Prevent overwriting previous declarations

Currently pybind11 doesn't check when you define a new object (e.g. a
class, function, or exception) that overwrites an existing one.  If the
thing being overwritten is a class, this leads to a segfault (because
pybind still thinks the type is defined, even though Python no longer
has the type).  In other cases this is harmless (e.g. replacing a
function with an exception), but even in that case it's most likely a
bug.

This code doesn't prevent you from actively doing something harmful,
like deliberately overwriting a previous definition, but detects
overwriting with a run-time error if it occurs in the standard
class/function/exception/def registration interfaces.

All of the additions are in non-template code; the result is actually a
tiny decrease in .so size compared to master without the new test code
(977304 to 977272 bytes), and about 4K higher with the new tests.

include/pybind11/pybind11.h

@@ -253,7 +253,8 @@ protected:
 #endif
 
         detail::function_record *chain = nullptr, *chain_start = rec;
-        if (rec->sibling && PyCFunction_Check(rec->sibling.ptr())) {
+        if (rec->sibling) {
+            if (PyCFunction_Check(rec->sibling.ptr())) {
                 capsule rec_capsule(PyCFunction_GetSelf(rec->sibling.ptr()), true);
                 chain = (detail::function_record *) rec_capsule;
                 /* Never append a method to an overload chain of a parent class;
@@ -261,6 +262,11 @@ protected:
                 if (chain->class_ != rec->class_)
                     chain = nullptr;
             }
+            // Don't trigger for things like the default __init__, which are wrapper_descriptors that we are intentionally replacing
+            else if (!rec->sibling.is_none() && rec->name[0] != '_')
+                pybind11_fail("Cannot overload existing non-function object \"" + std::string(rec->name) +
+                        "\" with a function of the same name");
+        }
 
         if (!chain) {
             /* No existing overload was found, create a new function object */
@@ -546,8 +552,9 @@ public:
     module &def(const char *name_, Func &&f, const Extra& ... extra) {
         cpp_function func(std::forward<Func>(f), name(name_), scope(*this),
                           sibling(getattr(*this, name_, none())), extra...);
-        /* PyModule_AddObject steals a reference to 'func' */
-        PyModule_AddObject(ptr(), name_, func.inc_ref().ptr());
+        // NB: allow overwriting here because cpp_function sets up a chain with the intention of
+        // overwriting (and has already checked internally that it isn't overwriting non-functions).
+        add_object(name_, func, true /* overwrite */);
         return *this;
     }
 
@@ -567,6 +574,20 @@ public:
             throw import_error("Module \"" + std::string(name) + "\" not found!");
         return module(obj, false);
     }
+
+    // Adds an object to the module using the given name.  Throws if an object with the given name
+    // already exists.
+    //
+    // overwrite should almost always be false: attempting to overwrite objects that pybind11 has
+    // established will, in most cases, break things.
+    PYBIND11_NOINLINE void add_object(const char *name, object &obj, bool overwrite = false) {
+        if (!overwrite && hasattr(*this, name))
+            pybind11_fail("Error during initialization: multiple incompatible definitions with name \"" +
+                    std::string(name) + "\"");
+
+        obj.inc_ref(); // PyModule_AddObject() steals a reference
+        PyModule_AddObject(ptr(), name, obj.ptr());
+    }
 };
 
 NAMESPACE_BEGIN(detail)
@@ -614,6 +635,10 @@ protected:
         object name(PYBIND11_FROM_STRING(rec->name), false);
         object scope_module;
         if (rec->scope) {
+            if (hasattr(rec->scope, rec->name))
+                pybind11_fail("generic_type: cannot initialize type \"" + std::string(rec->name) +
+                        "\": an object with that name is already defined");
+
             if (hasattr(rec->scope, "__module__")) {
                 scope_module = rec->scope.attr("__module__");
             } else if (hasattr(rec->scope, "__name__")) {
@@ -1357,8 +1382,7 @@ public:
                 + std::string(".") + name;
         char* exception_name = const_cast<char*>(full_name.c_str());
         m_ptr = PyErr_NewException(exception_name, base, NULL);
-        inc_ref(); // PyModule_AddObject() steals a reference
-        PyModule_AddObject(m.ptr(), name.c_str(), m_ptr);
+        m.add_object(name.c_str(), *this);
     }
 
     // Sets the current python exception to this exception object with the given message

tests/test_issues.cpp

@@ -36,6 +36,18 @@ OpTest2 operator+(const OpTest2 &, const OpTest1 &) {
     return OpTest2();
 }
 
+// #461
+class Dupe1 {
+public:
+    Dupe1(int v) : v_{v} {}
+    int get_value() const { return v_; }
+private:
+    int v_;
+};
+class Dupe2 {};
+class Dupe3 {};
+class DupeException : public std::runtime_error {};
+
 void init_issues(py::module &m) {
     py::module m2 = m.def_submodule("issues");
 
@@ -237,7 +249,49 @@ void init_issues(py::module &m) {
     static std::vector<int> list = { 1, 2, 3 };
     m2.def("make_iterator_1", []() { return py::make_iterator<py::return_value_policy::copy>(list); });
     m2.def("make_iterator_2", []() { return py::make_iterator<py::return_value_policy::automatic>(list); });
+
+    static std::vector<std::string> nothrows;
+    // Issue 461: registering two things with the same name:
+    py::class_<Dupe1>(m2, "Dupe1")
+        .def("get_value", &Dupe1::get_value)
+        ;
+    m2.def("dupe1_factory", [](int v) { return new Dupe1(v); });
+
+    py::class_<Dupe2>(m2, "Dupe2");
+    py::exception<DupeException>(m2, "DupeException");
+
+    try {
+        m2.def("Dupe1", [](int v) { return new Dupe1(v); });
+        nothrows.emplace_back("Dupe1");
+    }
+    catch (std::runtime_error &) {}
+    try {
+        py::class_<Dupe3>(m2, "dupe1_factory");
+        nothrows.emplace_back("dupe1_factory");
+    }
+    catch (std::runtime_error &) {}
+    try {
+        py::exception<Dupe3>(m2, "Dupe2");
+        nothrows.emplace_back("Dupe2");
+    }
+    catch (std::runtime_error &) {}
+    try {
+        m2.def("DupeException", []() { return 30; });
+        nothrows.emplace_back("DupeException1");
+    }
+    catch (std::runtime_error &) {}
+    try {
+        py::class_<DupeException>(m2, "DupeException");
+        nothrows.emplace_back("DupeException2");
+    }
+    catch (std::runtime_error &) {}
+    m2.def("dupe_exception_failures", []() {
+        py::list l;
+        for (auto &e : nothrows) l.append(py::cast(e));
+        return l;
+    });
 }
 
+
 // MSVC workaround: trying to use a lambda here crashes MSCV
 test_initializer issues(&init_issues);

tests/test_issues.py

@@ -182,3 +182,8 @@ def test_iterator_rvpolicy():
     assert list(make_iterator_1()) == [1, 2, 3]
     assert list(make_iterator_2()) == [1, 2, 3]
     assert(type(make_iterator_1()) != type(make_iterator_2()))
+
+def test_dupe_assignment():
+    """ Issue 461: overwriting a class with a function """
+    from pybind11_tests.issues import dupe_exception_failures
+    assert dupe_exception_failures() == []