Interpreter exit blocks waiting for ThreadPoolExecutor.map

[e183a343-e457-4baf-97bd-5e6a2e096b0d]

BPO	42409
Files	threadpool_map.py

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GitHub fields: ```python assignee = None closed_at = None created_at = labels = ['3.7', '3.8', 'type-bug', 'library'] title = 'Interpreter exit blocks waiting for ThreadPoolExecutor.map' updated_at = user = 'https://bugs.python.org/gsakkis' ``` bugs.python.org fields: ```python activity = actor = 'stefosgiwrgos' assignee = 'none' closed = False closed_date = None closer = None components = ['Library (Lib)'] creation = creator = 'gsakkis' dependencies = [] files = ['49608'] hgrepos = [] issue_num = 42409 keywords = [] message_count = 1.0 messages = ['381433'] nosy_count = 2.0 nosy_names = ['gsakkis', 'stefosgiwrgos'] pr_nums = [] priority = 'normal' resolution = None stage = None status = 'open' superseder = None type = 'behavior' url = 'https://bugs.python.org/issue42409' versions = ['Python 3.6', 'Python 3.7', 'Python 3.8'] ```

[e183a343-e457-4baf-97bd-5e6a2e096b0d]

ThreadPoolExecutor.map() prevents interpreter exit if there is a reference to the generator it returns. In the attached script:

• python threadpool_map.py run1 exits as soon as the exception is raised on the main thread. This is the desired behavior in our case.

• python threadpool_map.py run2 keeps running until the thread worker processes all queued work items. The only difference from run1 is that the result of ThreadPoolExecutor.map() is assigned to a variable.

• python threadpool_map.py run3 has a finally block that shuts down the executor without waiting. Still the worker thread keeps running even after the shutdown.

Initially it seemed like https://bugs.python.org/issue36780 but there is no change in the behavior after commenting out the atexit.register(_python_exit) call (for the run2 case at least).