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Fix TSS key exhaustion in implicitly_convertible() #6020

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Merged
rwgk merged 1 commit into from
Mar 29, 2026
Merged

Fix TSS key exhaustion in implicitly_convertible() #6020

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11 changes: 4 additions & 7 deletions include/pybind11/pybind11.h
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Original file line number Diff line number Diff line change
Expand Up @@ -3557,13 +3557,10 @@ typing::Iterator<ValueType> make_value_iterator(Type &value, Extra &&...extra) {

template <typename InputType, typename OutputType>
void implicitly_convertible() {
static int tss_sentinel_pointee = 1; // arbitrary value
struct set_flag {
thread_specific_storage<int> &flag;
explicit set_flag(thread_specific_storage<int> &flag_) : flag(flag_) {
flag = &tss_sentinel_pointee; // trick: the pointer itself is the sentinel
}
~set_flag() { flag.reset(nullptr); }
bool &flag;
explicit set_flag(bool &flag_) : flag(flag_) { flag_ = true; }
~set_flag() { flag = false; }

// Prevent copying/moving to ensure RAII guard is used safely
set_flag(const set_flag &) = delete;
Expand All @@ -3572,7 +3569,7 @@ void implicitly_convertible() {
set_flag &operator=(set_flag &&) = delete;
};
auto implicit_caster = [](PyObject *obj, PyTypeObject *type) -> PyObject * {
static thread_specific_storage<int> currently_used;
thread_local bool currently_used = false;
if (currently_used) { // implicit conversions are non-reentrant
return nullptr;
}
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