Empty (but defined) bridges.*.interfaces = [ ] doesn't trigger restart of network-setup service on config switch

[jficz]

Describe the bug
When a bridge is declared (in this case for container networking purposes) like this:

  networking = {                                                                
    interfaces.${brName} = {                                                    
      virtual = true;                                                           
      ipv4.addresses = [ {                                                      
        address      = hostIP;                                                  
        prefixLength = 24;                                                      
      } ];                                                                                                                                                                                     
    };                                                                          
                                                                                
    bridges.${brName}.interfaces = [ ];
(...)

it does not get created and set-up on nixos-rebuild switch. According to #50208 this is because the interfaces list is empty.

This breaks declarative containers and possibly other use cases where bridge creation is static (i.e. declarative) but config is done on-the-fly. One has to manually restart the network-setup service, the bridge-netdev service and, in case of containers, all the containers' services.

It also basically kills all the containers connected to the bridge.

To Reproduce
Steps to reproduce the behavior:

1. Declare a bridge as above
2. nixos-rebuild switch

Expected behavior
The declared bridge should be up with proper IP, etc. The containers connected to the bridge should be up and connected.

Notify maintainers

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 5.8.9, NixOS, 21.03pre243690.441a7da8080 (Okapi)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.7`
 - channels(root): `"nixos-21.03pre243690.441a7da8080, nixos-hardware"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
- containers
# a list of nixos modules affected by the problem
module:

[stale]

I marked this as stale due to inactivity. → More info

[vs49688]

Still an issue, just hit this on 21.05.4014.96ded94eb0a.

[misuzu]

Still reproducible