bpo-35943: Prevent PyImport_GetModule() from returning a partially-initialized module (GH-15057)

Misc/NEWS.d/next/Library/2019-07-31-15-52-51.bpo-35943.-KswoB.rst

+The function :c:func:`PyImport_GetModule` now ensures any module it returns is fully initialized.
+Patch by Joannah Nanjekye.

Python/import.c

@@ -387,11 +387,33 @@ import_get_module(PyThreadState *tstate, PyObject *name)
 }
 
 
-PyObject *
-PyImport_GetModule(PyObject *name)
+static int
+import_ensure_initialized(PyThreadState *tstate, PyObject *mod, PyObject *name)
 {
-    PyThreadState *tstate = _PyThreadState_GET();
-    return import_get_module(tstate, name);
+    PyInterpreterState *interp = tstate->interp;
+    PyObject *spec;
+
+    _Py_IDENTIFIER(__spec__);
+    _Py_IDENTIFIER(_lock_unlock_module);
+
+    /* Optimization: only call _bootstrap._lock_unlock_module() if
+       __spec__._initializing is true.
+       NOTE: because of this, initializing must be set *before*
+       stuffing the new module in sys.modules.
+    */
+    spec = _PyObject_GetAttrId(mod, &PyId___spec__);
+    int busy = _PyModuleSpec_IsInitializing(spec);
+    Py_XDECREF(spec);
+    if (busy) {
+        /* Wait until module is done importing. */
+        PyObject *value = _PyObject_CallMethodIdOneArg(
+            interp->importlib, &PyId__lock_unlock_module, name);
+        if (value == NULL) {
+            return -1;
+        }
+        Py_DECREF(value);
+    }
+    return 0;
 }
 
 
@@ -1461,6 +1483,7 @@ PyImport_ImportModule(const char *name)
     return result;
 }
 
+
 /* Import a module without blocking
  *
  * At first it tries to fetch the module from sys.modules. If the module was
@@ -1762,6 +1785,23 @@ import_find_and_load(PyThreadState *tstate, PyObject *abs_name)
     return mod;
 }
 
+PyObject *
+PyImport_GetModule(PyObject *name)
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+    PyObject *mod;
+
+    mod = import_get_module(tstate, name);
+    if (mod != NULL && mod != Py_None) {
+        if (import_ensure_initialized(tstate, mod, name) < 0) {
+            Py_DECREF(mod);
+            remove_importlib_frames(tstate);
+            return NULL;
+        }
+    }
+    return mod;
+}
+
 PyObject *
 PyImport_ImportModuleLevelObject(PyObject *name, PyObject *globals,
                                  PyObject *locals, PyObject *fromlist,
@@ -1817,26 +1857,9 @@ PyImport_ImportModuleLevelObject(PyObject *name, PyObject *globals,
     }
 
     if (mod != NULL && mod != Py_None) {
-        _Py_IDENTIFIER(__spec__);
-        _Py_IDENTIFIER(_lock_unlock_module);
-        PyObject *spec;
-
-        /* Optimization: only call _bootstrap._lock_unlock_module() if
-           __spec__._initializing is true.
-           NOTE: because of this, initializing must be set *before*
-           stuffing the new module in sys.modules.
-         */
-        spec = _PyObject_GetAttrId(mod, &PyId___spec__);
-        if (_PyModuleSpec_IsInitializing(spec)) {
-            PyObject *value = _PyObject_CallMethodIdOneArg(
-                interp->importlib, &PyId__lock_unlock_module, abs_name);
-            if (value == NULL) {
-                Py_DECREF(spec);
-                goto error;
-            }
-            Py_DECREF(value);
+        if (import_ensure_initialized(tstate, mod, name) < 0) {
+            goto error;
         }
-        Py_XDECREF(spec);
     }
     else {
         Py_XDECREF(mod);