Add _PyThreadState_UncheckedGet()

Issue #26154: Add a new private _PyThreadState_UncheckedGet() function which
gets the current thread state, but don't call Py_FatalError() if it is NULL.

Python 3.5.1 removed the _PyThreadState_Current symbol from the Python C API to
no more expose complex and private atomic types. Atomic types depends on the
compiler or can even depend on compiler options. The new function
_PyThreadState_UncheckedGet() allows to get the variable value without having
to care of the exact implementation of atomic types.

Changes:

* Replace direct usage of the _PyThreadState_Current variable with a call to
  _PyThreadState_UncheckedGet().
* In pystate.c, replace direct usage of the _PyThreadState_Current variable
  with the PyThreadState_GET() macro for readability.
* Document also PyThreadState_Get() in pystate.h

Include/pystate.h

@@ -168,7 +168,17 @@ PyAPI_FUNC(void) PyThreadState_DeleteCurrent(void);
 PyAPI_FUNC(void) _PyGILState_Reinit(void);
 #endif
 
+/* Return the current thread state. The global interpreter lock must be held.
+ * When the current thread state is NULL, this issues a fatal error (so that
+ * the caller needn't check for NULL). */
 PyAPI_FUNC(PyThreadState *) PyThreadState_Get(void);
+
+#ifdef WITH_THREAD
+/* Similar to PyThreadState_Get(), but don't issue a fatal error
+ * if it is NULL. */
+PyAPI_FUNC(PyThreadState *) _PyThreadState_UncheckedGet(void);
+#endif
+
 PyAPI_FUNC(PyThreadState *) PyThreadState_Swap(PyThreadState *);
 PyAPI_FUNC(PyObject *) PyThreadState_GetDict(void);
 PyAPI_FUNC(int) PyThreadState_SetAsyncExc(long, PyObject *);

Misc/NEWS

@@ -10,6 +10,13 @@ Release date: tba
 Core and Builtins
 -----------------
 
+- Issue #26154: Add a new private _PyThreadState_UncheckedGet() function to get
+  the current Python thread state, but don't issue a fatal error if it is NULL.
+  This new function must be used instead of accessing directly the
+  _PyThreadState_Current variable.  The variable is no more exposed since
+  Python 3.5.1 to hide the exact implementation of atomic C types, to avoid
+  compiler issues.
+
 - Issue #25731: Fix set and deleting __new__ on a class.
 
 - Issue #22995: [UPDATE] Comment out the one of the pickleability tests in

Modules/faulthandler.c

@@ -490,7 +490,7 @@ faulthandler_thread(void *unused)
         assert(st == PY_LOCK_FAILURE);
 
         /* get the thread holding the GIL, NULL if no thread hold the GIL */
-        current = (PyThreadState*)_Py_atomic_load_relaxed(&_PyThreadState_Current);
+        current = _PyThreadState_UncheckedGet();
 
         _Py_write_noraise(thread.fd, thread.header, (int)thread.header_len);
 

Objects/dictobject.c

@@ -1064,8 +1064,7 @@ PyDict_GetItem(PyObject *op, PyObject *key)
        Let's just hope that no exception occurs then...  This must be
        _PyThreadState_Current and not PyThreadState_GET() because in debug
        mode, the latter complains if tstate is NULL. */
-    tstate = (PyThreadState*)_Py_atomic_load_relaxed(
-        &_PyThreadState_Current);
+    tstate = _PyThreadState_UncheckedGet();
     if (tstate != NULL && tstate->curexc_type != NULL) {
         /* preserve the existing exception */
         PyObject *err_type, *err_value, *err_tb;
@@ -1102,8 +1101,7 @@ _PyDict_GetItem_KnownHash(PyObject *op, PyObject *key, Py_hash_t hash)
        Let's just hope that no exception occurs then...  This must be
        _PyThreadState_Current and not PyThreadState_GET() because in debug
        mode, the latter complains if tstate is NULL. */
-    tstate = (PyThreadState*)_Py_atomic_load_relaxed(
-        &_PyThreadState_Current);
+    tstate = _PyThreadState_UncheckedGet();
     if (tstate != NULL && tstate->curexc_type != NULL) {
         /* preserve the existing exception */
         PyObject *err_type, *err_value, *err_tb;

Python/errors.c

@@ -152,13 +152,7 @@ PyErr_SetString(PyObject *exception, const char *string)
 PyObject *
 PyErr_Occurred(void)
 {
-    /* If there is no thread state, PyThreadState_GET calls
-       Py_FatalError, which calls PyErr_Occurred.  To avoid the
-       resulting infinite loop, we inline PyThreadState_GET here and
-       treat no thread as no error. */
-    PyThreadState *tstate =
-        ((PyThreadState*)_Py_atomic_load_relaxed(&_PyThreadState_Current));
-
+    PyThreadState *tstate = _PyThreadState_UncheckedGet();
     return tstate == NULL ? NULL : tstate->curexc_type;
 }
 

Python/pystate.c

@@ -3,6 +3,12 @@
 
 #include "Python.h"
 
+#ifndef Py_BUILD_CORE
+/* ensure that PyThreadState_GET() is a macro, not an alias to
+ * PyThreadState_Get() */
+#  error "pystate.c must be compiled with Py_BUILD_CORE defined"
+#endif
+
 /* --------------------------------------------------------------------------
 CAUTION
 
@@ -423,7 +429,7 @@ tstate_delete_common(PyThreadState *tstate)
 void
 PyThreadState_Delete(PyThreadState *tstate)
 {
-    if (tstate == (PyThreadState*)_Py_atomic_load_relaxed(&_PyThreadState_Current))
+    if (tstate == PyThreadState_GET())
         Py_FatalError("PyThreadState_Delete: tstate is still current");
 #ifdef WITH_THREAD
     if (autoInterpreterState && PyThread_get_key_value(autoTLSkey) == tstate)
@@ -437,8 +443,7 @@ PyThreadState_Delete(PyThreadState *tstate)
 void
 PyThreadState_DeleteCurrent()
 {
-    PyThreadState *tstate = (PyThreadState*)_Py_atomic_load_relaxed(
-        &_PyThreadState_Current);
+    PyThreadState *tstate = PyThreadState_GET();
     if (tstate == NULL)
         Py_FatalError(
             "PyThreadState_DeleteCurrent: no current tstate");
@@ -488,11 +493,17 @@ _PyThreadState_DeleteExcept(PyThreadState *tstate)
 }
 
 
+PyThreadState *
+_PyThreadState_UncheckedGet(void)
+{
+    return PyThreadState_GET();
+}
+
+
 PyThreadState *
 PyThreadState_Get(void)
 {
-    PyThreadState *tstate = (PyThreadState*)_Py_atomic_load_relaxed(
-        &_PyThreadState_Current);
+    PyThreadState *tstate = PyThreadState_GET();
     if (tstate == NULL)
         Py_FatalError("PyThreadState_Get: no current thread");
 
@@ -503,8 +514,7 @@ PyThreadState_Get(void)
 PyThreadState *
 PyThreadState_Swap(PyThreadState *newts)
 {
-    PyThreadState *oldts = (PyThreadState*)_Py_atomic_load_relaxed(
-        &_PyThreadState_Current);
+    PyThreadState *oldts = PyThreadState_GET();
 
     _Py_atomic_store_relaxed(&_PyThreadState_Current, newts);
     /* It should not be possible for more than one thread state
@@ -535,8 +545,7 @@ PyThreadState_Swap(PyThreadState *newts)
 PyObject *
 PyThreadState_GetDict(void)
 {
-    PyThreadState *tstate = (PyThreadState*)_Py_atomic_load_relaxed(
-        &_PyThreadState_Current);
+    PyThreadState *tstate = PyThreadState_GET();
     if (tstate == NULL)
         return NULL;
 
@@ -682,7 +691,7 @@ PyThreadState_IsCurrent(PyThreadState *tstate)
 {
     /* Must be the tstate for this thread */
     assert(PyGILState_GetThisThreadState()==tstate);
-    return tstate == (PyThreadState*)_Py_atomic_load_relaxed(&_PyThreadState_Current);
+    return tstate == PyThreadState_GET();
 }
 
 /* Internal initialization/finalization functions called by
@@ -774,9 +783,7 @@ PyGILState_GetThisThreadState(void)
 int
 PyGILState_Check(void)
 {
-    /* can't use PyThreadState_Get() since it will assert that it has the GIL */
-    PyThreadState *tstate = (PyThreadState*)_Py_atomic_load_relaxed(
-        &_PyThreadState_Current);
+    PyThreadState *tstate = PyThreadState_GET();
     return tstate && (tstate == PyGILState_GetThisThreadState());
 }
 

Python/sysmodule.c

@@ -1397,7 +1397,7 @@ error:
     Py_XDECREF(name);
     Py_XDECREF(value);
     /* No return value, therefore clear error state if possible */
-    if (_Py_atomic_load_relaxed(&_PyThreadState_Current))
+    if (_PyThreadState_UncheckedGet())
         PyErr_Clear();
 }