bpo-36854: gcmodule.c gets its state from tstate (GH-17285)

* Add GCState type for readability
* gcmodule.c now gets its gcstate from tstate
* _PyGC_DumpShutdownStats() now expects tstate rather than runtime
* Rename "state" to "gcstate" for readability: to avoid confusion
  between "state" and "tstate" for example.
* collect() now only expects tstate: it gets gcstate from tstate.
* Pass tstate to _PyErr_xxx() functions

Author: vstinner

Include/internal/pycore_object.h

@@ -18,7 +18,7 @@ PyAPI_FUNC(int) _PyDict_CheckConsistency(PyObject *mp, int check_content);
  * NB: While the object is tracked by the collector, it must be safe to call the
  * ob_traverse method.
  *
- * Internal note: _PyRuntime.gc.generation0->_gc_prev doesn't have any bit flags
+ * Internal note: interp->gc.generation0->_gc_prev doesn't have any bit flags
  * because it's not object header.  So we don't use _PyGCHead_PREV() and
  * _PyGCHead_SET_PREV() for it to avoid unnecessary bitwise operations.
  *
@@ -37,11 +37,13 @@ static inline void _PyObject_GC_TRACK_impl(const char *filename, int lineno,
                           "object is in generation which is garbage collected",
                           filename, lineno, "_PyObject_GC_TRACK");
 
-    PyGC_Head *last = (PyGC_Head*)(_PyRuntime.gc.generation0->_gc_prev);
+    PyThreadState *tstate = _PyThreadState_GET();
+    PyGC_Head *generation0 = tstate->interp->runtime->gc.generation0;
+    PyGC_Head *last = (PyGC_Head*)(generation0->_gc_prev);
     _PyGCHead_SET_NEXT(last, gc);
     _PyGCHead_SET_PREV(gc, last);
-    _PyGCHead_SET_NEXT(gc, _PyRuntime.gc.generation0);
-    _PyRuntime.gc.generation0->_gc_prev = (uintptr_t)gc;
+    _PyGCHead_SET_NEXT(gc, generation0);
+    generation0->_gc_prev = (uintptr_t)gc;
 }
 
 #define _PyObject_GC_TRACK(op) \

Include/internal/pycore_pylifecycle.h

@@ -89,7 +89,7 @@ extern void _PyWarnings_Fini(PyInterpreterState *interp);
 extern void _PyGILState_Init(PyThreadState *tstate);
 extern void _PyGILState_Fini(PyThreadState *tstate);
 
-PyAPI_FUNC(void) _PyGC_DumpShutdownStats(struct pyruntimestate *runtime);
+PyAPI_FUNC(void) _PyGC_DumpShutdownStats(PyThreadState *tstate);
 
 PyAPI_FUNC(PyStatus) _Py_PreInitializeFromPyArgv(
     const PyPreConfig *src_config,