fix: stop Qidi slot parse throwing on null

src/slic3r/Utils/QidiPrinterAgent.cpp

@@ -193,20 +193,10 @@ bool QidiPrinterAgent::fetch_slot_info(const std::string&        base_url,
         return false;
     }
 
-    auto json = nlohmann::json::parse(response_body, nullptr, false, true);
+    nlohmann::json status;
-    if (json.is_discarded()) {
+    nlohmann::json variables;
-        error = "Invalid JSON response";
+    if (!parse_slot_response(response_body, status, variables, error))
         return false;
-    }
-
-    if (!json.contains("result") || !json["result"].contains("status") || !json["result"]["status"].contains("save_variables") ||
-        !json["result"]["status"]["save_variables"].contains("variables")) {
-        error = "Unexpected JSON structure";
-        return false;
-    }
-
-    auto& variables = json["result"]["status"]["save_variables"]["variables"];
-    auto& status    = json["result"]["status"];
 
     box_count = variables.value("box_count", 1);
     if (box_count < 0) {
@@ -281,6 +271,31 @@ bool QidiPrinterAgent::fetch_slot_info(const std::string&        base_url,
     return true;
 }
 
+bool QidiPrinterAgent::parse_slot_response(const std::string& response_body,
+                                           nlohmann::json&    status,
+                                           nlohmann::json&    variables,
+                                           std::string&       error)
+{
+    auto json = nlohmann::json::parse(response_body, nullptr, false, true);
+    if (json.is_discarded()) {
+        error = "Invalid JSON response";
+        return false;
+    }
+
+    if (!json.is_object() || !json.contains("result") || !json["result"].is_object() || !json["result"].contains("status") ||
+        !json["result"]["status"].is_object() || !json["result"]["status"].contains("save_variables") ||
+        !json["result"]["status"]["save_variables"].is_object() || !json["result"]["status"]["save_variables"].contains("variables") ||
+        !json["result"]["status"]["save_variables"]["variables"].is_object()) {
+        // why: Qidi firmware may send null here, but json::value() throws for it.
+        error = "Unexpected JSON structure: save_variables.variables must be an object";
+        return false;
+    }
+
+    status    = json["result"]["status"];
+    variables = status["save_variables"]["variables"];
+    return true;
+}
+
 bool QidiPrinterAgent::fetch_filament_dict(const std::string& base_url,
                                            const std::string& api_key,
                                            QidiFilamentDict& dict,

src/slic3r/Utils/QidiPrinterAgent.hpp

@@ -2,6 +2,7 @@
 #define __QIDI_PRINTER_AGENT_HPP__
 
 #include "MoonrakerPrinterAgent.hpp"
+#include "nlohmann/json_fwd.hpp"
 
 #include <map>
 #include <string>
@@ -21,6 +22,11 @@ public:
     // Override filament sync (Qidi-specific implementation)
     bool fetch_filament_info(std::string dev_id) override;
 
+    static bool parse_slot_response(const std::string& response_body,
+                                    nlohmann::json&    status,
+                                    nlohmann::json&    variables,
+                                    std::string&       error);
+
     // Print operations — emit QiDi multi-color box config, then delegate to base.
     int start_print(PrintParams params, OnUpdateStatusFn update_fn, WasCancelledFn cancel_fn, OnWaitFn wait_fn) override;
     int start_local_print(PrintParams params, OnUpdateStatusFn update_fn, WasCancelledFn cancel_fn) override;

tests/slic3rutils/CMakeLists.txt

@@ -10,6 +10,7 @@ add_executable(${_TEST_NAME}_tests
     test_plugin_capabilities_in_use.cpp
     test_plugin_status.cpp
     test_printer_agent.cpp
+    test_qidi_printer_agent.cpp
     test_plugin_install.cpp
     test_plugin_lifecycle.cpp
     test_slicing_pipeline_bindings.cpp

tests/slic3rutils/test_qidi_printer_agent.cpp

@@ -0,0 +1,131 @@
+#include <catch2/catch_all.hpp>
+
+#include <nlohmann/json.hpp>
+
+#include <slic3r/Utils/QidiPrinterAgent.hpp>
+
+#include <string>
+
+using namespace Slic3r;
+
+TEST_CASE("Qidi slot response rejects null variables without throwing", "[QidiPrinterAgent]")
+{
+    const std::string response = R"({
+        "result": {
+            "status": {
+                "save_variables": {
+                    "variables": null
+                }
+            }
+        }
+    })";
+    nlohmann::json status;
+    nlohmann::json variables;
+    std::string    error;
+    bool           parsed = true;
+
+    REQUIRE_NOTHROW(parsed = QidiPrinterAgent::parse_slot_response(response, status, variables, error));
+    CHECK_FALSE(parsed);
+    CHECK_THAT(error, Catch::Matchers::ContainsSubstring("variables"));
+    CHECK_THAT(error, Catch::Matchers::ContainsSubstring("object"));
+}
+
+TEST_CASE("Qidi slot response rejects missing and non-object fields without throwing", "[QidiPrinterAgent]")
+{
+    std::string response;
+
+    SECTION("missing result")
+    {
+        response = R"({})";
+    }
+
+    SECTION("non-object result")
+    {
+        response = R"({"result":null})";
+    }
+
+    SECTION("missing status")
+    {
+        response = R"({"result":{}})";
+    }
+
+    SECTION("non-object status")
+    {
+        response = R"({"result":{"status":null}})";
+    }
+
+    SECTION("missing save_variables")
+    {
+        response = R"({"result":{"status":{}}})";
+    }
+
+    SECTION("non-object save_variables")
+    {
+        response = R"({"result":{"status":{"save_variables":null}}})";
+    }
+
+    SECTION("missing variables")
+    {
+        response = R"({"result":{"status":{"save_variables":{}}}})";
+    }
+
+    SECTION("scalar")
+    {
+        response = R"({"result":{"status":{"save_variables":{"variables":42}}}})";
+    }
+
+    SECTION("array")
+    {
+        response = R"({"result":{"status":{"save_variables":{"variables":[]}}}})";
+    }
+
+    nlohmann::json status;
+    nlohmann::json variables;
+    std::string    error;
+    bool           parsed = true;
+
+    REQUIRE_NOTHROW(parsed = QidiPrinterAgent::parse_slot_response(response, status, variables, error));
+    CHECK_FALSE(parsed);
+}
+
+TEST_CASE("Qidi slot response exposes valid status and variables", "[QidiPrinterAgent]")
+{
+    const std::string response = R"({
+        "result": {
+            "status": {
+                "save_variables": {
+                    "variables": {
+                        "box_count": 2,
+                        "color_slot0": 3
+                    }
+                },
+                "box_stepper slot0": {
+                    "runout_button": 0
+                }
+            }
+        }
+    })";
+    nlohmann::json status;
+    nlohmann::json variables;
+    std::string    error;
+    bool           parsed = false;
+
+    REQUIRE_NOTHROW(parsed = QidiPrinterAgent::parse_slot_response(response, status, variables, error));
+    REQUIRE(parsed);
+    CHECK(status.is_object());
+    CHECK(variables.is_object());
+    CHECK(variables.at("box_count") == 2);
+    CHECK(status.contains("box_stepper slot0"));
+}
+
+TEST_CASE("Qidi slot response rejects invalid JSON", "[QidiPrinterAgent]")
+{
+    nlohmann::json status;
+    nlohmann::json variables;
+    std::string    error;
+    bool           parsed = true;
+
+    REQUIRE_NOTHROW(parsed = QidiPrinterAgent::parse_slot_response("{not json", status, variables, error));
+    CHECK_FALSE(parsed);
+    CHECK(error == "Invalid JSON response");
+}