nixos/mosquitto: restore passwordless system feature

during the rewrite the checkPasswords=false feature of the old module
was lost. restore it, and with it systems that allow any client to use
any username.

nixos/modules/services/networking/mosquitto.nix

@@ -264,6 +264,15 @@ let
         default = {};
       };
 
+      omitPasswordAuth = mkOption {
+        type = bool;
+        description = ''
+          Omits password checking, allowing anyone to log in with any user name unless
+          other mandatory authentication methods (eg TLS client certificates) are configured.
+        '';
+        default = false;
+      };
+
       acl = mkOption {
         type = listOf str;
         description = ''
@@ -294,9 +303,9 @@ let
   formatListener = idx: listener:
     [
       "listener ${toString listener.port} ${toString listener.address}"
-      "password_file ${cfg.dataDir}/passwd-${toString idx}"
       "acl_file ${makeACLFile idx listener.users listener.acl}"
     ]
+    ++ optional (! listener.omitPasswordAuth) "password_file ${cfg.dataDir}/passwd-${toString idx}"
     ++ formatFreeform {} listener.settings
     ++ concatMap formatAuthPlugin listener.authPlugins;
 

nixos/tests/mosquitto.nix

@@ -3,6 +3,7 @@ import ./make-test-python.nix ({ pkgs, lib, ... }:
 let
   port = 1888;
   tlsPort = 1889;
+  anonPort = 1890;
   password = "VERY_secret";
   hashedPassword = "$7$101$/WJc4Mp+I+uYE9sR$o7z9rD1EYXHPwEP5GqQj6A7k4W1yVbePlb8TqNcuOLV9WNCiDgwHOB0JHC1WCtdkssqTBduBNUnUGd6kmZvDSw==";
   topic = "test/foo";
@@ -63,7 +64,7 @@ in {
     };
   in {
     server = { pkgs, ... }: {
-      networking.firewall.allowedTCPPorts = [ port tlsPort ];
+      networking.firewall.allowedTCPPorts = [ port tlsPort anonPort ];
       services.mosquitto = {
         enable = true;
         settings = {
@@ -112,6 +113,18 @@ in {
               use_identity_as_username = true;
             };
           }
+          {
+            port = anonPort;
+            omitPasswordAuth = true;
+            settings.allow_anonymous = true;
+            acl = [ "pattern read #" ];
+            users = {
+              anonWriter = {
+                password = "<ignored>" + password;
+                acl = [ "write ${topic}" ];
+              };
+            };
+          }
         ];
       };
     };
@@ -182,5 +195,14 @@ in {
                 topic="$SYS/#",
                 port=${toString tlsPort},
                 user="no_such_user"))
+
+    with subtest("check omitPasswordAuth"):
+        parallel(
+            lambda: client1.succeed(subscribe("-i fd56032c-d9cb-4813-a3b4-6be0e04c8fc3",
+                "anonReader", port=${toString anonPort})),
+            lambda: [
+                server.wait_for_console_text("fd56032c-d9cb-4813-a3b4-6be0e04c8fc3"),
+                client2.succeed(publish("-m test", "anonWriter", port=${toString anonPort}))
+            ])
   '';
 })