gh-140550: allow slots that repeat information from PyModuleDef

When integrating slots-based module creation is with the inittab,
which currently requires PyModuleDef, it would be convenient to
reuse the the same slots array for the MethodDef.

Allow slots that match what's already present in the PyModuleDef.

Author: encukou

Doc/c-api/module.rst

@@ -725,10 +725,11 @@ remove it.
 
       An array of additional slots, terminated by a ``{0, NULL}`` entry.
 
-      This array may not contain slots corresponding to :c:type:`PyModuleDef`
-      members.
-      For example, you cannot use :c:macro:`Py_mod_name` in :c:member:`!m_slots`;
-      the module name must be given as :c:member:`PyModuleDef.m_name`.
+      If the array contains slots corresponding to :c:type:`PyModuleDef`
+      members, the values must match.
+      For example, if you use :c:macro:`Py_mod_name` in :c:member:`!m_slots`;
+      :c:member:`PyModuleDef.m_name` must be set to the same pointer
+      (not just an equal string).
 
       .. versionchanged:: 3.5
 

Lib/test/test_capi/test_module.py

@@ -122,8 +122,7 @@ def test_create(self):
             _testcapi.pymodule_get_token(mod)
 
     def test_def_slot(self):
-        """Slots that replace PyModuleDef fields can't be used with PyModuleDef
-        """
+        """Slots cannot contradict PyModuleDef fields"""
         for name in DEF_SLOTS:
             with self.subTest(name):
                 spec = FakeSpec()
@@ -133,6 +132,11 @@ def test_def_slot(self):
                 self.assertIn(name, str(cm.exception))
                 self.assertIn("PyModuleDef", str(cm.exception))
 
+    def test_def_slot_parrot(self):
+        """Slots with same value as PyModuleDef fields are allowed"""
+        spec = FakeSpec()
+        _testcapi.module_from_def_slot_parrot(spec)
+
     def test_repeated_def_slot(self):
         """Slots that replace PyModuleDef fields can't be repeated"""
         for name in (*DEF_SLOTS, 'Py_mod_exec'):

Misc/NEWS.d/next/Core_and_Builtins/2026-01-30-10-38-07.gh-issue-140550.Us9vPD.rst

@@ -0,0 +1,2 @@
+In :c:member:`PyModuleDef.m_slots`, allow slots that repeat information
+present in :c:type:`PyModuleDef`.

Modules/_testcapi/module.c

@@ -1,5 +1,6 @@
 #include "parts.h"
 #include "util.h"
+#include <stdbool.h>
 
 // Test PyModule_* API
 
@@ -270,6 +271,47 @@ module_from_def_slot(PyObject *self, PyObject *spec)
     return result;
 }
 
+static PyModuleDef parrot_def = {
+    PyModuleDef_HEAD_INIT,
+    .m_name = "test_capi/parrot",
+    .m_doc = "created from redundant information",
+    .m_size = 123,
+    .m_methods = a_methoddef_array,
+    .m_traverse = noop_traverse,
+    .m_clear = noop_clear,
+    .m_free = (void*)noop_free,
+    .m_slots = NULL /* set below */,
+};
+static PyModuleDef_Slot parrot_slots[] = {
+    {Py_mod_name, "test_capi/parrot"},
+    {Py_mod_doc, "created from redundant information"},
+    {Py_mod_state_size, (void*)123},
+    {Py_mod_methods, a_methoddef_array},
+    {Py_mod_state_traverse, noop_traverse},
+    {Py_mod_state_clear, noop_clear},
+    {Py_mod_state_free, (void*)noop_free},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
+    {Py_mod_token, &parrot_def},
+    {Py_mod_gil, Py_MOD_GIL_NOT_USED},
+    {0},
+};
+
+static PyObject *
+module_from_def_slot_parrot(PyObject *self, PyObject *spec)
+{
+    parrot_def.m_slots = parrot_slots;
+    PyObject *module = PyModule_FromDefAndSpec(&parrot_def, spec);
+    if (!module || (PyModule_Exec(module) < 0)) {
+        return NULL;
+    }
+    Py_ssize_t size;
+    assert(PyModule_GetStateSize(module, &size) == 0);
+    assert(size == 123);
+    PyModuleDef *def = PyModule_GetDef(module);
+    assert(def == &parrot_def);
+    return module;
+}
+
 static int
 another_exec(PyObject *module)
 {
@@ -368,6 +410,7 @@ static PyMethodDef test_methods[] = {
     {"module_from_slots_null_slot", module_from_slots_null_slot, METH_O},
     {"module_from_def_multiple_exec", module_from_def_multiple_exec, METH_O},
     {"module_from_def_slot", module_from_def_slot, METH_O},
+    {"module_from_def_slot_parrot", module_from_def_slot_parrot, METH_O},
     {"pymodule_get_token", pymodule_get_token, METH_O},
     {"pymodule_get_def", pymodule_get_def, METH_O},
     {"pymodule_get_state_size", pymodule_get_state_size, METH_O},

Objects/moduleobject.c

@@ -410,35 +410,78 @@ module_from_def_and_spec(
         goto error;
     }
 
+    bool seen_m_name_slot = false;
+    bool seen_m_doc_slot = false;
+    bool seen_m_size_slot = false;
+    bool seen_m_methods_slot = false;
+    bool seen_m_traverse_slot = false;
+    bool seen_m_clear_slot = false;
+    bool seen_m_free_slot = false;
     for (cur_slot = def_like->m_slots; cur_slot && cur_slot->slot; cur_slot++) {
-        // Macro to copy a non-NULL, non-repeatable slot that's unusable with
-        // PyModuleDef. The destination must be initially NULL.
-#define COPY_COMMON_SLOT(SLOT, TYPE, DEST)                              \
+
+        // Macro to copy a non-NULL, non-repeatable slot.
+#define COPY_NONNULL_SLOT(SLOTNAME, TYPE, DEST)                         \
         do {                                                            \
             if (!(TYPE)(cur_slot->value)) {                             \
                 PyErr_Format(                                           \
                     PyExc_SystemError,                                  \
-                    "module %s: " #SLOT " must not be NULL",            \
-                    name);                                              \
+                    "module %s: %s must not be NULL",                   \
+                    name, SLOTNAME);                                    \
                 goto error;                                             \
             }                                                           \
-            if (original_def) {                                         \
+            DEST = (TYPE)(cur_slot->value);                             \
+        } while (0);                                                    \
+        /////////////////////////////////////////////////////////////////
+
+        // Macro to copy a non-NULL, non-repeatable slot to def_like.
+#define COPY_DEF_SLOT(SLOTNAME, TYPE, MEMBER)                           \
+        do {                                                            \
+            if (seen_ ## MEMBER ## _slot) {                             \
                 PyErr_Format(                                           \
                     PyExc_SystemError,                                  \
-                    "module %s: " #SLOT " used with PyModuleDef",       \
-                    name);                                              \
+                    "module %s has more than one %s slot",              \
+                    name, SLOTNAME);                                    \
                 goto error;                                             \
             }                                                           \
+            seen_ ## MEMBER ## _slot = true;                            \
+            if (original_def) {                                         \
+                TYPE orig_value = (TYPE)original_def->MEMBER;           \
+                TYPE new_value = (TYPE)cur_slot->value;                 \
+                if (orig_value != new_value) {                          \
+                    PyErr_Format(                                       \
+                        PyExc_SystemError,                              \
+                        "module %s: %s conflicts with "                 \
+                        "PyModuleDef." #MEMBER,                         \
+                        name, SLOTNAME);                                \
+                    goto error;                                         \
+                }                                                       \
+            }                                                           \
+            COPY_NONNULL_SLOT(SLOTNAME, TYPE, (def_like->MEMBER))       \
+        } while (0);                                                    \
+        /////////////////////////////////////////////////////////////////
+
+        // Macro to copy a non-NULL, non-repeatable slot without a
+        // corresponding PyModuleDef member.
+        // DEST must be initially NULL (so we don't need a seen_* flag).
+#define COPY_NONDEF_SLOT(SLOTNAME, TYPE, DEST)                          \
+        do {                                                            \
             if (DEST) {                                                 \
                 PyErr_Format(                                           \
                     PyExc_SystemError,                                  \
-                    "module %s has more than one " #SLOT " slot",       \
-                    name);                                              \
+                    "module %s has more than one %s slot",              \
+                    name, SLOTNAME);                                    \
                 goto error;                                             \
             }                                                           \
-            DEST = (TYPE)(cur_slot->value);                             \
+            COPY_NONNULL_SLOT(SLOTNAME, TYPE, DEST)                     \
         } while (0);                                                    \
         /////////////////////////////////////////////////////////////////
+
+        // Define the whole common case
+#define DEF_SLOT_CASE(SLOT, TYPE, MEMBER)                               \
+        case SLOT:                                                      \
+            COPY_DEF_SLOT(#SLOT, TYPE, MEMBER);                         \
+            break;                                                      \
+        /////////////////////////////////////////////////////////////////
         switch (cur_slot->slot) {
             case Py_mod_create:
                 if (create) {
@@ -453,14 +496,15 @@ module_from_def_and_spec(
             case Py_mod_exec:
                 has_execution_slots = 1;
                 if (!original_def) {
-                    COPY_COMMON_SLOT(Py_mod_exec, _Py_modexecfunc, m_exec);
+                    COPY_NONDEF_SLOT("Py_mod_exec", _Py_modexecfunc, m_exec);
                 }
                 break;
             case Py_mod_multiple_interpreters:
                 if (has_multiple_interpreters_slot) {
                     PyErr_Format(
                         PyExc_SystemError,
-                        "module %s has more than one 'multiple interpreters' slots",
+                        "module %s has more than one 'multiple interpreters' "
+                        "slots",
                         name);
                     goto error;
                 }
@@ -483,34 +527,23 @@ module_from_def_and_spec(
                     goto error;
                 }
                 break;
-            case Py_mod_name:
-                COPY_COMMON_SLOT(Py_mod_name, char*, def_like->m_name);
-                break;
-            case Py_mod_doc:
-                COPY_COMMON_SLOT(Py_mod_doc, char*, def_like->m_doc);
-                break;
-            case Py_mod_state_size:
-                COPY_COMMON_SLOT(Py_mod_state_size, Py_ssize_t,
-                                 def_like->m_size);
-                break;
-            case Py_mod_methods:
-                COPY_COMMON_SLOT(Py_mod_methods, PyMethodDef*,
-                                 def_like->m_methods);
-                break;
-            case Py_mod_state_traverse:
-                COPY_COMMON_SLOT(Py_mod_state_traverse, traverseproc,
-                                 def_like->m_traverse);
-                break;
-            case Py_mod_state_clear:
-                COPY_COMMON_SLOT(Py_mod_state_clear, inquiry,
-                                 def_like->m_clear);
-                break;
-            case Py_mod_state_free:
-                COPY_COMMON_SLOT(Py_mod_state_free, freefunc,
-                                 def_like->m_free);
-                break;
+            DEF_SLOT_CASE(Py_mod_name, char*, m_name)
+            DEF_SLOT_CASE(Py_mod_doc, char*, m_doc)
+            DEF_SLOT_CASE(Py_mod_state_size, Py_ssize_t, m_size)
+            DEF_SLOT_CASE(Py_mod_methods, PyMethodDef*, m_methods)
+            DEF_SLOT_CASE(Py_mod_state_traverse, traverseproc, m_traverse)
+            DEF_SLOT_CASE(Py_mod_state_clear, inquiry, m_clear)
+            DEF_SLOT_CASE(Py_mod_state_free, freefunc, m_free)
             case Py_mod_token:
-                COPY_COMMON_SLOT(Py_mod_token, void*, token);
+                if (original_def && original_def != cur_slot->value) {
+                    PyErr_Format(
+                        PyExc_SystemError,
+                        "module %s: arbitrary Py_mod_token not "
+                        "allowed with PyModuleDef",
+                        name);
+                    goto error;
+                }
+                COPY_NONDEF_SLOT("Py_mod_token", void*, token);
                 break;
             default:
                 assert(cur_slot->slot < 0 || cur_slot->slot > _Py_mod_LAST_SLOT);
@@ -520,7 +553,10 @@ module_from_def_and_spec(
                     name, cur_slot->slot);
                 goto error;
         }
-#undef COPY_COMMON_SLOT
+#undef DEF_SLOT_CASE
+#undef COPY_DEF_SLOT
+#undef COPY_NONDEF_SLOT
+#undef _COPY_COMMON_SLOT
     }
 
 #ifdef Py_GIL_DISABLED
@@ -590,7 +626,7 @@ module_from_def_and_spec(
         mod->md_state = NULL;
         module_copy_members_from_deflike(mod, def_like);
         if (original_def) {
-            assert (!token);
+            assert (!token || token == original_def);
             mod->md_token = original_def;
             mod->md_token_is_def = 1;
         }