lib/config/options: add assertions for switch links

config/switch.nix

@@ -246,7 +246,7 @@
       links = {
         switch-d1 = {
           group = "1";
-          ports = [ "1" ];
+          ports = [ "12" ];
         };
         ap3.ports = [ "1" ];
         ap59.ports = [ "2" ];

nix/lib/config/options.nix

@@ -680,5 +680,45 @@ in
     in map (vlan: {
       assertion = builtins.length vlanNets.${vlan} == 1;
       message = "VLAN ${vlan} is used by more than one network: ${lib.concatStringsSep " " vlanNets.${vlan}}";
-    }) (builtins.attrNames vlanNets));
+    }) (builtins.attrNames vlanNets))
+    ++
+    # Duplicate switch port check
+    builtins.concatMap (hostName:
+      let
+        ports = lib.unique (
+          builtins.concatMap (linkName:
+            config.site.hosts.${hostName}.links.${linkName}.ports
+          ) (builtins.attrNames config.site.hosts.${hostName}.links)
+        );
+        linksOfPort = port:
+          builtins.attrNames (
+            lib.filterAttrs (_: { ports, ... }: builtins.elem port ports)
+              config.site.hosts.${hostName}.links
+          );
+      in map (port: {
+        assertion = builtins.length (linksOfPort port) == 1;
+        message = "${hostName}: port ${port} is used in more than one link: ${lib.concatStringsSep " " (linksOfPort port)}";
+      }) ports
+    ) (builtins.attrNames config.site.hosts)
+    ++
+    # Duplicate switch port group check
+    builtins.concatMap (hostName:
+      let
+        groups = lib.unique (
+          builtins.filter builtins.isString (
+            builtins.map (linkName:
+              config.site.hosts.${hostName}.links.${linkName}.group
+            ) (builtins.attrNames config.site.hosts.${hostName}.links)
+          )
+        );
+        linksOfGroup = wantedGroup:
+          builtins.attrNames (
+            lib.filterAttrs (_: { group, ... }: group == wantedGroup)
+              config.site.hosts.${hostName}.links
+          );
+      in map (group: {
+        assertion = builtins.length (linksOfGroup group) == 1;
+        message = "${hostName}: group ${group} is used in more than one link: ${lib.concatStringsSep " " (linksOfGroup group)}";
+      }) groups
+    ) (builtins.attrNames config.site.hosts);
 }