nixos/mosquitto: Refactor integration test code

nixos/tests/mosquitto.nix

@@ -5,17 +5,6 @@ let
   username = "mqtt";
   password = "VERY_secret";
   topic = "test/foo";
-
-  cmd = bin: pkgs.lib.concatStringsSep " " [
-    "${pkgs.mosquitto}/bin/mosquitto_${bin}"
-    "-V mqttv311"
-    "-h server"
-    "-p ${toString port}"
-    "-u ${username}"
-    "-P '${password}'"
-    "-t ${topic}"
-  ];
-
 in {
   name = "mosquitto";
   meta = with pkgs.stdenv.lib; {
@@ -49,9 +38,27 @@ in {
 
   testScript = let
     file = "/tmp/msg";
-    sub = args:
-      "(${cmd "sub"} -C 1 ${args} | tee ${file} &)";
   in ''
+    def mosquitto_cmd(binary):
+        return (
+            "${pkgs.mosquitto}/bin/mosquitto_{} "
+            "-V mqttv311 "
+            "-h server "
+            "-p ${toString port} "
+            "-u ${username} "
+            "-P '${password}' "
+            "-t ${topic}"
+        ).format(binary)
+
+
+    def publish(args):
+        return "{} {}".format(mosquitto_cmd("pub"), args)
+
+
+    def subscribe(args):
+        return "({} -C 1 {} | tee ${file} &)".format(mosquitto_cmd("sub"), args)
+
+
     start_all()
     server.wait_for_unit("mosquitto.service")
 
@@ -59,37 +66,25 @@ in {
         machine.fail("test -f ${file}")
 
     # QoS = 0, so only one subscribers should get it
-    server.execute(
-        "${sub "-q 0"}"
-    )
+    server.execute(subscribe("-q 0"))
 
     # we need to give the subscribers some time to connect
     client2.execute("sleep 5")
-    client2.succeed(
-        "${cmd "pub"} -m FOO -q 0"
-    )
+    client2.succeed(publish("-m FOO -q 0"))
 
     server.wait_until_succeeds("grep -q FOO ${file}")
     server.execute("rm ${file}")
 
     # QoS = 1, so both subscribers should get it
-    server.execute(
-        "${sub "-q 1"}"
-    )
-    client1.execute(
-        "${sub "-q 1"}"
-    )
+    server.execute(subscribe("-q 1"))
+    client1.execute(subscribe("-q 1"))
 
     # we need to give the subscribers some time to connect
     client2.execute("sleep 5")
-    client2.succeed(
-        "${cmd "pub"} -m BAR -q 1"
-    )
+    client2.succeed(publish("-m BAR -q 1"))
 
-    server.wait_until_succeeds("grep -q BAR ${file}")
-    server.execute("rm ${file}")
-
-    client1.wait_until_succeeds("grep -q BAR ${file}")
-    client1.execute("rm ${file}")
+    for machine in server, client1:
+        machine.wait_until_succeeds("grep -q BAR ${file}")
+        machine.execute("rm ${file}")
   '';
 })