Following issue #13390, fix compilation --without-pymalloc, and make...

Following issue #13390, fix compilation --without-pymalloc, and make sys.getallocatedblocks() return 0 in that situation.

Doc/library/sys.rst

@@ -396,15 +396,16 @@ always available.
 .. function:: getallocatedblocks()
 
    Return the number of memory blocks currently allocated by the interpreter,
-   regardless of their size.  This function is mainly useful for debugging
-   small memory leaks.  Because of the interpreter's internal caches, the
-   result can vary from call to call; you may have to call
-   :func:`_clear_type_cache()` to get more predictable results.
+   regardless of their size.  This function is mainly useful for tracking
+   and debugging memory leaks.  Because of the interpreter's internal
+   caches, the result can vary from call to call; you may have to call
+   :func:`_clear_type_cache()` and :func:`gc.collect()` to get more
+   predictable results.
 
-   .. versionadded:: 3.4
+   If a Python build or implementation cannot reasonably compute this
+   information, :func:`getallocatedblocks()` is allowed to return 0 instead.
 
-   .. impl-detail::
-      Not all Python implementations may be able to return this information.
+   .. versionadded:: 3.4
 
 
 .. function:: getcheckinterval()

Lib/test/test_sys.py

@@ -7,6 +7,7 @@ import warnings
 import operator
 import codecs
 import gc
+import sysconfig
 
 # count the number of test runs, used to create unique
 # strings to intern in test_intern()
@@ -616,9 +617,13 @@ class SysModuleTest(unittest.TestCase):
                          "sys.getallocatedblocks unavailable on this build")
     def test_getallocatedblocks(self):
         # Some sanity checks
+        with_pymalloc = sysconfig.get_config_var('WITH_PYMALLOC')
         a = sys.getallocatedblocks()
         self.assertIs(type(a), int)
+        if with_pymalloc:
             self.assertGreater(a, 0)
+        else:
+            self.assertEqual(a, 0)
         try:
             # While we could imagine a Python session where the number of
             # multiple buffer objects would exceed the sharing of references,

Objects/obmalloc.c

@@ -1316,6 +1316,13 @@ PyObject_Free(void *p)
 {
     PyMem_FREE(p);
 }
+
+Py_ssize_t
+_Py_GetAllocatedBlocks(void)
+{
+    return 0;
+}
+
 #endif /* WITH_PYMALLOC */
 
 #ifdef PYMALLOC_DEBUG