Revert "nixos/wireless: make wireless.interfaces mandatory"

This reverts commit 030a521adc9510207dd9f06b8d8b552ff7d999f9.

(cherry picked from commit eba5f5c1e5a61ef65a970c699b9f93aff87448bf)

nixos/modules/services/networking/wpa_supplicant.nix

@@ -40,7 +40,8 @@ in {
         default = [];
         example = [ "wlan0" "wlan1" ];
         description = ''
-          The interfaces <command>wpa_supplicant</command> will use.
+          The interfaces <command>wpa_supplicant</command> will use. If empty, it will
+          automatically use all wireless interfaces.
         '';
       };
 
@@ -219,14 +220,7 @@ in {
   };
 
   config = mkIf cfg.enable {
-    assertions = [
-      { assertion = cfg.interfaces != [];
-        message = ''
-          No network interfaces for wpa_supplicant have been configured.
-          Please, specify at least one using networking.wireless.interfaces.
-        '';
-      }
-    ] ++ flip mapAttrsToList cfg.networks (name: cfg: {
+    assertions = flip mapAttrsToList cfg.networks (name: cfg: {
       assertion = with cfg; count (x: x != null) [ psk pskRaw auth ] <= 1;
       message = ''options networking.wireless."${name}".{psk,pskRaw,auth} are mutually exclusive'';
     });
@@ -261,7 +255,20 @@ in {
         then echo >&2 "<3>/etc/wpa_supplicant.conf present but ignored. Generated ${configFile} is used instead."
         fi
         iface_args="-s -u -D${cfg.driver} ${configStr}"
-        args="${concatMapStringsSep " -N " (i: "-i${i} $iface_args") ifaces}"
+        ${if ifaces == [] then ''
+          for i in $(cd /sys/class/net && echo *); do
+            DEVTYPE=
+            UEVENT_PATH=/sys/class/net/$i/uevent
+            if [ -e "$UEVENT_PATH" ]; then
+              source "$UEVENT_PATH"
+              if [ "$DEVTYPE" = "wlan" -o -e /sys/class/net/$i/wireless ]; then
+                args+="''${args:+ -N} -i$i $iface_args"
+              fi
+            fi
+          done
+        '' else ''
+          args="${concatMapStringsSep " -N " (i: "-i${i} $iface_args") ifaces}"
+        ''}
         exec wpa_supplicant $args
       '';
     };