machined: return recognizable error when we try to register the same machine name twice

src/machine/machine-varlink.c

@@ -153,6 +153,8 @@ int vl_method_register(Varlink *link, JsonVariant *parameters, VarlinkMethodFlag
         }
 
         r = machine_link(manager, machine);
+        if (r == -EEXIST)
+                return varlink_error(link, "io.systemd.Machine.MachineExists", NULL);
         if (r < 0)
                 return r;
 

src/shared/varlink-io.systemd.Machine.c

@@ -16,7 +16,10 @@ static VARLINK_DEFINE_METHOD(
                 VARLINK_DEFINE_INPUT(sshAddress,        VARLINK_STRING, VARLINK_NULLABLE),
                 VARLINK_DEFINE_INPUT(sshPrivateKeyPath, VARLINK_STRING, VARLINK_NULLABLE));
 
+static VARLINK_DEFINE_ERROR(MachineExists);
+
 VARLINK_DEFINE_INTERFACE(
                 io_systemd_Machine,
                 "io.systemd.Machine",
-                &vl_method_Register);
+                &vl_method_Register,
+                &vl_error_MachineExists);

src/vmspawn/vmspawn-register.c

@@ -58,7 +58,6 @@ int register_machine(
         if (r < 0)
                 return log_error_errno(r, "Failed to connect to machined on /run/systemd/machine/io.systemd.Machine: %m");
 
-
         return varlink_callb_and_log(vl,
                         "io.systemd.Machine.Register",
                         NULL,