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fix: Replace bare static exception<T> with gil_safe_call_once_and_store. #4897

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Oct 23, 2023
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fix: Replace bare static exception<T> with gil_safe_call_once_and_store. #4897

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20 changes: 6 additions & 14 deletions include/pybind11/pybind11.h
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@
#include "detail/init.h"
#include "attr.h"
#include "gil.h"
#include "gil_safe_call_once.h"
#include "options.h"
#include "typing.h"

Expand Down Expand Up @@ -2609,23 +2610,14 @@ class exception : public object {
};

PYBIND11_NAMESPACE_BEGIN(detail)
// Returns a reference to a function-local static exception object used in the simple
// register_exception approach below. (It would be simpler to have the static local variable
// directly in register_exception, but that makes clang <3.5 segfault - issue #1349).
template <typename CppException>
exception<CppException> &get_exception_object() {
static exception<CppException> ex;
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Does this default constructor call into CPython (and release/reacquire the GIL)?

The deadlock scenario can only arise if there's some non-trivial lock interaction in the initializer of the static variable.

I suppose this change would also deal with some end-of-program lifetime situations, though, e.g. if this object gets destroyed too early or too late.

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Does this default constructor call into CPython (and release/reacquire the GIL)?

No, it just copies nullptr to m_ptr (

pybind11/include/pybind11/pytypes.h

Line 297 in 3414c56

PyObject *m_ptr = nullptr;
).

The deadlock scenario can only arise if there's some non-trivial lock interaction in the initializer of the static variable.

Yes, that was on my mind.

I suppose this change would also deal with some end-of-program lifetime situations, though, e.g. if this object gets destroyed too early or too late.

That was the only reason I got into here.

I considered simply changing this to use new (with a boilerplate "intentional leak" comment), but then decided: the universally safe solution

  1. is easy,
  2. inexpensive,
  3. expressive (call once is exactly what we want),
  4. reads nicely,
  5. future proof (in case the exception<T> implementation is changed),
  6. and a great pattern for people to remember and follow,

let's use it!

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Yes, makes sense! Using the new facility just for the lifetime is certainly also a good move.

return ex;
}

// Helper function for register_exception and register_local_exception
template <typename CppException>
exception<CppException> &
register_exception_impl(handle scope, const char *name, handle base, bool isLocal) {
auto &ex = detail::get_exception_object<CppException>();
if (!ex) {
ex = exception<CppException>(scope, name, base);
}
PYBIND11_CONSTINIT static gil_safe_call_once_and_store<exception<CppException>> exc_storage;
exc_storage.call_once_and_store_result(
[&]() { return exception<CppException>(scope, name, base); });

auto register_func
= isLocal ? &register_local_exception_translator : &register_exception_translator;
Expand All @@ -2637,10 +2629,10 @@ register_exception_impl(handle scope, const char *name, handle base, bool isLoca
try {
std::rethrow_exception(p);
} catch (const CppException &e) {
set_error(detail::get_exception_object<CppException>(), e.what());
set_error(exc_storage.get_stored(), e.what());
}
});
return ex;
return exc_storage.get_stored();
}

PYBIND11_NAMESPACE_END(detail)
Expand Down