bpo-38733: PyErr_Occurred() caller must hold the GIL (GH-17080)

bpo-3605, bpo-38733: Optimize _PyErr_Occurred(): remove "tstate ==
NULL" test.

Py_FatalError() no longer calls PyErr_Occurred() if called without
holding the GIL. So PyErr_Occurred() no longer has to support
tstate==NULL case.

_Py_CheckFunctionResult(): use directly _PyErr_Occurred() to avoid
explicit "!= NULL" test.

Doc/c-api/exceptions.rst

@@ -374,6 +374,8 @@ Querying the error indicator
    own a reference to the return value, so you do not need to :c:func:`Py_DECREF`
    it.
 
+   The caller must hold the GIL.
+
    .. note::
 
       Do not compare the return value to a specific exception; use

Include/internal/pycore_pyerrors.h

@@ -10,7 +10,8 @@ extern "C" {
 
 static inline PyObject* _PyErr_Occurred(PyThreadState *tstate)
 {
-    return tstate == NULL ? NULL : tstate->curexc_type;
+    assert(tstate != NULL);
+    return tstate->curexc_type;
 }
 
 

Objects/call.c

@@ -30,12 +30,10 @@ PyObject*
 _Py_CheckFunctionResult(PyThreadState *tstate, PyObject *callable,
                         PyObject *result, const char *where)
 {
-    int err_occurred = (_PyErr_Occurred(tstate) != NULL);
-
     assert((callable != NULL) ^ (where != NULL));
 
     if (result == NULL) {
-        if (!err_occurred) {
+        if (!_PyErr_Occurred(tstate)) {
             if (callable)
                 _PyErr_Format(tstate, PyExc_SystemError,
                               "%R returned NULL without setting an error",
@@ -52,7 +50,7 @@ _Py_CheckFunctionResult(PyThreadState *tstate, PyObject *callable,
         }
     }
     else {
-        if (err_occurred) {
+        if (_PyErr_Occurred(tstate)) {
             Py_DECREF(result);
 
             if (callable) {

Objects/obmalloc.c

@@ -2313,13 +2313,14 @@ _PyMem_DebugRawRealloc(void *ctx, void *p, size_t nbytes)
     return data;
 }
 
-static void
+static inline void
 _PyMem_DebugCheckGIL(void)
 {
-    if (!PyGILState_Check())
+    if (!PyGILState_Check()) {
         Py_FatalError("Python memory allocator called "
                       "without holding the GIL");
     }
+}
 
 static void *
 _PyMem_DebugMalloc(void *ctx, size_t nbytes)

Python/errors.c

@@ -218,6 +218,9 @@ PyErr_SetString(PyObject *exception, const char *string)
 PyObject* _Py_HOT_FUNCTION
 PyErr_Occurred(void)
 {
+    /* The caller must hold the GIL. */
+    assert(PyGILState_Check());
+
     PyThreadState *tstate = _PyThreadState_GET();
     return _PyErr_Occurred(tstate);
 }