Lock Config and Preset Files Across Instances and Write Them Atomically

Every running instance shares one OrcaSlicer.conf and one user preset
tree, and nothing kept their writers apart. Two instances saving at the
same moment, or the cloud preset sync thread writing while the GUI thread
saved, could interleave, and a reader in another instance could open a
preset JSON or .info file between truncate and close and get a partial
file, dropping that preset for the session with a parse error.

Add InstanceLock, a scoped guard that serialises the threads of one
process through a recursive mutex and other processes through an advisory
OS file lock, which the OS releases when its holder dies. It is best
effort: when the lock file cannot be created, or another instance still
holds it after two seconds, the guard logs a warning and lets the write
proceed rather than letting a hung instance block every other one.

AppConfig::save() and load() hold OrcaSlicer.conf.lock; load is included
because the Windows path restores from the .bak copy, and because Windows
cannot replace a file another process has open, so an unlocked reader
there made the other instance's save fail. Every user preset writer and
reader holds user.lock: Preset::save(), save_info(), load_info() and
remove_files(), the whole directory scan in load_presets(), the bundle
metadata file, the .info removal after a cloud-confirmed delete and the
bundle folder removal on unsubscribe. The guard sits at the leaf writers
on purpose: set_sync_info_and_save() calls save_info() under the preset
collection mutex, so a batch lock around save_user_presets(), which takes
that mutex through delete_preset(), would invert the order.

Write preset JSON, .info and bundle metadata files through a temporary
file beside the target that is renamed over it, so a reader that never
waits still sees a complete old or new file. On Linux and macOS
rename_file() deleted the target before renaming, leaving a window in
which the file did not exist at all, and returned a meaningless error
code on failure; rename() replaces atomically, so call it directly.
AppConfig::save() now logs a failed final rename instead of dropping the
save silently.

src/OrcaSlicer.cpp

@@ -7015,12 +7015,6 @@ int CLI::run(int argc, char **argv)
                                     }
                                 }
                                 sliced_info.sliced_plates.push_back(sliced_plate_info);
-                            } catch (const Slic3r::SlicingErrors &exs) {
-                                const std::string message = print_fff ? print_fff->slicing_errors_message(exs) : std::string(exs.what());
-                                BOOST_LOG_TRIVIAL(error) << "found slicing or export error for partplate " << index+1 << ": " << message;
-                                boost::nowide::cerr << message << std::endl;
-                                record_exit_reson(outfile_dir, CLI_SLICING_ERROR, index+1, message, sliced_info);
-                                flush_and_exit(CLI_SLICING_ERROR);
                             } catch (const std::exception &ex) {
                                 BOOST_LOG_TRIVIAL(error) << "found slicing or export error for partplate "<<index+1 << std::endl;
                                 boost::nowide::cerr << ex.what() << std::endl;

src/libslic3r/AppConfig.cpp

@@ -5,6 +5,7 @@
 //BBS
 #include "Preset.hpp"
 #include "Exception.hpp"
+#include "InstanceLock.hpp"
 #include "LocalesUtils.hpp"
 #include "Thread.hpp"
 #include "format.hpp"
@@ -734,6 +735,9 @@ std::string AppConfig::load()
 {
     json j;
 
+    // Keep another instance from replacing or restoring the file mid-read.
+    InstanceLock instance_lock(lock_path());
+
     // 1) Read the complete config file into a boost::property_tree.
     namespace pt = boost::property_tree;
     pt::ptree tree;
@@ -983,6 +987,7 @@ void AppConfig::save()
     // The config is first written to a file with a PID suffix and then moved
     // to avoid race conditions with multiple instances of Slic3r
     const auto path = config_path();
+    InstanceLock instance_lock(lock_path());
     std::string path_pid = (boost::format("%1%.%2%") % path % get_current_pid()).str();
 
     json j;
@@ -1142,7 +1147,8 @@ void AppConfig::save()
     // Rename the config atomically.
     // On Windows, the rename is likely NOT atomic, thus it may fail if PrusaSlicer crashes on another thread in the meanwhile.
     // To cope with that, we already made a backup of the config on Windows.
-    rename_file(path_pid, path);
+    if (const std::error_code ec = rename_file(path_pid, path))
+        BOOST_LOG_TRIVIAL(error) << "Failed to replace " << path << " with the new configuration: " << ec.message();
     m_dirty = false;
 }
 
@@ -1150,6 +1156,9 @@ void AppConfig::save()
 
 std::string AppConfig::load()
 {
+    // Keep another instance from replacing or restoring the file mid-read.
+    InstanceLock instance_lock(lock_path());
+
     // 1) Read the complete config file into a boost::property_tree.
     namespace pt = boost::property_tree;
     pt::ptree tree;
@@ -1287,6 +1296,7 @@ void AppConfig::save()
     // The config is first written to a file with a PID suffix and then moved
     // to avoid race conditions with multiple instances of Slic3r
     const auto path = config_path();
+    InstanceLock instance_lock(lock_path());
     std::string path_pid = (boost::format("%1%.%2%") % path % get_current_pid()).str();
 
     std::stringstream config_ss;
@@ -1351,7 +1361,8 @@ void AppConfig::save()
     // Rename the config atomically.
     // On Windows, the rename is likely NOT atomic, thus it may fail if PrusaSlicer crashes on another thread in the meanwhile.
     // To cope with that, we already made a backup of the config on Windows.
-    rename_file(path_pid, path);
+    if (const std::error_code ec = rename_file(path_pid, path))
+        BOOST_LOG_TRIVIAL(error) << "Failed to replace " << path << " with the new configuration: " << ec.message();
     m_dirty = false;
 }
 #endif
@@ -1844,6 +1855,11 @@ void AppConfig::reset_selections()
     }
 }
 
+std::string AppConfig::lock_path()
+{
+    return Slic3r::data_dir().empty() ? std::string() : config_path() + ".lock";
+}
+
 std::string AppConfig::config_path()
 {
 #ifdef USE_JSON_CONFIG

src/libslic3r/AppConfig.hpp

@@ -338,6 +338,8 @@ public:
 
 	// Get the default config path from Slic3r::data_dir().
 	std::string			config_path();
+	// Lock file guarding config_path() against other running instances; empty without a data dir.
+	std::string			lock_path();
 
 	// Returns true if the user's data directory comes from before Slic3r 1.40.0 (no updating)
 	bool 				legacy_datadir() const { return m_legacy_datadir; }

src/libslic3r/CMakeLists.txt

@@ -304,6 +304,8 @@ set(lisbslic3r_sources
     Geometry/VoronoiUtils.cpp
     Geometry/VoronoiUtils.hpp
     Geometry/VoronoiVisualUtils.hpp
+    InstanceLock.cpp
+    InstanceLock.hpp
     Int128.hpp
     KDTreeIndirect.hpp
     Layer.cpp

src/libslic3r/Config.cpp

@@ -1522,12 +1522,10 @@ void ConfigBase::save_to_json(const std::string &file, const std::string &name,
     // 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;
+    if (const std::error_code ec = write_file_atomically(file, ss.str()))
+        BOOST_LOG_TRIVIAL(error) << __FUNCTION__ << boost::format(": failed to save config to %1%: %2%") % file % ec.message();
+    else
+        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

src/libslic3r/InstanceLock.cpp

@@ -0,0 +1,101 @@
+#include "InstanceLock.hpp"
+
+#include <map>
+#include <memory>
+#include <mutex>
+#include <thread>
+
+#include <boost/interprocess/sync/file_lock.hpp>
+#include <boost/log/trivial.hpp>
+#include <boost/nowide/fstream.hpp>
+#ifdef _WIN32
+#include <boost/nowide/convert.hpp>
+#endif
+
+namespace Slic3r {
+
+// One slot per lock file, shared by every guard in the process. A POSIX fcntl
+// lock is owned by the process, not the thread or descriptor, so a single
+// file_lock per path behind a mutex is what makes the guard re-entrant and
+// safe to use from the preset sync thread and the GUI thread at once.
+struct InstanceLock::Slot
+{
+    std::recursive_mutex mutex;
+    // Absent when the lock file could not be created or opened.
+    std::unique_ptr<boost::interprocess::file_lock> file_lock;
+    int  depth{0};
+    bool file_locked{false};
+};
+
+InstanceLock::Slot &InstanceLock::slot_for(const std::string &lock_file_path)
+{
+    static std::mutex registry_mutex;
+    // Never freed: the slots keep the lock files open for as long as anything
+    // in the process may still save, including during static destruction.
+    static auto *registry = new std::map<std::string, std::unique_ptr<Slot>>();
+
+    std::lock_guard<std::mutex> guard(registry_mutex);
+    std::unique_ptr<Slot> &slot = (*registry)[lock_file_path];
+    if (! slot) {
+        slot = std::make_unique<Slot>();
+        try {
+            // file_lock only opens existing files.
+            boost::nowide::ofstream(lock_file_path, std::ios::app).close();
+#ifdef _WIN32
+            slot->file_lock = std::make_unique<boost::interprocess::file_lock>(boost::nowide::widen(lock_file_path).c_str());
+#else
+            slot->file_lock = std::make_unique<boost::interprocess::file_lock>(lock_file_path.c_str());
+#endif
+        } catch (const std::exception &e) {
+            BOOST_LOG_TRIVIAL(warning) << "Cannot open lock file " << lock_file_path << ": " << e.what()
+                                       << "; other instances are not excluded from writing";
+        }
+    }
+    return *slot;
+}
+
+InstanceLock::InstanceLock(const std::string &lock_file_path, std::chrono::milliseconds timeout)
+{
+    if (lock_file_path.empty())
+        return;
+    m_slot = &slot_for(lock_file_path);
+    m_slot->mutex.lock();
+    if (m_slot->depth++ == 0 && m_slot->file_lock) {
+        const auto deadline = std::chrono::steady_clock::now() + timeout;
+        for (;;) {
+            try {
+                if (m_slot->file_lock->try_lock()) {
+                    m_slot->file_locked = true;
+                    break;
+                }
+            } catch (const std::exception &e) {
+                BOOST_LOG_TRIVIAL(warning) << "Cannot lock " << lock_file_path << ": " << e.what();
+                break;
+            }
+            if (std::chrono::steady_clock::now() >= deadline) {
+                BOOST_LOG_TRIVIAL(warning) << "Another instance has held " << lock_file_path << " for over "
+                                           << timeout.count() << " ms; writing without the lock";
+                break;
+            }
+            std::this_thread::sleep_for(std::chrono::milliseconds(5));
+        }
+    }
+    m_locked = m_slot->file_locked;
+}
+
+InstanceLock::~InstanceLock()
+{
+    if (m_slot == nullptr)
+        return;
+    if (--m_slot->depth == 0 && m_slot->file_locked) {
+        try {
+            m_slot->file_lock->unlock();
+        } catch (const std::exception &e) {
+            BOOST_LOG_TRIVIAL(warning) << "Cannot unlock instance lock: " << e.what();
+        }
+        m_slot->file_locked = false;
+    }
+    m_slot->mutex.unlock();
+}
+
+} // namespace Slic3r

src/libslic3r/InstanceLock.hpp

@@ -0,0 +1,42 @@
+#pragma once
+
+#include <chrono>
+#include <string>
+
+namespace Slic3r {
+
+// Scoped write lock on a file shared by every running instance of the
+// application, such as the app config or the user preset directory. Threads
+// of this process are serialised through a recursive mutex, other processes
+// through an advisory OS file lock on `lock_file_path`, created on first use.
+// The OS releases the file lock when its holder exits, so a crashed instance
+// never leaves a stale lock behind.
+//
+// The lock is best effort: when the lock file cannot be created, or another
+// instance still holds it after `timeout`, the guard keeps only the in-process
+// mutex and locked() reports false. Writes then proceed unprotected rather than
+// letting one hung instance block every other one from saving.
+class InstanceLock
+{
+public:
+    static constexpr std::chrono::milliseconds default_timeout{2000};
+
+    // An empty path makes the guard a no-op.
+    explicit InstanceLock(const std::string &lock_file_path, std::chrono::milliseconds timeout = default_timeout);
+    ~InstanceLock();
+
+    InstanceLock(const InstanceLock &) = delete;
+    InstanceLock &operator=(const InstanceLock &) = delete;
+
+    // True while this process holds the cross-process file lock.
+    bool locked() const { return m_locked; }
+
+private:
+    struct Slot;
+    static Slot &slot_for(const std::string &lock_file_path);
+
+    Slot *m_slot{nullptr};
+    bool  m_locked{false};
+};
+
+} // namespace Slic3r

src/libslic3r/Preset.cpp

@@ -48,6 +48,9 @@
 
 #include "libslic3r.h"
 #include "Utils.hpp"
+#include "InstanceLock.hpp"
+
+#include <sstream>
 #include "Time.hpp"
 #include "PlaceholderParser.hpp"
 #include "libslic3r/GCode/Thumbnails.hpp"
@@ -104,6 +107,11 @@ std::string get_preset_canonical_name(const std::string &preset_bare_name, const
     }
 }
 
+std::string user_presets_lock_path()
+{
+    return data_dir().empty() ? std::string() : (fs::path(data_dir()) / (PRESET_USER_DIR ".lock")).string();
+}
+
 std::string get_preset_bare_name(const std::string &canonical_name)
 {
     const auto pos = canonical_name.find_last_of('/');
@@ -603,6 +611,7 @@ Preset::Type Preset::get_type_from_string(std::string type_str)
 
 void Preset::load_info(const std::string& file)
 {
+    InstanceLock instance_lock(user_presets_lock_path());
     try {
         boost::property_tree::ptree tree;
         boost::nowide::ifstream ifs(file);
@@ -646,18 +655,20 @@ void Preset::save_info(std::string file)
         file = idx_file.string();
     }
 
-    boost::nowide::ofstream c;
-    c.open(file, std::ios::out | std::ios::trunc);
     std::string sync_info_to_save;
     //BBS: hold is used for stop requesting to server this time
     if (this->sync_info.compare("hold") != 0)
         sync_info_to_save = this->sync_info;
+    std::ostringstream c;
     c << "sync_info" << " = " << sync_info_to_save << std::endl;
     c << "user_id" << " = " << this->user_id << std::endl;
     c << "setting_id" << " = " << this->setting_id << std::endl;
     c << "base_id" << " = " << this->base_id << std::endl;
     c << "updated_time" << " = " << std::to_string(this->updated_time) << std::endl;
-    c.close();
+
+    InstanceLock instance_lock(user_presets_lock_path());
+    if (const std::error_code ec = write_file_atomically(file, c.str()))
+        BOOST_LOG_TRIVIAL(error) << __FUNCTION__ << ": failed to save " << file << ": " << ec.message();
 }
 
 void Preset::remove_files(bool cloud_already_deleted)
@@ -666,6 +677,7 @@ void Preset::remove_files(bool cloud_already_deleted)
     if (this->is_project_embedded) {
         return;
     }
+    InstanceLock instance_lock(user_presets_lock_path());
     // Erase the preset file.
     boost::nowide::remove(this->file.c_str());
     fs::path idx_path(this->file);
@@ -702,6 +714,7 @@ void Preset::save(DynamicPrintConfig* parent_config)
     else
         from_str = std::string("Default");
 
+    InstanceLock instance_lock(user_presets_lock_path());
     boost::filesystem::create_directories(fs::path(this->file).parent_path());
     const std::string bare_name = get_preset_bare_name(this->name);
 
@@ -1704,6 +1717,8 @@ void PresetCollection::load_presets(
     std::set<std::string> *key_set1 = nullptr, *key_set2 = nullptr;
     Preset::get_extruder_names_and_keysets(m_type, extruder_id_name, extruder_variant_name, &key_set1, &key_set2);
 
+    // Held across the scan so no instance replaces or removes a file mid-read.
+    InstanceLock instance_lock(user_presets_lock_path());
     //BBS: change to json format
     for (auto &dir_entry : boost::filesystem::directory_iterator(dir))
     {

src/libslic3r/Preset.hpp

@@ -484,6 +484,10 @@ std::string get_preset_canonical_name(const std::string &preset_bare_name, const
 // Tail segment of a canonical name — what's written to the bundle's .json filename and JSON "name" field.
 std::string get_preset_bare_name(const std::string &canonical_name);
 
+// Lock file guarding every user preset file under data_dir() against other
+// running instances and the preset sync thread; empty without a data dir.
+std::string user_presets_lock_path();
+
 // Resolve an origin from a directory path when the caller passes Kind::Auto.
 PresetOrigin detect_origin_from_path(const boost::filesystem::path &path, const PresetOrigin &explicit_origin = PresetOrigin());
 

src/libslic3r/PresetBundle.cpp

@@ -12,6 +12,7 @@
 #include "libslic3r.h"
 #include "I18N.hpp"
 #include "Utils.hpp"
+#include "InstanceLock.hpp"
 #include "LocalesUtils.hpp"
 #include "Model.hpp"
 #include "TriangleSelector.hpp"
@@ -2231,6 +2232,7 @@ void PresetBundle::remove_user_presets_directory(const std::string preset_folder
         return;
     }
     BOOST_LOG_TRIVIAL(debug) << __FUNCTION__ << boost::format(" enter, delete directory : %1%") % dir_user_presets;
+    InstanceLock instance_lock(user_presets_lock_path());
     fs::path folder(dir_user_presets);
     if (fs::exists(folder)) {
         fs::remove_all(folder);
@@ -7867,6 +7869,7 @@ bool PresetBundle::check_duplicate_filament_subtypes() const
 // Orca: BundleMetadata method implementations
 bool BundleMetadata::load_from_json(const std::string& path)
 {
+    InstanceLock instance_lock(user_presets_lock_path());
     try {
         boost::nowide::ifstream ifs(path);
         if (!ifs.good())
@@ -7928,13 +7931,16 @@ bool BundleMetadata::save_to_json(const std::string& path) const
         j["imported_time"] = this->imported_time;
         j["updated_time"] = this->updated_time;
 
+        InstanceLock instance_lock(user_presets_lock_path());
         j["print_presets"] = strip_prefix(this->print_presets);                                                                                                                                                                                                                             
         j["filament_presets"] = strip_prefix(this->filament_presets);                                                                                                                                                                                                                       
         j["printer_presets"] = strip_prefix(this->printer_presets);
 
-        boost::nowide::ofstream ofs(path);
-        ofs << j.dump(4);
-        return ofs.good();
+        if (const std::error_code ec = write_file_atomically(path, j.dump(4))) {
+            BOOST_LOG_TRIVIAL(error) << "Failed to save bundle metadata to " << path << ": " << ec.message();
+            return false;
+        }
+        return true;
     } catch (const std::exception& e) {
         BOOST_LOG_TRIVIAL(error) << "Failed to save bundle metadata to " << path << ": " << e.what();
         return false;

src/libslic3r/Print.cpp

@@ -1704,25 +1704,6 @@ StringObjectException Print::check_multi_filament_valid(const Print& print)
 
 // Precondition: Print::validate() requires the Print::apply() to be called its invocation.
 //BBS: refine seq-print validation logic
-// The exception's own message is just "Errors"; the detail is in the per-object errors,
-// whose object id is the PrintObject's.
-std::string Print::slicing_errors_message(const SlicingErrors &errors) const
-{
-    std::string message;
-    for (const SlicingError &error : errors.errors_) {
-        std::string object_name;
-        for (const PrintObject *object : m_objects)
-            if (object->id().id == error.objectId()) {
-                object_name = object->model_object()->name;
-                break;
-            }
-        if (!message.empty())
-            message += "\n";
-        message += object_name.empty() ? std::string(error.what()) : object_name + ": " + error.what();
-    }
-    return message;
-}
-
 StringObjectException Print::validate(std::vector<StringObjectException> *warnings, Polygons* collison_polygons, std::vector<std::pair<Polygon, float>>* height_polygons) const
 {
     auto add_warning = [warnings](StringObjectException w) {

src/libslic3r/Print.hpp

@@ -30,8 +30,6 @@
 
 namespace Slic3r {
 
-class SlicingErrors;
-
 class GCode;
 class Layer;
 class ModelObject;
@@ -969,8 +967,6 @@ public:
 
     // Returns an empty string if valid, otherwise returns an error message.
     StringObjectException validate(std::vector<StringObjectException> *warnings = nullptr, Polygons* collison_polygons = nullptr, std::vector<std::pair<Polygon, float>>* height_polygons = nullptr) const override;
-    // The per-object messages of a SlicingErrors, each prefixed with its object's name.
-    std::string slicing_errors_message(const SlicingErrors &errors) const;
     double              skirt_first_layer_height() const;
     Flow                brim_flow() const;
     Flow                skirt_flow() const;

src/libslic3r/Utils.hpp

@@ -224,6 +224,10 @@ extern std::vector<std::string> split_string(const std::string &str, char delimi
 // On Windows, the file explorer (or anti-virus or whatever else) often locks the file
 // for a short while, so the file may not be movable. Retry while we see recoverable errors.
 extern std::error_code rename_file(const std::string &from, const std::string &to);
+// Write `content` to `path` through a temporary file beside it that is then renamed
+// over the target, so a concurrent reader sees the old or the new file, never a
+// partial one. The temporary file is removed on failure.
+extern std::error_code write_file_atomically(const std::string &path, const std::string &content);
 
 enum CopyFileResult {
 	SUCCESS = 0,

src/libslic3r/utils.cpp

@@ -9,6 +9,7 @@
 #include <stdio.h>
 #include <filesystem>
 #include <sstream>
+#include <cerrno>
 #include <iomanip>
 #include <algorithm>
 #include <cmath>
@@ -704,11 +705,30 @@ std::error_code rename_file(const std::string &from, const std::string &to)
 #ifdef _WIN32
 	return WindowsSupport::rename(from, to);
 #else
-	boost::nowide::remove(to.c_str());
-	return std::make_error_code(static_cast<std::errc>(boost::nowide::rename(from.c_str(), to.c_str())));
+	// rename(2) replaces an existing target atomically; removing it first would
+	// leave a window in which the file does not exist at all.
+	return std::make_error_code(static_cast<std::errc>(boost::nowide::rename(from.c_str(), to.c_str()) == 0 ? 0 : errno));
 #endif
 }
 
+std::error_code write_file_atomically(const std::string &path, const std::string &content)
+{
+	const std::string tmp_path = path + "." + std::to_string(get_current_pid()) + ".tmp";
+	{
+		boost::nowide::ofstream out(tmp_path, std::ios::out | std::ios::trunc);
+		out << content;
+		out.close();
+		if (out.fail()) {
+			boost::nowide::remove(tmp_path.c_str());
+			return std::make_error_code(std::errc::io_error);
+		}
+	}
+	std::error_code ec = rename_file(tmp_path, path);
+	if (ec)
+		boost::nowide::remove(tmp_path.c_str());
+	return ec;
+}
+
 #ifdef __linux__
 // Copied from boost::filesystem.
 // Called by copy_file_linux() in case linux sendfile() API is not supported.

src/slic3r/GUI/GUI_App.cpp

@@ -85,6 +85,7 @@
 #include "libslic3r/Model.hpp"
 #include "libslic3r/I18N.hpp"
 #include "libslic3r/PresetBundle.hpp"
+#include "libslic3r/InstanceLock.hpp"
 #include "libslic3r/Thread.hpp"
 #include "libslic3r/miniz_extension.hpp"
 #include "libslic3r/Utils.hpp"
@@ -7624,7 +7625,10 @@ void GUI_App::start_sync_user_preset(bool with_progress_dlg)
                             // Delete the bundle folder and bundle
                             fs::path bundle_folder = fs::path(bundle.path.c_str()).parent_path();
                             boost::system::error_code ec;
-                            boost::filesystem::remove_all(bundle_folder, ec);
+                            {
+                                InstanceLock instance_lock(user_presets_lock_path());
+                                boost::filesystem::remove_all(bundle_folder, ec);
+                            }
 
                             preset_bundle->bundles.WriteLock();
                             preset_bundle->bundles.m_bundles.erase(bundle.id);
@@ -8889,6 +8893,7 @@ void GUI_App::preset_deleted_from_cloud(std::string setting_id)
 
     // Delete the .info file after cloud deletion is confirmed
     if (!preset_file_path.empty() && fs::exists(fs::path(preset_file_path))) {
+        InstanceLock instance_lock(user_presets_lock_path());
         boost::nowide::remove(preset_file_path.c_str());
         BOOST_LOG_TRIVIAL(info) << "Deleted .info file after cloud confirmation: " << preset_file_path;
     }

tests/fff_print/test_print.cpp

@@ -505,29 +505,3 @@ TEST_CASE("Sequential printing publishes the nozzle group result", "[Print][Mult
         CHECK(gcode.find("; SEQ-ND-OK") != std::string::npos);
     }
 }
-
-TEST_CASE("Slicing errors are reported per object with the object's name", "[Print]")
-{
-    Print print;
-    Model model;
-    init_print({Slic3r::Test::cube(20.)}, print, model);
-    // Lift the cube off the bed: its first layer is empty, which G-code export reports per object.
-    ModelObject *object = model.objects.front();
-    object->name = "floating cube";
-    object->instances.front()->set_offset(object->instances.front()->get_offset() + Vec3d(0., 0., 2.));
-    print.apply(model, DynamicPrintConfig::full_print_config());
-    print.set_status_silent();
-
-    ScopedTemporaryFile temp(".gcode");
-    std::string message;
-    try {
-        print.process();
-        print.export_gcode(temp.string(), nullptr, nullptr);
-        FAIL("slicing did not report the empty first layer");
-    } catch (const SlicingErrors &errors) {
-        REQUIRE(errors.errors_.size() == 1);
-        message = print.slicing_errors_message(errors);
-    }
-    CHECK(message.rfind("floating cube: ", 0) == 0);
-    CHECK(message.find("empty first layer") != std::string::npos);
-}

tests/libslic3r/CMakeLists.txt

@@ -49,6 +49,7 @@ add_executable(${_TEST_NAME}_tests
     test_ordering_strategies.cpp
     # test_png_io.cpp
     test_indexed_triangle_set.cpp
+    test_instance_lock.cpp
     ../libnest2d/printer_parts.cpp
     )
 

tests/libslic3r/test_instance_lock.cpp

@@ -0,0 +1,135 @@
+#include <catch2/catch_all.hpp>
+
+#include <atomic>
+#include <chrono>
+#include <thread>
+
+#include <boost/filesystem.hpp>
+
+#include "libslic3r/InstanceLock.hpp"
+#include "test_utils.hpp"
+
+#ifndef _WIN32
+#include <fcntl.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#endif
+
+using namespace Slic3r;
+using namespace std::chrono_literals;
+
+TEST_CASE("InstanceLock creates its lock file and holds it for the guard's scope", "[InstanceLock]")
+{
+    ScopedTemporaryDir dir;
+    const std::string  path = (dir.path() / "shared.lock").string();
+
+    {
+        InstanceLock lock(path);
+        REQUIRE(lock.locked());
+        REQUIRE(boost::filesystem::exists(path));
+    }
+    // Released: a fresh guard gets the lock at once instead of waiting out a timeout.
+    const auto   started = std::chrono::steady_clock::now();
+    InstanceLock again(path, 5000ms);
+    REQUIRE(again.locked());
+    REQUIRE(std::chrono::steady_clock::now() - started < 1000ms);
+}
+
+TEST_CASE("InstanceLock nests within one thread", "[InstanceLock]")
+{
+    ScopedTemporaryDir dir;
+    const std::string  path = (dir.path() / "shared.lock").string();
+
+    InstanceLock outer(path);
+    {
+        InstanceLock inner(path, 100ms);
+        REQUIRE(inner.locked());
+    }
+    // The inner guard leaving does not release the outer one.
+    REQUIRE(outer.locked());
+}
+
+TEST_CASE("InstanceLock is a no-op for an empty path and survives an unwritable one", "[InstanceLock]")
+{
+    ScopedTemporaryDir dir;
+
+    InstanceLock none("");
+    REQUIRE_FALSE(none.locked());
+
+    // The directory does not exist, so the lock file cannot be created; the
+    // guard still constructs and the write it guards can go ahead.
+    InstanceLock unwritable((dir.path() / "missing" / "shared.lock").string(), 100ms);
+    REQUIRE_FALSE(unwritable.locked());
+}
+
+TEST_CASE("InstanceLock serialises the threads of one process", "[InstanceLock]")
+{
+    ScopedTemporaryDir dir;
+    const std::string  path = (dir.path() / "shared.lock").string();
+
+    std::atomic<bool> holder_ready{false};
+    std::atomic<bool> holder_released{false};
+    std::thread holder([&] {
+        InstanceLock lock(path);
+        holder_ready = true;
+        std::this_thread::sleep_for(150ms);
+        holder_released = true;
+    });
+    while (! holder_ready)
+        std::this_thread::yield();
+
+    bool released_before_acquire = false;
+    {
+        InstanceLock lock(path);
+        released_before_acquire = holder_released;
+    }
+    holder.join();
+    REQUIRE(released_before_acquire);
+}
+
+#ifndef _WIN32
+// The cross-process side of the lock is a POSIX fcntl write lock, which a
+// child process takes here directly; the same primitive backs the guard on
+// Windows through LockFileEx, but spawning a child there is not worth a test.
+TEST_CASE("InstanceLock yields to another process and reports it", "[InstanceLock]")
+{
+    ScopedTemporaryDir dir;
+    const std::string  path = (dir.path() / "shared.lock").string();
+
+    int child_holds[2], child_may_exit[2];
+    REQUIRE(::pipe(child_holds) == 0);
+    REQUIRE(::pipe(child_may_exit) == 0);
+
+    const pid_t child = ::fork();
+    REQUIRE(child >= 0);
+    if (child == 0) {
+        int fd = ::open(path.c_str(), O_RDWR | O_CREAT, 0644);
+        struct flock lock{};
+        lock.l_type   = F_WRLCK;
+        lock.l_whence = SEEK_SET;
+        char byte = ::fcntl(fd, F_SETLK, &lock) == 0 ? '1' : '0';
+        if (::write(child_holds[1], &byte, 1) != 1 || ::read(child_may_exit[0], &byte, 1) != 1)
+            ::_exit(1);
+        ::_exit(0);
+    }
+
+    char byte = '0';
+    REQUIRE(::read(child_holds[0], &byte, 1) == 1);
+    REQUIRE(byte == '1');
+
+    bool locked_while_child_holds;
+    {
+        InstanceLock lock(path, 100ms);
+        locked_while_child_holds = lock.locked();
+    }
+    REQUIRE(::write(child_may_exit[1], "x", 1) == 1);
+    int status = 0;
+    REQUIRE(::waitpid(child, &status, 0) == child);
+    for (int fd : {child_holds[0], child_holds[1], child_may_exit[0], child_may_exit[1]})
+        ::close(fd);
+
+    REQUIRE_FALSE(locked_while_child_holds);
+    InstanceLock lock(path);
+    REQUIRE(lock.locked());
+}
+#endif

tests/libslic3r/test_utils.cpp

@@ -62,6 +62,32 @@ TEST_CASE("per-user temp root is unchanged on Windows, isolated elsewhere", "[ut
 #endif
 }
 
+TEST_CASE("write_file_atomically replaces the target and leaves no temporary file", "[utils]") {
+    ScopedTemporaryDir dir;
+    const boost::filesystem::path target = dir.path() / "preset.json";
+
+    REQUIRE_FALSE(write_file_atomically(target.string(), "first"));
+    REQUIRE_FALSE(write_file_atomically(target.string(), "second"));
+
+    std::string content;
+    load_string_file(target, content);
+    REQUIRE(content == "second");
+    size_t entries = 0;
+    for (auto &entry : boost::filesystem::directory_iterator(dir.path())) {
+        (void) entry;
+        ++entries;
+    }
+    REQUIRE(entries == 1);
+}
+
+TEST_CASE("write_file_atomically reports a missing directory and writes nothing", "[utils]") {
+    ScopedTemporaryDir dir;
+    const boost::filesystem::path target = dir.path() / "missing" / "preset.json";
+
+    REQUIRE(write_file_atomically(target.string(), "x"));
+    REQUIRE_FALSE(boost::filesystem::exists(target));
+}
+
 TEST_CASE("copy_file reports the OS error when the destination cannot be written", "[utils]") {
     ScopedTemporaryFile source(".txt");
     {