fix: synchronize lifecycle hook shutdown with active dispatches

src/libslic3r/LifecycleEvents.hpp

@@ -8,6 +8,9 @@
 // the slicing engine itself; see fire_lifecycle_event() below.
 
 #include <functional>
+#include <condition_variable>
+#include <cstddef>
+#include <mutex>
 #include <string>
 #include <utility>
 
@@ -165,17 +168,79 @@ namespace Slic3r
     // require callers to hold a Print& just to report an event.
     using LifecycleHookFn = std::function<void(LifecycleEvent, const LifecycleEventContext&)>;
 
-    inline LifecycleHookFn& lifecycle_hook_fn()
+    namespace detail {
+
+    struct LifecycleHookState
     {
-        static LifecycleHookFn fn;
-        return fn;
+        std::mutex              mutex;
+        std::condition_variable cv;
+        LifecycleHookFn         fn;
+        std::size_t             active_dispatches = 0;
+        bool                    accepting        = false;
+    };
+
+    inline LifecycleHookState& lifecycle_hook_state()
+    {
+        static LifecycleHookState state;
+        return state;
     }
 
-    inline void set_lifecycle_hook_fn(LifecycleHookFn fn) { lifecycle_hook_fn() = std::move(fn); }
+    class LifecycleDispatchGuard
+    {
+    public:
+        explicit LifecycleDispatchGuard(LifecycleHookState& state) : m_state(state) {}
+
+        ~LifecycleDispatchGuard()
+        {
+            std::lock_guard<std::mutex> lock(m_state.mutex);
+            --m_state.active_dispatches;
+            if (m_state.active_dispatches == 0)
+                m_state.cv.notify_all();
+        }
+
+        LifecycleDispatchGuard(const LifecycleDispatchGuard&)            = delete;
+        LifecycleDispatchGuard& operator=(const LifecycleDispatchGuard&) = delete;
+
+    private:
+        LifecycleHookState& m_state;
+    };
+
+    } // namespace detail
+
+    // Installing a hook starts accepting dispatches. Passing an empty function stops accepting
+    // new dispatches, detaches the hook, and waits for callbacks already in progress to finish.
+    // This is used during plugin shutdown so plugin code cannot be unloaded while a lifecycle
+    // callback is still executing. The empty-function path must not be called from inside the
+    // lifecycle callback itself.
+    inline void set_lifecycle_hook_fn(LifecycleHookFn fn)
+    {
+        detail::LifecycleHookState& state = detail::lifecycle_hook_state();
+        if (fn) {
+            std::lock_guard<std::mutex> lock(state.mutex);
+            state.fn        = std::move(fn);
+            state.accepting = true;
+            return;
+        }
+
+        std::unique_lock<std::mutex> lock(state.mutex);
+        state.accepting = false;
+        state.fn        = nullptr;
+        state.cv.wait(lock, [&state] { return state.active_dispatches == 0; });
+    }
 
     inline void fire_lifecycle_event(LifecycleEvent event, const LifecycleEventContext& ctx)
     {
-        if (const LifecycleHookFn& fn = lifecycle_hook_fn(); fn)
-            fn(event, ctx);
+        detail::LifecycleHookState& state = detail::lifecycle_hook_state();
+        LifecycleHookFn             fn;
+        {
+            std::lock_guard<std::mutex> lock(state.mutex);
+            if (!state.accepting || !state.fn)
+                return;
+            fn = state.fn;
+            ++state.active_dispatches;
+        }
+
+        detail::LifecycleDispatchGuard guard(state);
+        fn(event, ctx);
     }
 }

src/slic3r/plugin/PluginHooks.hpp

@@ -14,8 +14,9 @@ namespace Slic3r::plugin_hooks {
 void install();
 
 // Reset every hook to null so none can enter Python after the interpreter
-// finalizes. Called from PluginManager::shutdown(); callers must have stopped
-// background slicing first (resetting a hook while process() runs is a race).
+// finalizes. The lifecycle-event hook drains callbacks already in progress
+// before returning. Other hooks retain their existing caller-side shutdown
+// requirements.
 void uninstall();
 
 } // namespace Slic3r::plugin_hooks

src/slic3r/plugin/PluginManager.cpp

@@ -135,7 +135,8 @@ void PluginManager::shutdown()
     BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ": PluginManager shutdown enter";
 
     // Detach the libslic3r hooks first so nothing dispatches into Python while (or after) plugins
-    // unload. Callers stop background slicing before this.
+    // unload. The lifecycle-event hook also drains callbacks already in progress before returning;
+    // the remaining hook seams retain their existing shutdown requirements.
     plugin_hooks::uninstall();
 
     // Reject new plugin loads before we drain.

tests/slic3rutils/test_plugin_lifecycle.cpp

@@ -1,5 +1,6 @@
 #include <catch2/catch_all.hpp>
 
+#include <libslic3r/LifecycleEvents.hpp>
 #include <libslic3r/Utils.hpp>
 #include <slic3r/plugin/PluginDescriptor.hpp>
 #include <slic3r/plugin/PluginManager.hpp>
@@ -12,8 +13,11 @@
 
 #include <algorithm>
 #include <chrono>
+#include <condition_variable>
 #include <fstream>
+#include <mutex>
 #include <string>
+#include <thread>
 #include <vector>
 
 using namespace Slic3r;
@@ -114,6 +118,72 @@ PluginDescriptor descriptor_of(PluginManager& manager, const std::string& plugin
 
 } // namespace
 
+TEST_CASE("Lifecycle hook shutdown drains concurrent dispatch", "[PluginLifecycle][LifecycleEvents]")
+{
+    std::mutex              mutex;
+    std::condition_variable cv;
+    bool                    callback_entered = false;
+    bool                    release_callback = false;
+    bool                    shutdown_started = false;
+    bool                    shutdown_finished = false;
+
+    set_lifecycle_hook_fn([&](LifecycleEvent, const LifecycleEventContext&) {
+        std::unique_lock<std::mutex> lock(mutex);
+        callback_entered = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return release_callback; });
+    });
+
+    std::thread dispatch_thread([] {
+        LifecycleEventContext ctx;
+        fire_lifecycle_event(LifecycleEvent::ProjectOpened, ctx);
+    });
+
+    bool callback_was_entered = false;
+    {
+        std::unique_lock<std::mutex> lock(mutex);
+        callback_was_entered = cv.wait_for(lock, std::chrono::seconds(5), [&] { return callback_entered; });
+    }
+    if (!callback_was_entered) {
+        {
+            std::lock_guard<std::mutex> lock(mutex);
+            release_callback = true;
+            cv.notify_all();
+        }
+        dispatch_thread.join();
+        set_lifecycle_hook_fn(nullptr);
+        FAIL("Lifecycle callback did not start");
+    }
+
+    std::thread shutdown_thread([&] {
+        {
+            std::lock_guard<std::mutex> lock(mutex);
+            shutdown_started = true;
+            cv.notify_all();
+        }
+        set_lifecycle_hook_fn(nullptr);
+        std::lock_guard<std::mutex> lock(mutex);
+        shutdown_finished = true;
+        cv.notify_all();
+    });
+
+    bool shutdown_was_started = false;
+    {
+        std::unique_lock<std::mutex> lock(mutex);
+        shutdown_was_started = cv.wait_for(lock, std::chrono::seconds(5), [&] { return shutdown_started; });
+        if (shutdown_was_started)
+            CHECK_FALSE(cv.wait_for(lock, std::chrono::milliseconds(100), [&] { return shutdown_finished; }));
+        release_callback = true;
+        cv.notify_all();
+    }
+
+    dispatch_thread.join();
+    shutdown_thread.join();
+    if (!shutdown_was_started)
+        FAIL("Lifecycle hook shutdown did not start");
+    CHECK(shutdown_finished);
+}
+
 TEST_CASE("A discovered script plugin loads and materializes its capability", "[PluginLifecycle][Python]")
 {
     ScopedPluginManager plugin_system;