Fix unsafe "block" implementation in test_eigen.cpp

Author: rwgk

tests/test_eigen.cpp

@@ -201,7 +201,14 @@ TEST_SUBMODULE(eigen, m) {
              int start_row,
              int start_col,
              int block_rows,
-             int block_cols) { return x.block(start_row, start_col, block_rows, block_cols); });
+             int block_cols) {
+              // In case x is a copy made in the type_caster for Eigen::Ref
+              // (https://pybind11.readthedocs.io/en/stable/advanced/cast/eigen.html#pass-by-reference),
+              // returning the Eigen::Ref returned by x.block() will lead to heap-use-after-free,
+              // because the block references the copy, which is destroyed when this function
+              // returns. Therefore the block needs to be returned by value.
+              return Eigen::MatrixXd{x.block(start_row, start_col, block_rows, block_cols)};
+          });
 
     // test_eigen_return_references, test_eigen_keepalive
     // return value referencing/copying tests:

tests/test_eigen.py

@@ -223,7 +223,6 @@ def test_nonunit_stride_to_python():
     for i in range(-5, 7):
         assert np.all(m.diagonal_n(ref, i) == ref.diagonal(i)), f"m.diagonal_n({i})"
 
-    return  # TODO(rwgk): Fix ASAN failure.
     assert np.all(m.block(ref, 2, 1, 3, 3) == ref[2:5, 1:4])
     assert np.all(m.block(ref, 1, 4, 4, 2) == ref[1:, 4:])
     assert np.all(m.block(ref, 1, 4, 3, 2) == ref[1:4, 4:])