> why was that only an issue on 2.7?
I added PyMem_RawMalloc() to Python 3.4 and this function must be thread-safe.
https://docs.python.org/dev/c-api/memory.html#raw-memory-interface
"These functions are thread-safe, the GIL does not need to be held."
I replaced PyMem_RawMalloc() with PyMem_Malloc() when I backported the change, but I didn't know that PyMem_Malloc() isn't thread-safe *in debug mode*!
Gregory: do you think that it would be be crazy to fix PyMem_Malloc() to make it thread-safe in debug mode as well? I implemented a fix: PR 10828.