O(n) RemoveAt(0) risk
Code quality	✅ Good, minor duplication

Bottom line: The per-buffer sort fix (introsort → merge sort) is correct and well-implemented. However, the multi-buffer merge in PopulateColumnSortIndicesWithHeap has a remaining stability bug that all three models independently identified with equivalent reproduction traces. This should be fixed before merge.

Address issues identified by multimodal code review (Claude Sonnet 4.6, GPT-5.2, Gemini 3 Pro): 1. Fix cross-buffer stability: When re-inserting a buffer's next element with the same key into the heap, use AddFirst instead of Add(append). The popped buffer has a lower index than remaining entries, so its next element must come before them to preserve stable order. 2. Fix O(n) RemoveAt(0) performance: Replace List<ValueTuple<int,int>> with LinkedList<ValueTuple<int,int>> as the SortedDictionary value type. LinkedList provides O(1) RemoveFirst/AddFirst vs List's O(n) RemoveAt(0)/Insert(0,...). 3. Rename misleading test: TestOrderBy_StableSort_LargeDataset (100 rows) → TestOrderBy_StableSort_ManyDuplicates. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Round 2 multimodal review (GPT-5.2 + Gemini 3 Pro) identified that AddFirst is incorrect when a higher-indexed buffer reaches a key after a lower-indexed buffer already queued it. Example: Buffer0=[100], Buffer1=[10,100] Pop 10(Buf1) → advance → 100 → AddFirst puts Buf1 before Buf0 → outputs 100(Buf1) before 100(Buf0) — stability violated. Fix: insert new entries in buffer-index order via linear scan of the LinkedList. The list is bounded by buffer count (tiny), so O(k) scan is negligible. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

kubaflo · 2026-03-07T22:38:29Z

🔍 Multimodal Re-Review (Round 2) — PR #7586

Reviewed independently by 3 AI models: Claude Sonnet 4.6 · GPT-5.2 · Gemini 3 Pro Preview

Round 1 Issues — Status

Issue	Status	Details
🔴 Multi-buffer stability (`Add` → tail)	⚠️ Partially fixed	`AddFirst` solved the consecutive-duplicate case but introduced a new interleaved-buffer bug (see below)
🟡 `RemoveAt(0)` O(n) perf	✅ Fixed	`LinkedList` with O(1) `RemoveFirst()`
🟡 Misleading test name	✅ Fixed	Renamed to `ManyDuplicates`

🔴 NEW BUG Found in Round 2: `AddFirst` breaks interleaved buffer stability

Identified by: GPT-5.2 ✅ · Gemini 3 Pro ✅ (Claude Sonnet missed it)

existingList.AddFirst(...) unconditionally puts the newly advancing buffer at the front. This is wrong when a higher-indexed buffer reaches a key after a lower-indexed buffer already has it queued.

Reproduction trace (both GPT-5.2 and Gemini produced equivalent):

Buffer0: [100]          (original rows 0)
Buffer1: [10, 100]      (original rows 1-2)

Heap init: {10 → [Buf1], 100 → [Buf0]}
Pop min=10(Buf1). Advance Buf1 → next=100.
AddFirst puts (Buf1) before (Buf0): list = [Buf1, Buf0]
Pop 100(Buf1) before 100(Buf0) ← STABILITY VIOLATED
Expected: 100(Buf0) then 100(Buf1) since Buf0 rows come first

Fix applied (commit 1339df0): Replace AddFirst with ordered insertion by bufferIndex. Linear scan of the LinkedList to find correct position — O(k) where k = number of buffers (tiny).

var node = existingList.First;
while (node != null && node.Value.Item2 < bufferIndex)
    node = node.Next;
if (node == null)
    existingList.AddLast((...));
else
    existingList.AddBefore(node, (...));

✅ Confirmed Correct (All/Majority Agreement)

Area	Verdict	Models
Merge sort `<= 0` comparison	✅ Stable (left half wins ties)	All 3
`ArrayPool.Shared.Rent/Return`	✅ Correct	All 3
Reversed comparer for descending	✅ Sound	All 3
`LinkedList` migration (callers consistent)	✅ Complete	All 3
Null-index position logic	✅ Correct	All 3
Per-buffer sort (single-buffer path)	✅ Fully stable	All 3

🟡 Remaining Suggestions

Model	Finding	Severity
Claude Sonnet	`AddFirst`/ordered-insert code path has zero test coverage — all 5 tests use single-buffer data (< MaxCapacity rows). Multi-buffer path is untestable without >536M rows or test hooks.	🟡 Warning
Gemini	`LinkedList` allocates a `LinkedListNode` per row processed. For very large datasets this increases GC pressure vs `List` (where the list is bounded by buffer count). Acceptable trade-off given O(1) removal.	ℹ️ Note
Claude Sonnet	Code duplication between `Merge` (Span) and `MergeList` (IList)	🔵 Suggestion

Summary Scorecard (Post-Fix)

Category	Rating
Correctness (single-buffer)	✅ Solid
Correctness (multi-buffer)	✅ Fixed (ordered insert by bufferIndex)
Test coverage	🟡 Good for single-buffer; multi-buffer untestable
Performance	✅ O(1) removal, O(k) insertion (k=buffer count)
Code quality	✅ Good

All 17 sort tests pass. Fix pushed.

codecov · 2026-03-08T00:20:20Z

Codecov Report

❌ Patch coverage is 93.47826% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.06%. Comparing base (70d7603) to head (1339df0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Microsoft.Data.Analysis/DataFrameColumn.cs	88.28%	12 Missing and 1 partial ⚠️
...Analysis/DataFrameColumns/StringDataFrameColumn.cs	85.71%	1 Missing ⚠️
...oft.Data.Analysis/PrimitiveDataFrameColumn.Sort.cs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7586      +/-   ##
==========================================
- Coverage   69.07%   69.06%   -0.02%     
==========================================
  Files        1483     1483              
  Lines      274513   274722     +209     
  Branches    28285    28305      +20     
==========================================
+ Hits       189625   189733     +108     
- Misses      77503    77614     +111     
+ Partials     7385     7375      -10

Flag	Coverage Δ
Debug	`69.06% <93.47%> (-0.02%)`	⬇️
production	`63.31% <88.00%> (-0.03%)`	⬇️
test	`89.54% <100.00%> (+0.01%)`	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...crosoft.Data.Analysis.Tests/DataFrameTests.Sort.cs	`100.00% <100.00%> (ø)`
...Analysis/DataFrameColumns/StringDataFrameColumn.cs	`68.24% <85.71%> (+0.08%)`	⬆️
...oft.Data.Analysis/PrimitiveDataFrameColumn.Sort.cs	`88.09% <85.71%> (+0.29%)`	⬆️
src/Microsoft.Data.Analysis/DataFrameColumn.cs	`47.40% <88.28%> (-18.06%)`	⬇️

... and 6 files with indirect coverage changes

🚀 New features to boost your workflow:

❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

Address Codecov report showing 15 uncovered lines — all in the multi-buffer merge path of PopulateColumnSortIndicesWithHeap. Trick: set StringDataFrameColumn.MaxCapacity to a small value (2-3) to force multiple internal buffers with only a few elements, then verify stable sort across buffer boundaries. 3 new tests: - TestOrderBy_StableSort_MultipleStringBuffers: 2 buffers, duplicate keys spanning both, verifies ascending stability - TestOrderBy_StableSort_MultipleBuffers_InterleavedKeys: exercises the interleaved scenario (higher buffer reaches a key after lower buffer already queued it) that broke AddFirst in round 2 - TestOrderByDescending_StableSort_MultipleBuffers: reversed comparer with multi-buffer data Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

kubaflo · 2026-03-08T10:40:54Z

🔍 Multimodal Review — Round 3 — PR #7586

Reviewed by: Claude Sonnet 4.6 ✅ · GPT-5.2 ✅ · Gemini 3 Pro ⏱️ (timed out)

Round 2 Issues — Status

Issue	Status
🔴 Interleaved-buffer bug (`AddFirst`)	✅ Fixed — ordered insertion by `bufferIndex`
🟡 Zero multi-buffer test coverage	✅ Fixed — 3 new tests via `MaxCapacity` trick
ℹ️ LinkedList GC pressure	Accepted trade-off
🔵 Code duplication (Merge/MergeList)	Deferred (cosmetic)

✅ No Bugs Found (Round 3)

Both Claude Sonnet 4.6 and GPT-5.2 confirm the ordered insertion by bufferIndex is correct.

Claude Sonnet 4.6 — Full Trace Verification

"I traced all three tests end-to-end through the getBufferSortIndex lambda and confirmed every expected ID. Both AddBefore and AddLast paths are exercised. The Round-2 interleaved bug is genuinely exercised. No significant issues found."

Key findings:

✅ AddLast path hit in InterleavedKeys test (buf1 reaches "z" after buf0 already queued it)
✅ AddBefore path hit in Descending test (buf0 arrives after buf1 is already queued)
✅ DisableTestParallelization = true is present in AssemblyInfo.cs — MaxCapacity mutation is safe
✅ All assertions arithmetically verified through the getBufferSortIndex lambda

GPT-5.2 — Confirmed Fix

Verified the ordered insertion handles all cases correctly. The InterleavedKeys test specifically targets the R2 scenario.

🟡 One Minor Concern Raised

Thread safety of `MaxCapacity` mutation (GPT-5.2)

Severity: 🟡 Low (mitigated)

GPT-5.2 flagged that mutating the static StringDataFrameColumn.MaxCapacity could cause flakiness if tests run in parallel. However, Claude Sonnet independently verified that DisableTestParallelization = true exists in test/Microsoft.Data.Analysis.Tests/Properties/AssemblyInfo.cs, meaning all tests in this assembly run sequentially. The try/finally pattern ensures cleanup even on test failure.

Verdict: Safe as-is. No action needed.

✅ Confirmed Correct — Full PR (All Models Agree)

Area	Verdict
Merge sort (`<= 0` stability)	✅ Correct
ArrayPool usage	✅ Correct
Reversed comparer for descending	✅ Sound
LinkedList migration	✅ Complete and consistent
Ordered insertion by bufferIndex	✅ Correct (traced end-to-end)
Multi-buffer test coverage	✅ Both AddBefore and AddLast exercised
MaxCapacity trick safety	✅ Safe (parallelization disabled)
Null handling	✅ Correct

Final Summary

Category	R1	R2	R3
Correctness (single-buffer)	✅	✅	✅
Correctness (multi-buffer)	🔴	✅ Fixed	✅ Verified
Performance	🟡	✅ Fixed	✅
Test coverage	🟡	🟡	✅ Fixed
Code quality	✅	✅	✅

Bottom line: All bugs identified across 3 rounds of multimodal review have been fixed and verified. The PR is ready for merge. 🚀

Copilot AI review requested due to automatic review settings March 7, 2026 21:57

Copilot started reviewing on behalf of kubaflo March 7, 2026 21:57 View session

dotnet-policy-service bot added the community-contribution label Mar 7, 2026

kubaflo force-pushed the fix/stable-sort-6443 branch from ae879f2 to 5c2e34b Compare March 7, 2026 22:01

Copilot AI reviewed Mar 7, 2026

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Fix DataFrame.OrderBy to perform stable sorting#7586

Fix DataFrame.OrderBy to perform stable sorting#7586
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kubaflo commented Mar 7, 2026

Summary

Root Cause

Fix

Per-buffer sort: IntrospectiveSort → MergeSortIndices

Descending sort fix

Files Changed

Tests

Multi-Model AI Review (Cross-Pollination)

Gemini 3 Pro Review

GPT-5.2 Review

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Pull request overview

Reviewed changes

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kubaflo commented Mar 7, 2026

🔍 Multimodal Code Review — PR #7586: Stable Sort for DataFrame.OrderBy

🚨 Cross-Model Consensus (All 3 Models Agree)

🔴 BUG: Multi-buffer merge still breaks stability for equal keys spanning buffers

🟡 WARNING: RemoveAt(0) on List<T> is O(n) — quadratic risk

🟡 WARNING: Tests don't cover the multi-buffer merge path

✅ Confirmed Correct (All 3 Models Agree)

🔵 Suggestions (Individual Model Findings)

Summary Scorecard

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kubaflo commented Mar 7, 2026

🔍 Multimodal Re-Review (Round 2) — PR #7586

Round 1 Issues — Status

🔴 NEW BUG Found in Round 2: AddFirst breaks interleaved buffer stability

✅ Confirmed Correct (All/Majority Agreement)

🟡 Remaining Suggestions

Summary Scorecard (Post-Fix)

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codecov bot commented Mar 8, 2026

Codecov Report

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kubaflo commented Mar 8, 2026

🔍 Multimodal Review — Round 3 — PR #7586

Round 2 Issues — Status

✅ No Bugs Found (Round 3)

Claude Sonnet 4.6 — Full Trace Verification

GPT-5.2 — Confirmed Fix

🟡 One Minor Concern Raised

Thread safety of MaxCapacity mutation (GPT-5.2)

✅ Confirmed Correct — Full PR (All Models Agree)

Final Summary

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Per-buffer sort: `IntrospectiveSort` → `MergeSortIndices`

🟡 WARNING: `RemoveAt(0)` on `List<T>` is O(n) — quadratic risk

🔴 NEW BUG Found in Round 2: `AddFirst` breaks interleaved buffer stability

Thread safety of `MaxCapacity` mutation (GPT-5.2)