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

3 files changed, 22 insertions, 21 deletions

diff --git a/Python/errors.c b/Python/errors.c
index 7b67566727..5ff1e4c81a 100644
--- a/Python/errors.c
+++ b/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;
 }
 
diff --git a/Python/pystate.c b/Python/pystate.c
index 7e0267ae1d..83f15fd671 100644
--- a/Python/pystate.c
+++ b/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");
@@ -489,10 +494,16 @@ _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());
 }
 
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 334f5d0c8e..8d7e05a465 100644
--- a/Python/sysmodule.c
+++ b/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();
 }