There several bugs in the memory allocator.

Incorrectly detected the case when realloc() resizes a memory block in-place. Wrong address is used for filling the extra memory with DEADBYTE.

-    if (q == oldq && nbytes < original_nbytes) {
+    if (q == oldq - 2*SST && nbytes < original_nbytes) {
         /* shrinking:  mark old extra memory dead */
-        memset(q + nbytes, DEADBYTE, original_nbytes - nbytes);
+        memset(q + 2*SST + nbytes, DEADBYTE, original_nbytes - nbytes);
     }

But fixing this exposes other problem. _PyMem_DebugRawRealloc() is called recursively. _PyMem_DebugRawRealloc calls api->alloc.realloc which is _PyMem_DebugRawRealloc. There are two nested debug allocators. The block is nested in other block, both have their own header and footer.

|header1|header2|------------------------------|footer2|footer1|

_PyMem_DebugRawRealloc fills the extra memory with DEADBYTE.

|header|---------------------------..unused..|footer|
|header|---------------------------|footer|XXXXXXXXX|

But in case of nested _PyMem_DebugRawRealloc's, the outer one (which reallocates the inner block), overwrites the footer of the outer block.

|header1|header2|--------------------..unused..|footer2|footer1|
|header1|header2|--------------------..unused|footer1|XXXXXXXXX| after inner realloc
|header1|header2|--------------------|footer2|YYYYYYYYY|XXXXXXX| after outher realloc

XXX are DEADBYTEs written by the inner allocator, YYY are DEADBYTEs written by the outer allocator.