Use finally to terminate parallel processes

[pmeier]

Subject: Correctly terminate parallel processes.

Feature or Bugfix

• Feature

Purpose

While debugging an issue with parallel docs building, I encountered zombie parallel processes. This happened after I used ctrl+c to stop a hanging build. With this change, all parallel process will be terminated correctly regardless of the reason that terminated the build.

Detail

The reason is simple: except Exception does not catch everything. For example, KeyboardInterrupt which is raised through ctrl+c will not be caught:

try:
    raise KeyboardInterrupt
except Exception:
    print("Terminating processes", flush=True)
    raise

Traceback (most recent call last):
  File "/home/user/main.py", line 2, in <module>
    raise KeyboardInterrupt
KeyboardInterrupt

See the exception hierarchy for details.

Since we don't do anything with the caught exception other than re-raising it, we can simply use the finally clause:

try:
    raise KeyboardInterrupt
finally:
    print("Terminating processes", flush=True)

Terminating processes
Traceback (most recent call last):
  File "/home/user/main.py", line 2, in <module>
    raise KeyboardInterrupt
KeyboardInterrupt