bpo-32030: Rework memory allocators (#4625)

* Fix _PyMem_SetupAllocators("debug"): always restore allocators to
  the defaults, rather than only caling _PyMem_SetupDebugHooks().
* Add _PyMem_SetDefaultAllocator() helper to set the "default"
  allocator.
* Add _PyMem_GetAllocatorsName(): get the name of the allocators
* main() now uses debug hooks on memory allocators if Py_DEBUG is
  defined, rather than calling directly malloc()
* Document default memory allocators in C API documentation
* _Py_InitializeCore() now fails with a fatal user error if
  PYTHONMALLOC value is an unknown memory allocator, instead of
  failing with a fatal internal error.
* Add new tests on the PYTHONMALLOC environment variable
* Add support.with_pymalloc()
* Add the _testcapi.WITH_PYMALLOC constant and expose it as
   support.with_pymalloc().
* sysconfig.get_config_var('WITH_PYMALLOC') doesn't work on Windows, so
   replace it with support.with_pymalloc().
* pythoninfo: add _testcapi collector for pymem

Doc/c-api/memory.rst

@@ -100,9 +100,10 @@ The following function sets are wrappers to the system allocator. These
 functions are thread-safe, the :term:`GIL <global interpreter lock>` does not
 need to be held.
 
-The default raw memory block allocator uses the following functions:
-:c:func:`malloc`, :c:func:`calloc`, :c:func:`realloc` and :c:func:`free`; call
-``malloc(1)`` (or ``calloc(1, 1)``) when requesting zero bytes.
+The :ref:`default raw memory allocator <default-memory-allocators>` uses
+the following functions: :c:func:`malloc`, :c:func:`calloc`, :c:func:`realloc`
+and :c:func:`free`; call ``malloc(1)`` (or ``calloc(1, 1)``) when requesting
+zero bytes.
 
 .. versionadded:: 3.4
 
@@ -165,7 +166,8 @@ The following function sets, modeled after the ANSI C standard, but specifying
 behavior when requesting zero bytes, are available for allocating and releasing
 memory from the Python heap.
 
-By default, these functions use :ref:`pymalloc memory allocator <pymalloc>`.
+The :ref:`default memory allocator <default-memory-allocators>` uses the
+:ref:`pymalloc memory allocator <pymalloc>`.
 
 .. warning::
 
@@ -270,7 +272,8 @@ The following function sets, modeled after the ANSI C standard, but specifying
 behavior when requesting zero bytes, are available for allocating and releasing
 memory from the Python heap.
 
-By default, these functions use :ref:`pymalloc memory allocator <pymalloc>`.
+The :ref:`default object allocator <default-memory-allocators>` uses the
+:ref:`pymalloc memory allocator <pymalloc>`.
 
 .. warning::
 
@@ -326,6 +329,31 @@ By default, these functions use :ref:`pymalloc memory allocator <pymalloc>`.
    If *p* is *NULL*, no operation is performed.
 
 
+.. _default-memory-allocators:
+
+Default Memory Allocators
+=========================
+
+Default memory allocators:
+
+===============================  ====================  ==================  =====================  ====================
+Configuration                    Name                  PyMem_RawMalloc     PyMem_Malloc           PyObject_Malloc
+===============================  ====================  ==================  =====================  ====================
+Release build                    ``"pymalloc"``        ``malloc``          ``pymalloc``           ``pymalloc``
+Debug build                      ``"pymalloc_debug"``  ``malloc`` + debug  ``pymalloc`` + debug   ``pymalloc`` + debug
+Release build, without pymalloc  ``"malloc"``          ``malloc``          ``malloc``             ``malloc``
+Release build, without pymalloc  ``"malloc_debug"``    ``malloc`` + debug  ``malloc`` + debug     ``malloc`` + debug
+===============================  ====================  ==================  =====================  ====================
+
+Legend:
+
+* Name: value for :envvar:`PYTHONMALLOC` environment variable
+* ``malloc``: system allocators from the standard C library, C functions:
+  :c:func:`malloc`, :c:func:`calloc`, :c:func:`realloc` and :c:func:`free`
+* ``pymalloc``: :ref:`pymalloc memory allocator <pymalloc>`
+* "+ debug": with debug hooks installed by :c:func:`PyMem_SetupDebugHooks`
+
+
 Customize Memory Allocators
 ===========================
 
@@ -431,7 +459,8 @@ Customize Memory Allocators
    displayed if :mod:`tracemalloc` is tracing Python memory allocations and the
    memory block was traced.
 
-   These hooks are installed by default if Python is compiled in debug
+   These hooks are :ref:`installed by default <default-memory-allocators>` if
+   Python is compiled in debug
    mode. The :envvar:`PYTHONMALLOC` environment variable can be used to install
    debug hooks on a Python compiled in release mode.
 
@@ -453,9 +482,9 @@ to 512 bytes) with a short lifetime. It uses memory mappings called "arenas"
 with a fixed size of 256 KiB. It falls back to :c:func:`PyMem_RawMalloc` and
 :c:func:`PyMem_RawRealloc` for allocations larger than 512 bytes.
 
-*pymalloc* is the default allocator of the :c:data:`PYMEM_DOMAIN_MEM` (ex:
-:c:func:`PyMem_Malloc`) and :c:data:`PYMEM_DOMAIN_OBJ` (ex:
-:c:func:`PyObject_Malloc`) domains.
+*pymalloc* is the :ref:`default allocator <default-memory-allocators>` of the
+:c:data:`PYMEM_DOMAIN_MEM` (ex: :c:func:`PyMem_Malloc`) and
+:c:data:`PYMEM_DOMAIN_OBJ` (ex: :c:func:`PyObject_Malloc`) domains.
 
 The arena allocator uses the following functions: