bpo-31356: Add context manager to temporarily disable GC (GH-4224)

Doc/library/gc.rst

@@ -33,6 +33,34 @@ The :mod:`gc` module provides the following functions:
    Disable automatic garbage collection.
 
 
+.. class:: ensure_disabled()
+
+   Return a context manager object that disables the garbage collector and reenables the previous
+   state upon completion of the block. This is basically equivalent to::
+
+     from gc import enable, disable, isenabled
+
+     @contextmanager
+     def ensure_disabled():
+         was_enabled_previously = isenabled()
+         gc.disable()
+         yield
+         if was_enabled_previously:
+             gc.enable()
+
+   And lets you write code like this::
+
+     with ensure_disabled():
+         run_some_timing()
+
+     with ensure_disabled():
+         # do_something_that_has_real_time_guarantees
+         # such as a pair trade, robotic braking, etc
+
+   without needing to explicitly enable and disable the garbage collector yourself.
+   This context manager is implemented in C to assure atomicity, thread safety and speed.
+
+
 .. function:: isenabled()
 
    Returns true if automatic collection is enabled.

Include/internal/mem.h

@@ -116,6 +116,7 @@ struct _gc_runtime_state {
 
     int enabled;
     int debug;
+    long disabled_threads;
     /* linked lists of container objects */
     struct gc_generation generations[NUM_GENERATIONS];
     PyGC_Head *generation0;

Lib/test/test_gc.py

 import unittest
 from test.support import (verbose, refcount_test, run_unittest,
                           strip_python_stderr, cpython_only, start_threads,
-                          temp_dir, requires_type_collecting)
+                          temp_dir, requires_type_collecting,reap_threads)
 from test.support.script_helper import assert_python_ok, make_script
 
 import sys
@@ -9,6 +9,8 @@ import time
 import gc
 import weakref
 import threading
+import warnings
+
 
 try:
     from _testcapi import with_tp_del
@@ -1007,6 +1009,73 @@ class GCTogglingTests(unittest.TestCase):
             # empty __dict__.
             self.assertEqual(x, None)
 
+    def test_ensure_disabled(self):
+        original_status = gc.isenabled()
+
+        with gc.ensure_disabled():
+            inside_status = gc.isenabled()
+
+        after_status = gc.isenabled()
+        self.assertEqual(original_status, True)
+        self.assertEqual(inside_status, False)
+        self.assertEqual(after_status, True)
+
+    def test_ensure_disabled_with_gc_disabled(self):
+        gc.disable()
+
+        original_status = gc.isenabled()
+
+        with gc.ensure_disabled():
+            inside_status = gc.isenabled()
+
+        after_status = gc.isenabled()
+        self.assertEqual(original_status, False)
+        self.assertEqual(inside_status, False)
+        self.assertEqual(after_status, False)
+
+    @reap_threads
+    def test_ensure_disabled_thread(self):
+
+        thread_original_status = None
+        thread_inside_status = None
+        thread_after_status = None
+
+        def disabling_thread():
+            nonlocal thread_original_status
+            nonlocal thread_inside_status
+            nonlocal thread_after_status
+            thread_original_status = gc.isenabled()
+
+            with gc.ensure_disabled():
+                time.sleep(0.01)
+                thread_inside_status = gc.isenabled()
+
+            thread_after_status = gc.isenabled()
+
+        original_status = gc.isenabled()
+
+        with warnings.catch_warnings(record=True) as w, gc.ensure_disabled():
+            inside_status_before_thread = gc.isenabled()
+            thread = threading.Thread(target=disabling_thread)
+            thread.start()
+            inside_status_after_thread = gc.isenabled()
+
+        after_status = gc.isenabled()
+        thread.join()
+
+        self.assertEqual(len(w), 1)
+        self.assertTrue(issubclass(w[-1].category, RuntimeWarning))
+        self.assertEqual("Garbage collector enabled while another thread is "
+                         "inside gc.ensure_enabled", str(w[-1].message))
+        self.assertEqual(original_status, True)
+        self.assertEqual(inside_status_before_thread, False)
+        self.assertEqual(thread_original_status, False)
+        self.assertEqual(thread_inside_status, True)
+        self.assertEqual(thread_after_status, False)
+        self.assertEqual(inside_status_after_thread, False)
+        self.assertEqual(after_status, True)
+
+
 def test_main():
     enabled = gc.isenabled()
     gc.disable()

Misc/NEWS.d/next/Core and Builtins/2017-11-02-00-34-42.bpo-31356.54Lb8U.rst

+Add a new contextmanager to the gc module that temporarily disables the GC
+and restores the previous state. The implementation is done in C to assure
+atomicity and speed.

Modules/gcmodule.c

@@ -1067,6 +1067,10 @@ static PyObject *
 gc_enable_impl(PyObject *module)
 /*[clinic end generated code: output=45a427e9dce9155c input=81ac4940ca579707]*/
 {
+    if(_PyRuntime.gc.disabled_threads){
+        PyErr_WarnEx(PyExc_RuntimeWarning, "Garbage collector enabled while another "
+            "thread is inside gc.ensure_enabled",1);
+    }
     _PyRuntime.gc.enabled = 1;
     Py_RETURN_NONE;
 }
@@ -1508,6 +1512,102 @@ static PyMethodDef GcMethods[] = {
     {NULL,      NULL}           /* Sentinel */
 };
 
+typedef struct {
+    PyObject_HEAD
+    int previous_gc_state;
+} ensure_disabled_object;
+
+
+static void
+ensure_disabled_object_dealloc(ensure_disabled_object *m_obj)
+{
+    Py_TYPE(m_obj)->tp_free((PyObject*)m_obj);
+}
+
+static PyObject *
+ensure_disabled__enter__method(ensure_disabled_object *self, PyObject *args)
+{
+    PyGILState_STATE gstate = PyGILState_Ensure();
+    ++_PyRuntime.gc.disabled_threads;
+    self->previous_gc_state = _PyRuntime.gc.enabled;
+    gc_disable_impl(NULL);
+    PyGILState_Release(gstate);
+    Py_RETURN_NONE;
+}
+
+static PyObject *
+ensure_disabled__exit__method(ensure_disabled_object *self, PyObject *args)
+{
+    PyGILState_STATE gstate = PyGILState_Ensure();
+    --_PyRuntime.gc.disabled_threads;
+    if(self->previous_gc_state){
+        gc_enable_impl(NULL);
+    }else{
+        gc_disable_impl(NULL);
+    }
+    PyGILState_Release(gstate);
+    Py_RETURN_NONE;
+}
+
+
+
+static struct PyMethodDef ensure_disabled_object_methods[] = {
+    {"__enter__",       (PyCFunction) ensure_disabled__enter__method,      METH_NOARGS},
+    {"__exit__",        (PyCFunction) ensure_disabled__exit__method,       METH_VARARGS},
+    {NULL,         NULL}       /* sentinel */
+};
+
+static PyObject *
+new_disabled_obj(PyTypeObject *type, PyObject *args, PyObject *kwdict){
+    ensure_disabled_object *self;
+    self = (ensure_disabled_object *)type->tp_alloc(type, 0);
+    return (PyObject *) self;
+};
+
+static PyTypeObject gc_ensure_disabled_type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "gc.ensure_disabled",                       /* tp_name */
+    sizeof(ensure_disabled_object),             /* tp_size */
+    0,                                          /* tp_itemsize */
+    /* methods */
+    (destructor) ensure_disabled_object_dealloc,/* tp_dealloc */
+    0,                                          /* tp_print */
+    0,                                          /* tp_getattr */
+    0,                                          /* tp_setattr */
+    0,                                          /* tp_reserved */
+    0,                                          /* tp_repr */
+    0,                                          /* tp_as_number */
+    0,                                          /*tp_as_sequence*/
+    0,                                          /*tp_as_mapping*/
+    0,                                          /*tp_hash*/
+    0,                                          /*tp_call*/
+    0,                                          /*tp_str*/
+    PyObject_GenericGetAttr,                    /*tp_getattro*/
+    0,                                          /*tp_setattro*/
+    0,                                          /*tp_as_buffer*/
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,   /*tp_flags*/
+    0,                                          /*tp_doc*/
+    0,                                          /* tp_traverse */
+    0,                                          /* tp_clear */
+    0,                                          /* tp_richcompare */
+    0,                                          /* tp_weaklistoffset */
+    0,                                          /* tp_iter */
+    0,                                          /* tp_iternext */
+    ensure_disabled_object_methods,             /* tp_methods */
+    0,                                          /* tp_members */
+    0,                                          /* tp_getset */
+    0,                                          /* tp_base */
+    0,                                          /* tp_dict */
+    0,                                          /* tp_descr_get */
+    0,                                          /* tp_descr_set */
+    0,                                          /* tp_dictoffset */
+    0,                                          /* tp_init */
+    PyType_GenericAlloc,                        /* tp_alloc */
+    new_disabled_obj,                           /* tp_new */
+    PyObject_Del,                               /* tp_free */
+};
+
+
 static struct PyModuleDef gcmodule = {
     PyModuleDef_HEAD_INIT,
     "gc",              /* m_name */
@@ -1548,6 +1648,12 @@ PyInit_gc(void)
     if (PyModule_AddObject(m, "callbacks", _PyRuntime.gc.callbacks) < 0)
         return NULL;
 
+    if (PyType_Ready(&gc_ensure_disabled_type) < 0)
+        return NULL;
+    if (PyModule_AddObject(m, "ensure_disabled", (PyObject*) &gc_ensure_disabled_type) < 0)
+        return NULL;
+
+
 #define ADD_INT(NAME) if (PyModule_AddIntConstant(m, #NAME, NAME) < 0) return NULL
     ADD_INT(DEBUG_STATS);
     ADD_INT(DEBUG_COLLECTABLE);