[BUG] set_cover Python: fix UB in the all_subsets property

[jg-codes]

What

Fixes undefined behaviour in the set_cover Python wrapper (ortools/set_cover/python/set_cover.cc) and adds a regression test.

Disclosure: drafted with AI assistance (Claude). See the verification section for exactly what was built and run.

The bug

SetCoverModel.all_subsets terminates the interpreter when read from Python. The property is set_cover.cc:201-213, and it has two independent defects in one statement:

1. begin() and end() come from different objects. SetCoverModel::all_subsets() returns by value (set_cover_model.h:211):

   std::vector<SubsetIndex> all_subsets() const { return all_subsets_; }

   The property calls it twice, so model.all_subsets().begin() and model.all_subsets().end() are iterators into two distinct temporaries. That pair does not delimit a valid range, and the transform runs off the end of the first temporary's buffer. This is the dominant defect, and it is why the property has never worked.

2. The output iterator is also invalid. subsets.begin() on a default-constructed, zero-capacity vector: a write through an iterator that was never valid.

The neighbouring columns and rows properties are fine: columns() and rows() return const references (:189, :192), so calling them twice is harmless. Their pre-size-and-write-through-begin() pattern is correct for a reference-returning accessor. all_subsets() is the only by-value accessor in the file, and it is the only one that crashes.

Reproduction

from ortools.set_cover.python import set_cover

model = set_cover.SetCoverModel()
model.add_empty_subset(1.0)
model.add_element_to_last_subset(0)
model.add_empty_subset(1.0)
model.add_element_to_last_subset(1)

print(model.all_subsets)  # process dies here

Segmentation fault, exit 139, on the released ortools 9.15.6755 wheel (macOS arm64, CPython 3.13.11). The std::transform statement at the v9.15 tag is identical to the one on main.

The fix

Bind the returned vector to a local, then append through back_inserter. Binding the local also removes a third full copy that reserve(model.all_subsets().size()) would otherwise make.

VectorIntToVectorSubsetIndex has the second defect too, and is fixed here for consistency, though it is not currently reachable from Python: all of its callers take absl::Span, and this module registers no absl::Span type caster. That is a separate, unrelated defect, reported independently rather than folded into this PR.

Verification

Built and tested on Linux (Bazel 8.7.0, x86_64) against main at d9c0910:

new test applied WITHOUT the fix:  FAIL (Segmentation fault)
new test applied WITH the fix:     PASSED

clang-format clean against the repo .clang-format.

Checklist

• Read CONTRIBUTING.md and the PR template, targeting main
• CLA signed
• Minimal diff: the all_subsets fix and its test
• No unrelated formatting or refactoring
• AI-assisted: disclosed above

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[jg-codes]

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[jg-codes]

@Mizux, rebased onto current main, just force-pushed. The diagnosis changed since I opened this, so the whole PR may be worth a fresh look.

a1e13b37 ("set_cover: export from google3") fixed the swapped GuidedTabuSearch::{Set,Get}LagrangianFactor bindings, so that hunk and its test are gone. Thanks for fixing it.

model.all_subsets kills the interpreter, and the empty-vector write is only half of why

The property is set_cover.cc:201-213. Two defects in one statement:

1. begin() and end() come from different objects. SetCoverModel::all_subsets() returns by value (set_cover_model.h:211):

std::vector<SubsetIndex> all_subsets() const { return all_subsets_; }

The property calls it twice, so model.all_subsets().begin() and model.all_subsets().end() are iterators into two distinct temporaries. That pair does not delimit a range, and the transform runs off the end of the first temporary's buffer.

2. The output iterator is invalid too. subsets.begin() on a default-constructed, zero-capacity vector.

I originally reported only the second one, but both need fixing.

The neighbouring columns and rows properties are fine: columns() and rows() return const references (:189, :192), so calling them twice is harmless. Their pre-size-and-write-through-begin() form is correct for a reference-returning accessor. all_subsets() is the only by-value one, and it is the only property that crashes.

Reproduction

48 elements on a 6×8 grid, 24 subsets, each covering the cells within L1 distance 2, all costs 1.0:

from ortools.set_cover.python import set_cover

ROWS, COLS = 6, 8
model = set_cover.SetCoverModel()
for r in range(ROWS):
    for c in range(COLS):
        if (r + c) % 2:
            continue
        model.add_empty_subset(1.0)
        for dr in range(-2, 3):
            for dc in range(-2, 3):
                nr, nc = r + dr, c + dc
                if abs(dr) + abs(dc) <= 2 and 0 <= nr < ROWS and 0 <= nc < COLS:
                    model.add_element_to_last_subset(nr * COLS + nc)

print("subsets:", model.num_subsets)              # 24
print("coverable:", model.compute_feasibility())  # True
print("ids:", model.all_subsets)                  # process dies here

subsets: 24
coverable: True
[1]    segmentation fault

Exit 139 on 6 of 6 runs. Solving the same model is fine: next_solution() returns True at cost 10.0, exit 0 on 5 of 5. So it is the property, not the model. ortools 9.15.6755, CPython 3.13.11, macOS 26.5.2 arm64; the statement at the v9.15 tag is identical to the one on main.

There is no exception to catch and nothing in the log, which is what makes it awkward in practice.

The fix

Bind the returned vector to a local, then append through back_inserter. Binding the local also removes a third full copy that reserve(model.all_subsets().size()) would otherwise make.

One more correction to my original description

I claimed the UB reached every Python call passing a focus list. That is wrong. Those overloads take absl::Span, and this module registers no absl::Span caster: pybind11 falls back to printing the raw C++ type and rejects any list, so VectorIntToVectorSubsetIndex is not reachable from Python at all. It has the same output-iterator defect and I fixed it here for consistency, but all_subsets is the reachable instance. The Span problem is filed separately.

That also means the focus paths cannot carry a regression test from Python.

Diff and verification

17 added, 12 removed in set_cover.cc, plus 6 lines of test. Branch jg-codes:fix-set-cover-python-bindings, compare. clang-format clean against the repo .clang-format.

Built and ran bazel test //ortools/set_cover/python:set_cover_test on Linux (Bazel 8.7.0, x86_64) against main at d9c0910:

new test applied WITHOUT the fix:  FAIL (Segmentation fault)
new test applied WITH the fix:     PASSED

Disclosure: drafted with AI assistance (Claude). Every claim above, the segfault, the exit codes, and the bazel result, is from a run I executed myself.

Happy to close this if you would rather take it internally.

[jg-codes]

Filed the Span/focus-API defect as its own issue: #5278.