[issue41699] Potential memory leak with asyncio and run_in_executor

[Sophia Wisdom]

Sophia Wisdom <sophia at reduct.video> added the comment:

While not calling executor.shutdown() may leave some resources still used, it should be small and fixed. Regularly calling executor.shutdown() and then instantiating a new ThreadPoolExecutor in order to run an asyncio program does not seem like a good API to me.

You mention there appear to be both an event loop and a futures leak -- I think I have a good test case for the futures, without using threads at all. This seems to be leaking `future._result`s somehow even though their __del__ is called.

```
import asyncio
from concurrent.futures import Executor, Future
import gc

result_gcs = 0
suture_gcs = 0

class ResultHolder:
    def __init__(self, mem_size):
        self.mem = list(range(mem_size)) # so we can see the leak
    
    def __del__(self):
        global result_gcs
        result_gc += 1

class Suture(Future):
   def __del__(self):
       global suture_gcs
       suture_gcs += 1

class SimpleExecutor(Executor):
    def submit(self, fn):
        future = Suture()
        future.set_result(ResultHolder(1000))
        return future

async def function():
    loop = asyncio.get_running_loop()
    for i in range(10000):
        loop.run_in_executor(SimpleExecutor(), lambda x:x)

def run():
    asyncio.run(function())
    print(suture_gcs, result_gcs)
```
10MB

```
> run()
10000 10000
```
100MB

Both result_gcs and suture_gcs are 10000 every time. My best guess for why this would happen (for me it doesn't seem to happen without the loop.run_in_executor) is the conversion from a concurrent.Future to an asyncio.Future, which involves callbacks to check on the result, but that doesn't make sense, because the result itself has __del__ called on it but somehow it doesn't free the memory!

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41699>
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