Switch builtins.sort to a custom stable PeekSort #12623
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Unlike std::sort and std::stable_sort, this implementation does not lead to out-of-bounds memory reads or other undefined behavior when the predicate is not strict weak ordering. This makes it possible to use this function in libexpr for builtins.sort, where an incorrectly implemented comparator in the user nix code currently can crash and burn the evaluator by invoking C++ UB.
This prevents C++ level undefined behavior from affecting the evaluator. Stdlib implementation details should not affect eval, regardless of the build platform. Even erroneous usage of `builtins.sort` should not make it possible to crash the evaluator or produce results that depend on the host platform.
…16 elements libstdc++'s std::stable_sort and new builtins.sort implementation special-case ranges with length less than or equal to 16 and delegate to insertionsort. Having a larger e2e test would allow catching sort stability issues at functional level as well.
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xokdvium
commented
Mar 10, 2025
| comparator a b && comparator b c -> comparator a c | ||
| ``` | ||
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| 1. Irreflexivity |
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Forgot to add that the relation must also be asymmetric.
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Not sure if mentioning it here explicitly would be preferable. Irreflexivity together with transitivity already imply that the relation is asymmetric: a < b && b < a -> (a < a = false). Maybe including this in the doc is beneficial, even if it's somewhat redundant from the mathematics point of view?
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FWIW when I think about this stuff, I tend to start with a "total preorder" for <= (which is also a total order on equivalence classes for <=), and then define a total order from it.
xokdvium
commented
Mar 11, 2025
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| for (auto innerBegin = begin; innerBegin < innerEnd; ++innerBegin) { | ||
| ASSERT_LE(prevId, innerBegin->second); | ||
| prevId = innerBegin->second; | ||
| } |
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This can be simplified with std::is_sorted.
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Motivation
Unlike
std::sortandstd::stable_sort, this implementationdoes not lead to out-of-bounds memory reads or other undefined
behavior when the predicate is not strict weak ordering.
This makes it possible to use this function in libexpr for
builtins.sort, where an incorrectly implemented comparator
in the user nix code currently can crash and burn the evaluator
by invoking C++ UB.
This prevents C++ level undefined behavior from affecting
the evaluator. Stdlib implementation details should not affect
eval, regardless of the build platform. Even erroneous usage
of
builtins.sortshould not make it possible to crash theevaluator or produce results that depend on the host platform.
Context
#12106
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