nixos/nebula: wait for start notification to prevent startup race

[numinit]

Description of changes

Support slackhq/nebula#929. Marks the service as started when the tunnel is up.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[numinit]

cc @Jaculabilis

[numinit]

Enabled by #272581

[jian-lin]

LGTM

Can #273273 be reverted after we have this change?

[numinit]

@jian-lin I think #273273 is probably good to keep because it's defensive. We should wait for the service to be up -- yes, it may be redundant, but I'm not sure if it's redundant with respect to systemctl restart.

[jian-lin]

yes, it may be redundant, but I'm not sure if it's redundant with respect to systemctl restart

It is indeed redundant. systemctl restart is systemctl stop followed by systemctl start according to man systemctl.

I think #273273 is probably good to keep because it's defensive.

I see #273273 as a workaround for an issue in the nebula service and this PR fixes that issue properly.

[NickCao]

Should this be backported?

[numinit]

We will have to backport 1.8.0 as well.

[numinit]

I created #274087. Once it's merged I'll do a manual backport PR with all these changes, including Nebula 1.8 which adds the more predictable systemd service start behavior.