The following warning was being output when using `nats-server`:
```
trace: warning: getExe: Package "nats-server-2.9.21" does not have the meta.mainProgram attribute. We'll assume that the main program has the same name for now, but this behavior is deprecated, because it leads to surprising errors when the assumption does not hold. If the package has a main program, please set `meta.mainProgram` in its definition to make this warning go away. Otherwise, if the package does not have a main program, or if you don't control its definition, use getExe' to specify the name to the program, such as lib.getExe' foo "bar".
```
fixes#232505
Implements the new option `security.acme.maxConcurrentRenewals` to limit
the number of certificate generation (or renewal) jobs that can run in
parallel. This avoids overloading the system resources with many
certificates or running into acme registry rate limits and network
timeouts.
Architecture considerations:
- simplicity, lightweight: Concerns have been voiced about making this
already rather complex module even more convoluted. Additionally,
locking solutions shall not significantly increase performance and
footprint of individual job runs.
To accomodate these concerns, this solution is implemented purely in
Nix, bash, and using the light-weight `flock` util. To reduce
complexity, jobs are already assigned their lockfile slot at system
build time instead of dynamic locking and retrying. This comes at the
cost of not always maxing out the permitted concurrency at runtime.
- no stale locks: Limiting concurrency via locking mechanism is usually
approached with semaphores. Unfortunately, both SysV as well as
POSIX-Semaphores are *not* released when the process currently locking
them is SIGKILLed. This poses the danger of stale locks staying around
and certificate renewal being blocked from running altogether.
`flock` locks though are released when the process holding the file
descriptor of the lock file is KILLed or terminated.
- lockfile generation: Lock files could either be created at build time
in the Nix store or at script runtime in a idempotent manner.
While the latter would be simpler to achieve, we might exceed the number
of permitted concurrent runs during a system switch: Already running
jobs are still locked on the existing lock files, while jobs started
after the system switch will acquire locks on freshly created files,
not being blocked by the still running services.
For this reason, locks are generated and managed at runtime in the
shared state directory `/var/lib/locks/`.
nixos/security/acme: move locks to /run
also, move over permission and directory management to systemd-tmpfiles
nixos/security/acme: fix some linter remarks in my code
there are some remarks left for existing code, not touching that
nixos/security/acme: redesign script locking flow
- get rid of subshell
- provide function for wrapping scripts in a locked environment
nixos/acme: improve visibility of blocking on locks
nixos/acme: add smoke test for concurrency limitation
heavily inspired by m1cr0man
nixos/acme: release notes entry on new concurrency limits
nixos/acme: cleanup, clarifications
