[Tutor] Multiprocessing question

[Albert-Jan Roskam]

   Hi,
   I'm trying to create multiprocessing task that runs multiple SQL queries
   in parallel (COPY TO csv), then adds those csv files to a zip. In the code
   below, tge csv files don't end up in the zip. Why not?
   I'm using a buffer instead of a file for the zip because in my actual
   scenario, a Flask endpoint returns the .zip
   Thanks in advance!
   Albert-Jan
   import io
   import logging
   import multiprocessing as mp
   #from multiprocessing.shared_memory import SharedMemory
   from random import randint
   from time import sleep
   from zipfile import ZipFile, ZIP_DEFLATED
   mp.log_to_stderr(logging.DEBUG)
   def task(lock, archive):
       # Do some work in parellel (SQL), followed
       # by adding the query result to a zip
       name = mp.current_process().name
       print(f"==begin {name}")
       sleep(randint(1, 5))
       csv_buff = io.BytesIO(b"name = " + name.encode("utf-8"))
       with lock:
           print(f"* begin zipping {name}")
           sleep(randint(1, 5))
           data = csv_buff.getvalue()
           print("csv data", data)
           archive.writestr(name, data)
           print(f"* end zipping {name}")
       print(f"==end {name}")
   def main():
       #shm = SharedMemory(create=True, size=2048) # read-only! :-(
       #zip_buff = shm.buf
       zip_buff = io.BytesIO()
       archive = ZipFile(zip_buff, "w", ZIP_DEFLATED, False, 2)
       lock = mp.Lock()
       procs = [mp.Process(target=task, args=(lock, archive)) for _ in
   range(3)]
       for proc in procs:
           proc.start()
       for proc in procs:
           proc.join()
       with archive:
           archive.writestr("main", b"from main")
       zip_buff.seek(0)
       data = zip_buff.getvalue()
       assert len(data), "No data!"
       print("data:", data)

       with open("archive.zip", mode="wb") as archive:
           archive.write(data)
   if __name__ == "__main__":
       main()