network-interfaces: reload bridges on conf changes

And adopt the tests to add an interface and remove it again.

It should work when deactivating rstp, it will not work when activating
rstp for the first bridge as then the userspace daemon is not yet
available. But once one bridge is active with stp, it should work with
the reload for any further bridge.

Fixes #21745. Also see #22547.

nixos/modules/tasks/network-interfaces-scripted.nix

@@ -239,6 +239,10 @@ let
                 ip link set "${i}" master "${n}"
                 ip link set "${i}" up
               '')}
+              # Save list of enslaved interfaces
+              echo "${flip concatMapStrings v.interfaces (i: ''
+                ${i}
+              '')}" > /run/${n}.interfaces
 
               # Enable stp on the interface
               ${optionalString v.rstp ''
@@ -250,7 +254,28 @@ let
             postStop = ''
               ip link set "${n}" down || true
               ip link del "${n}" || true
+              rm -f /run/${n}.interfaces
             '';
+            reload = ''
+              # Un-enslave child interfaces (old list of interfaces)
+              for interface in `cat /run/${n}.interfaces`; do
+                ip link set "$interface" nomaster up
+              done
+
+              # Enslave child interfaces (new list of interfaces)
+              ${flip concatMapStrings v.interfaces (i: ''
+                ip link set "${i}" master "${n}"
+                ip link set "${i}" up
+              '')}
+              # Save list of enslaved interfaces
+              echo "${flip concatMapStrings v.interfaces (i: ''
+                ${i}
+              '')}" > /run/${n}.interfaces
+
+              # (Un-)set stp on the bridge
+              echo ${if v.rstp then "2" else "0"} > /sys/class/net/${n}/bridge/stp_state
+            '';
+            reloadIfChanged = true;
           });
 
         createVswitchDevice = n: v: nameValuePair "${n}-netdev"