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/**************************************************************************
* *
* Regina - A Normal Surface Theory Calculator *
* Python Interface *
* *
* Copyright (c) 1999-2025, Ben Burton *
* For further details contact Ben Burton (bab@debian.org). *
* *
* This program is free software; you can redistribute it and/or *
* modify it under the terms of the GNU General Public License as *
* published by the Free Software Foundation; either version 2 of the *
* License, or (at your option) any later version. *
* *
* As an exception, when this program is distributed through (i) the *
* App Store by Apple Inc.; (ii) the Mac App Store by Apple Inc.; or *
* (iii) Google Play by Google Inc., then that store may impose any *
* digital rights management, device limits and/or redistribution *
* restrictions that are required by its terms of service. *
* *
* This program is distributed in the hope that it will be useful, but *
* WITHOUT ANY WARRANTY; without even the implied warranty of *
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU *
* General Public License for more details. *
* *
* You should have received a copy of the GNU General Public License *
* along with this program. If not, see <https://www.gnu.org/licenses/>. *
* *
**************************************************************************/
/*! \file python/helpers/exception.h
* \brief Assists with wrapping Regina's exception classes.
*
* This header is _not_ included automatically by python/helpers.h.
* If you need it, you will need to include it yourself.
*/
#include <pybind11/pybind11.h>
#if REGINA_PYBIND11_VERSION == 3
#include <pybind11/subinterpreter.h>
#include <mutex>
#endif
#include "utilities/exception.h"
#include "../helpers/docstrings.h"
namespace regina::python {
#if REGINA_PYBIND11_VERSION == 3
/**
* For a single C++ exception type, holds a corresponding Python exception
* type for each subinterpreter.
*
* All insertion and lookup operations are thread-safe.
*/
class ExceptionCache {
private:
std::map<int64_t, PyObject*> cache_;
/**< A map from subinterpreter IDs to Python exception types. */
std::mutex mutex_;
/**< Protects cache_. */
public:
ExceptionCache() = default;
void insert(PyObject* exc) {
int64_t id = pybind11::subinterpreter::current().id();
std::scoped_lock lock(mutex_);
auto result = cache_.emplace(id, exc);
if (! result.second) {
// An exception is already registered for this subinterpreter.
// This should only happen if regina's module was loaded,
// unloaded, and is now being loaded again. We definitely
// want the new value -- the old value is a stale pointer
// which we can safely discard.
result.first->second = exc;
}
}
PyObject* lookup() {
// This function is non-const because we need to lock mutex_.
int64_t id = pybind11::subinterpreter::current().id();
std::scoped_lock lock(mutex_);
auto it = cache_.find(id);
return (it == cache_.end() ? nullptr : it->second);
}
};
#endif
/**
* Adds Python bindings for one of Regina's C++ exception types.
*
* This routine will create a corresponding Python exception type (using
* PyExc_RuntimeError as its base), and will register a translator between
* the two.
*
* Conceptually, this does a similar job to `pybind11::register_exception()`;
* however, it is also safe to use with subinterpreters (which, as of
* pybind11 3.0.0rc3, `pybind11::register_exception()` is not).
*/
template <typename ReginaExceptionType>
void registerReginaException(pybind11::module_& m, const char* className,
const char* docstring) {
#if REGINA_PYBIND11_VERSION == 3
// In pybind11 v3 (at least in 3.0.0rc3), the implementation of
// register_exception() uses a global singleton Python exception object,
// which breaks things when we load the module multiple times in different
// subinterpreters. We will need to implement things manually ourselves,
// with a different Python exception object for each subinterpreter.
//
// TODO: This code does come with a small memory leak, in that once a
// subinterpreter is destroyed, the corresponding entry in cache is not
// removed (although the Python exception type should be destroyed).
// For now, we live with this memory leak: it is only relevant when using
// subinterpreters in the GUI, and these are created manually by the user
// when opening a new python console -- this means there should never be
// so many that this becomes a serious problem.
static ExceptionCache cache;
// The following exception type should live for only as long as the module
// is loaded, since it is the reference in m.__dict__ that keeps it alive
// on the python side. In our own cache we just keep a borrowed reference.
pybind11::exception<ReginaExceptionType> exc(m, className, docstring,
PyExc_RuntimeError);
cache.insert(exc.ptr());
pybind11::register_exception_translator([](std::exception_ptr p) {
if (!p) {
return;
}
try {
std::rethrow_exception(p);
} catch (const ReginaExceptionType &e) {
if (PyObject* exc = cache.lookup())
PyErr_SetString(exc, e.what());
else {
// This should never happen. But.. just in case, translate
// this to a standard python RuntimeError instead.
PyErr_SetString(PyExc_RuntimeError, e.what());
}
}
});
#elif REGINA_PYBIND11_VERSION == 2
// pybind11 v2 does not support subinterpreters properly; however,
// register_exception() does appear to work perfectly fine with the
// subinterpreter hacks that we are using.
pybind11::register_exception<ReginaExceptionType>(m, className,
PyExc_RuntimeError).doc() = docstring;
#else
#error "Unsupported pybind11 version"
#endif
}
} // namespace regina::python
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