CLI: let --export-settings - write the merged config to stdout

--export-settings already writes the merged config as JSON at the right
point in the CLI flow. Passing - writes the same document to stdout.

- ConfigBase::save_to_json gains a stream overload. The file overload
  serializes through it before opening the file, so the format is
  unchanged and a config that cannot be serialized leaves an existing
  file untouched instead of truncating it.
- On stdout, invalid UTF-8 in string values is written as U+FFFD instead
  of ending the process with an uncaught type_error; files keep the
  strict behaviour.
- - is rejected up front when combined with an action or transform that
  can write to stdout or does real work, so stdout carries only the
  JSON.
- The unconditional "skip locked instance" stdout write during arrange
  now goes to the log.
- Tests in tests/libslic3r/test_config.cpp.

src/OrcaSlicer.cpp

@@ -1387,6 +1387,25 @@ int CLI::run(int argc, char **argv)
     if (downward_check_option)
         downward_check = downward_check_option->value;
 
+    // --export-settings - writes its JSON to stdout, so reject every action or transform that may write there
+    // too (--info, --help, --orient, slicing and exporting). The allowed ones do nothing when nothing is
+    // sliced or exported.
+    if (std::find(m_actions.begin(), m_actions.end(), "export_settings") != m_actions.end() && m_config.opt_string("export_settings") == "-") {
+        static const std::set<std::string> stdout_compatible = { "export_settings", "uptodate", "load_defaultfila", "min_save",
+                                                                 "mtcpp", "mstpp", "no_check", "normative_check", "pipe" };
+        for (const std::vector<std::string> *opt_keys : { &m_actions, &m_transforms }) {
+            for (const std::string &opt_key : *opt_keys) {
+                if (stdout_compatible.count(opt_key) == 0) {
+                    std::string flag = opt_key;
+                    std::replace(flag.begin(), flag.end(), '_', '-');
+                    boost::nowide::cerr << "--export-settings - cannot be combined with --" << flag << std::endl;
+                    record_exit_reson(outfile_dir, CLI_INVALID_PARAMS, 0, cli_errors[CLI_INVALID_PARAMS], sliced_info);
+                    flush_and_exit(CLI_INVALID_PARAMS);
+                }
+            }
+        }
+    }
+
     bool start_gui = m_actions.empty() && !downward_check;
     if (start_gui) {
         BOOST_LOG_TRIVIAL(info) << "no action, start gui directly" << std::endl;
@@ -5348,7 +5367,7 @@ int CLI::run(int argc, char **argv)
                                 //skip this object due to be locked in plate
                                 ap.itemid = locked_aps.size();
                                 locked_aps.emplace_back(ap);
-                                boost::nowide::cout <<__FUNCTION__ << boost::format(": skip locked instance, obj_id %1%, instance_id %2%") % oidx % inst_idx;
+                                BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << boost::format(": skip locked instance, obj_id %1%, instance_id %2%") % oidx % inst_idx;
                             }
                         }
                     }
@@ -5937,7 +5956,11 @@ int CLI::run(int argc, char **argv)
             //FIXME check for mixing the FFF / SLA parameters.
             // or better save fff_print_config vs. sla_print_config
             //m_print_config.save(m_config.opt_string("save"));
-            m_print_config.save_to_json(m_config.opt_string(opt_key), std::string("project_settings"), std::string("project"), std::string(SoftFever_VERSION));
+            const std::string &settings_file = m_config.opt_string(opt_key);
+            if (settings_file == "-")
+                m_print_config.save_to_json(boost::nowide::cout, "project_settings", "project", SoftFever_VERSION, /*replace_invalid_utf8=*/true);
+            else
+                m_print_config.save_to_json(settings_file, std::string("project_settings"), std::string("project"), std::string(SoftFever_VERSION));
         } else if (opt_key == "info") {
             // --info works on unrepaired model
             for (Model &model : m_models) {

src/libslic3r/Config.cpp

@@ -7,6 +7,7 @@
 #include <algorithm>
 #include <assert.h>
 #include <fstream>
+#include <sstream>
 #include <iostream>
 #include <iomanip>
 #include <regex>
@@ -1515,6 +1516,19 @@ std::optional<PluginCapabilityRef> parse_capability_ref(const std::string& value
 
 //BBS: add json support
 void ConfigBase::save_to_json(const std::string &file, const std::string &name, const std::string &from, const std::string &version) const
+{
+    // Serialize first: if that throws (invalid UTF-8), the existing file stays untouched.
+    std::ostringstream ss;
+    this->save_to_json(ss, name, from, version);
+    boost::nowide::ofstream c;
+    c.open(file, std::ios::out | std::ios::trunc);
+    c << ss.str();
+    c.close();
+
+    BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ":" <<__LINE__ << boost::format(", saved config to %1%\n")%file;
+}
+
+void ConfigBase::save_to_json(std::ostream &os, const std::string &name, const std::string &from, const std::string &version, bool replace_invalid_utf8) const
 {
     json j;
     //record the headers
@@ -1561,12 +1575,7 @@ void ConfigBase::save_to_json(const std::string &file, const std::string &name,
             j["plugins"] = unique_refs;
     }
 
-    boost::nowide::ofstream c;
+    os << j.dump(1, '\t', false, replace_invalid_utf8 ? json::error_handler_t::replace : json::error_handler_t::strict) << std::endl;
-    c.open(file, std::ios::out | std::ios::trunc);
-    c << j.dump(1, '\t') << std::endl;
-    c.close();
-
-    BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ":" <<__LINE__ << boost::format(", saved config to %1%\n")%file;
 }
 
 void ConfigBase::save(const std::string &file) const

src/libslic3r/Config.hpp

@@ -2825,6 +2825,9 @@ public:
 
     //BBS: add json support
     void save_to_json(const std::string &file, const std::string &name, const std::string &from, const std::string &version) const;
+    // Same document, written to a stream. Invalid UTF-8 in a string value throws nlohmann's type_error unless
+    // replace_invalid_utf8 is set, which writes U+FFFD instead (for callers such as stdout with no handler).
+    void save_to_json(std::ostream &os, const std::string &name, const std::string &from, const std::string &version, bool replace_invalid_utf8 = false) const;
 
     // Rebuild the in-memory "plugins" manifest (the "name;uuid;capability" references the plugin
     // dispatchers consume) from the plugin-backed options via the registered resolver. save_to_json()

src/libslic3r/PrintConfig.cpp

@@ -11916,7 +11916,7 @@ CLIActionsConfigDef::CLIActionsConfigDef()
 
     def = this->add("export_settings", coString);
     def->label = L("Export Settings");
-    def->tooltip = L("This exports settings to a file.");
+    def->tooltip = L("This exports settings to a file. Use - to write them to stdout.");
     def->cli_params = "settings.json";
     def->set_default_value(new ConfigOptionString("output.json"));
 

tests/libslic3r/test_config.cpp

@@ -15,6 +15,8 @@
 #include <boost/nowide/fstream.hpp>
 #include <nlohmann/json.hpp>
 
+#include <sstream>
+
 using namespace Slic3r;
 
 SCENARIO("Generic config validation performs as expected.", "[Config]") {
@@ -488,6 +490,59 @@ TEST_CASE("save_to_json round-trips plugin capability references as strings", "[
     CHECK(reloaded.option<ConfigOptionStrings>("slicing_pipeline_plugin")->values == refs);
 }
 
+TEST_CASE("save_to_json writes the same document to a stream as to a file", "[Config]") {
+    DynamicPrintConfig config;
+    config.set_key_value("layer_height", new ConfigOptionFloat(0.2));
+    config.set_key_value("wall_loops", new ConfigOptionInt(3));
+    config.set_key_value("filament_type", new ConfigOptionStrings({ "PLA", "PETG" }));
+    config.set_key_value("machine_start_gcode", new ConfigOptionString("G28\nG1 Z5"));
+
+    ScopedTemporaryFile tmp(".json");
+    config.save_to_json(tmp.string(), "test_preset", "User", "1.0.0.0");
+    std::string file_contents;
+    {
+        boost::nowide::ifstream ifs(tmp.string());
+        file_contents.assign(std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>());
+    }
+    // The file format: one tab per nesting level and a trailing newline.
+    REQUIRE_FALSE(file_contents.empty());
+    CHECK(file_contents.rfind("{\n\t\"", 0) == 0);
+    CHECK(file_contents.back() == '\n');
+
+    std::ostringstream strict, replaced;
+    config.save_to_json(strict, "test_preset", "User", "1.0.0.0");
+    config.save_to_json(replaced, "test_preset", "User", "1.0.0.0", true);
+    CHECK(strict.str() == file_contents);
+    CHECK(replaced.str() == file_contents);
+    CHECK(nlohmann::json::parse(strict.str())["machine_start_gcode"] == "G28\nG1 Z5");
+}
+
+TEST_CASE("save_to_json replaces invalid UTF-8 in a stream only when asked", "[Config]") {
+    DynamicPrintConfig config;
+    config.set_key_value("machine_start_gcode", new ConfigOptionString("G28 ; \xff"));
+
+    std::ostringstream strict, replaced;
+    CHECK_THROWS_AS(config.save_to_json(strict, "test_preset", "User", "1.0.0.0"), nlohmann::json::type_error);
+    REQUIRE_NOTHROW(config.save_to_json(replaced, "test_preset", "User", "1.0.0.0", true));
+    CHECK(nlohmann::json::parse(replaced.str())["machine_start_gcode"] == "G28 ; \xEF\xBF\xBD");
+}
+
+TEST_CASE("save_to_json leaves an existing file untouched when the config cannot be serialized", "[Config]") {
+    DynamicPrintConfig config;
+    config.set_key_value("machine_start_gcode", new ConfigOptionString("G28 ; \xff"));
+
+    ScopedTemporaryFile tmp(".json");
+    {
+        boost::nowide::ofstream ofs(tmp.string());
+        ofs << "previous";
+    }
+    CHECK_THROWS_AS(config.save_to_json(tmp.string(), "test_preset", "User", "1.0.0.0"), nlohmann::json::type_error);
+
+    boost::nowide::ifstream ifs(tmp.string());
+    const std::string contents((std::istreambuf_iterator<char>(ifs)), std::istreambuf_iterator<char>());
+    CHECK(contents == "previous");
+}
+
 TEST_CASE("plugin capability references survive string-map serialization", "[Config][plugins]") {
     const std::vector<std::string> refs = {
         "master_plugin;;header-stamp",