Unable to set numprocesses from configured environment variable

[fgimian]

Hello again, it doesn't appear that all environment variables defined in the config are parsed at the right time.

When using $(circus.env.<name>) for defining numprocesses, this only works when an environment variable is defined before running Circus, not when it's configured in the INI file in the [env:<name>] section. I can confirm this problem is present on the master branch of GitHub right now.

See below for an example.

(fastapi-deploy) [fots@workstation fotsies-fastapi-deploy-config]$ cat circus.ini
[watcher:app]
cmd = bash -c 'echo number is set to $number'
numprocesses = $(circus.env.number)

[env:app]
number = 8
(fastapi-deploy) [fots@workstation fotsies-fastapi-deploy-config]$ number=10 circusd circus.ini
2020-09-14 12:15:34 circus[100942] [INFO] Starting master on pid 100942
2020-09-14 12:15:34 circus[100942] [INFO] Arbiter now waiting for commands
number is set to 8
number is set to 8
number is set to 8
number is set to 8
number is set to 8
number is set to 8
number is set to 8
number is set to 8
number is set to 8
2020-09-14 12:15:34 circus[100942] [INFO] app started
number is set to 8
^C2020-09-14 12:15:34 circus[100942] [INFO] Got signal SIG_INT
2020-09-14 12:15:34 circus[100942] [INFO] Arbiter exiting
2020-09-14 12:15:34 circus[100942] [INFO] app stopped
(fastapi-deploy) [fots@workstation fotsies-fastapi-deploy-config]$ circusd circus.ini
Traceback (most recent call last):
  File "/home/fots/.virtualenv/fastapi-deploy/bin/circusd", line 11, in <module>
    load_entry_point('circus==0.16.1', 'console_scripts', 'circusd')()
  File "/home/fots/.virtualenv/fastapi-deploy/lib/python3.8/site-packages/circus/circusd.py", line 126, in main
    arbiter = Arbiter.load_from_config(args.config)
  File "/home/fots/.virtualenv/fastapi-deploy/lib/python3.8/site-packages/circus/arbiter.py", line 419, in load_from_config
    cfg = get_config(config_file)
  File "/home/fots/.virtualenv/fastapi-deploy/lib/python3.8/site-packages/circus/config.py", line 220, in get_config
    watcher['numprocesses'] = dget(section, 'numprocesses', 1,
  File "/home/fots/.virtualenv/fastapi-deploy/lib/python3.8/site-packages/circus/config.py", line 79, in dget
    value = int(value)
ValueError: invalid literal for int() with base 10: '$(circus.env.number)'

This doesn't seem to be a problem for cmd, only for numprocesses:

(fastapi-deploy) [fots@workstation fotsies-fastapi-deploy-config]$ cat circus.ini
[watcher:app]
cmd = bash -c 'echo number is set to $number'

[env:app]
number = 8
(fastapi-deploy) [fots@workstation fotsies-fastapi-deploy-config]$ circusd circus.ini
2020-09-14 12:15:57 circus[101119] [INFO] Starting master on pid 101119
2020-09-14 12:15:57 circus[101119] [INFO] Arbiter now waiting for commands
number is set to 8
2020-09-14 12:15:57 circus[101119] [INFO] app started
^C2020-09-14 12:15:58 circus[101119] [INFO] Got signal SIG_INT
2020-09-14 12:15:58 circus[101119] [INFO] Arbiter exiting
2020-09-14 12:15:58 circus[101119] [INFO] app stopped

Any help is appreciated
Fotis