New patch (version 2), more complete:

 * add "void *data" argument to all allocator functions
 * add "block" API used for pymalloc allocator to allocate arenas. Use mmap or malloc, but may use VirtualAlloc in a near future (see #13483). Callbacks prototype:

   - void block_malloc (size_t, void*);
   - void block_free (void*, size_t, void*);

 * remove PY_ALLOC_SYSTEM_API


Main API:
---
#define PY_ALLOC_MEM_API 'm'      /* PyMem_Malloc() API */
#define PY_ALLOC_OBJECT_API 'o'   /* PyObject_Malloc() API */

PyAPI_FUNC(int) Py_GetAllocators(
    char api,
    void* (**malloc_p) (size_t size, void *user_data),
    void* (**realloc_p) (void *ptr, size_t size, void *user_data),
    void (**free_p) (void *ptr, void *user_data),
    void **user_data_p
    );

PyAPI_FUNC(int) Py_SetAllocators(
    char api,
    void* (*malloc) (size_t size, void *user_data),
    void* (*realloc) (void *ptr, size_t size, void *user_data),
    void (*free) (void *ptr, void *user_data),
    void *user_data
    );

PyAPI_FUNC(void) Py_GetBlockAllocators(
    void* (**malloc_p) (size_t size, void *user_data),
    void (**free_p) (void *ptr, size_t size, void *user_data),
    void **user_data_p
    );

PyAPI_FUNC(int) Py_SetBlockAllocators(
    void* (*malloc) (size_t size, void *user_data),
    void (*free) (void *ptr, size_t size, void *user_data),
    void *user_data
    );
---


I see the following use cases using allocators:

* Don't use malloc nor mmap but your own allocator: replace PyMem and PyObject allocators
* Track memory leaks (my pytracemalloc project, or Antoine's simple _Py_AllocatedBlocks counter): hook PyMem and PyObject allocators
* Fill newly allocated memory with a pattern and check for buffer underflow and overflow: hook PyMem and PyObject allocators

"Hook" means adding extra code before and/or after calling the original function.

The final API should allow to hook the APIS multiple times and replacing allocators. So it should be possible to track memory leaks, detect buffer overflow and our your own allocators. It is not yet possible with the patch 2, because _PyMem_DebugMalloc() calls directly malloc().

_PyMem_DebugMalloc is no more used by PyObject_Malloc. This code should be rewritten to use the hook approach instead of replacing memory allocators.


Example tracing PyMem calls using the hook approach:
-----------------------------------
typedef struct {
    void* (*malloc) (size_t, void*);
    void* (*realloc) (void*, size_t, void*);
    void (*free) (void*, void*);
    void *data;
} allocators_t;

allocators_t pymem, pyobject;

void* trace_malloc (size_t size, void* data)
{
    allocators_t *alloc = (allocators_t *)data;
    printf("malloc(%z)\n", size);
    return alloc.malloc(size, alloc.data);
}

void* trace_realloc (void* ptr, size_t size, void* data)
{
    allocators_t *alloc = (allocators_t *)data;
    printf("realloc(%p, %z)\n", ptr, size);
    return alloc.realloc(ptr, size, alloc.data);
}

void trace_free (void* ptr, void* data)
{
    allocators_t *alloc = (allocators_t *)data;
    printf("free(%p)\n", ptr);
    alloc.free(ptr, alloc.data);
}

void hook_pymem(void)
{
   Py_GetAllocators(PY_ALLOC_MEM_API, &pymem.malloc, &pymem.realloc, &pymem.free, &pymem.data);
   Py_SetAllocators(PY_ALLOC_MEM_API, trace_malloc, trace_realloc, trace_free, &pymem);

   Py_GetAllocators(PY_ALLOC_OBJECT_API, &pyobject.malloc, &pyobject.realloc, &pyobject.free, &pyobject.data);
   Py_SetAllocators(PY_ALLOC_OBJECT_API, trace_malloc, trace_realloc, trace_free, &pyobject);
}
-----------------------------------

I didn't try the example :-p It is just to give you an idea of the API and how to use it.