I have run into a memory leak caused by using run_in_executor + ThreadPoolExecutor while running some stability tests with custom web services.

It was 1 MB leaked for 1k requests made for my case and I've extracted the root cause and converted it into minimal script with both mentioned parts + just NOP function to "run".

The script can easily eat up to 1 GB of memory in less then 1 minute now. It uses external psutil library to report the memory allocated but it can be easily commented out and the leak will stay anyway.

One can found that script attached + Dockerfile/Makefile for reproducibility. I've also reproduced it in my own conda-based 3.7 environment as well as the master branch of cpython.