Better error message if batou is already running

[frlan]

I was running a deployment in shell 1 and wanted to start a second one in shell 2

./batou deploy -P production
batou/2.4.1 (cpython 3.11.8-final0, Darwin 23.4.0 arm64)
Could not acquire lock .batou-lock
Traceback (most recent call last):
  File "/Users/me/myprj/.appenv/a2ac3148/lib/python3.11/site-packages/batou/utils.py", line 71, in locked
    fcntl.lockf(lockfile, fcntl.LOCK_EX | fcntl.LOCK_NB)
BlockingIOError: [Errno 35] Resource temporarily unavailable

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/me/myprj/./.appenv/a2ac3148/bin/batou", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/Users/me/myprj/.appenv/a2ac3148/lib/python3.11/site-packages/batou/main.py", line 221, in main
    return args.func(**func_args)
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/me/myprj/.appenv/a2ac3148/lib/python3.11/site-packages/batou/deploy.py", line 364, in main
    with locked(".batou-lock"):
  File "/Users/flanitz/.nix-profile/lib/python3.11/contextlib.py", line 137, in __enter__
    return next(self.gen)
           ^^^^^^^^^^^^^^
  File "/Users/me/myprj/.appenv/a2ac3148/lib/python3.11/site-packages/batou/utils.py", line 74, in locked
    raise RuntimeError(
RuntimeError: ('cannot create lock "<_io.TextIOWrapper name=\'.batou-lock\' mode=\'a+\' encoding=\'UTF-8\'>": more than one instance running concurrently?', <_io.TextIOWrapper name='.batou-lock' mode='a+' encoding='UTF-8'>)

[elikoga]

Currently, batou is locking the deployment host, so that only one pid can deploy the project on a host. Since frlan wanted to deploy two projects at the same time, the lock mechanism seems to be too restrictive.

We are currently locking on the deployment host, so that only one pid can deploy the project on a host. We are not locking on the target systems, and also not environment-granular.

What should be the semantics of the lock mechanism? I think a good design would be a lock per environment, so that only one deployment can run in an environment at a time, as well as a lock per target system, so that only one deployment can run on a target system at a time, which is #450

[frlan]

This report was intended not about the locking itself but of the handling of the (expected) exception.