Fix potential hang on exit due to deadlocks.

modules/terraman/library/terrain_library_merger_pcm.cpp

@@ -172,6 +172,11 @@ void TerrainLibraryMergerPCM::_material_cache_get_key(Ref<TerrainChunk> chunk) {
 			continue;
 		}
 
+		if (unlikely(_engine_quitting)) {
+			_material_cache_mutex.unlock();
+			return;
+		}
+
 		Ref<TerrainSurfaceMerger> nms = ms->duplicate();
 		nms->set_library(Ref<TerrainLibraryMergerPCM>(this));
 		nms->set_id(s);
@@ -186,6 +191,11 @@ void TerrainLibraryMergerPCM::_material_cache_get_key(Ref<TerrainChunk> chunk) {
 			continue;
 		}
 
+		if (unlikely(_engine_quitting)) {
+			_material_cache_mutex.unlock();
+			return;
+		}
+
 		Ref<Material> nm = m->duplicate();
 
 		cache->material_add(nm);
@@ -359,6 +369,11 @@ void TerrainLibraryMergerPCM::_liquid_material_cache_get_key(Ref<TerrainChunk> c
 			continue;
 		}
 
+		if (unlikely(_engine_quitting)) {
+			_liquid_material_cache_mutex.unlock();
+			return;
+		}
+
 		Ref<TerrainSurfaceMerger> nms = ms->duplicate();
 		nms->set_library(Ref<TerrainLibraryMergerPCM>(this));
 		nms->set_id(s);
@@ -373,6 +388,11 @@ void TerrainLibraryMergerPCM::_liquid_material_cache_get_key(Ref<TerrainChunk> c
 			continue;
 		}
 
+		if (unlikely(_engine_quitting)) {
+			_liquid_material_cache_mutex.unlock();
+			return;
+		}
+
 		Ref<Material> nm = m->duplicate();
 
 		cache->material_add(nm);
@@ -560,6 +580,11 @@ void TerrainLibraryMergerPCM::_prop_material_cache_get_key(Ref<TerrainChunk> chu
 			continue;
 		}
 
+		if (unlikely(_engine_quitting)) {
+			_prop_material_cache_mutex.unlock();
+			return;
+		}
+
 		Ref<Material> nm = m->duplicate();
 
 		cache->material_add(nm);
@@ -972,6 +997,8 @@ void TerrainLibraryMergerPCM::_setup_material_albedo(const int material_index, c
 }
 
 TerrainLibraryMergerPCM::TerrainLibraryMergerPCM() {
+	_engine_quitting = false;
+
 	_packer.instance();
 
 	_packer->set_texture_flags(Texture::FLAG_MIPMAPS | Texture::FLAG_FILTER);
@@ -1046,6 +1073,14 @@ bool TerrainLibraryMergerPCM::process_prop_textures(Ref<PropData> prop) {
 }
 #endif
 
+void TerrainLibraryMergerPCM::_notification(int p_what) {
+	switch (p_what) {
+		case MainLoop::NOTIFICATION_QUITTING: {
+			_engine_quitting = true;
+		} break;
+	}
+}
+
 void TerrainLibraryMergerPCM::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("get_texture_flags"), &TerrainLibraryMergerPCM::get_texture_flags);
 	ClassDB::bind_method(D_METHOD("set_texture_flags", "flags"), &TerrainLibraryMergerPCM::set_texture_flags);

modules/terraman/library/terrain_library_merger_pcm.h

@@ -125,6 +125,7 @@ protected:
 	bool process_prop_textures(Ref<PropData> prop);
 #endif
 
+	virtual void _notification(int p_what);
 	static void _bind_methods();
 
 	RBMap<int, Ref<TerrainMaterialCachePCM>> _material_cache;
@@ -143,6 +144,12 @@ protected:
 	Mutex _material_cache_mutex;
 	Mutex _liquid_material_cache_mutex;
 	Mutex _prop_material_cache_mutex;
+
+	// To fix potential deadlock on quit
+	// (Materials can't be duplicated after the engine started deinitializing itself)
+	// The notification that sets this should be immediate, the engine will start deinitialization later
+	// so materials that already started duplicating should be fine
+	bool _engine_quitting;
 };
 
 #endif

modules/terraman/world/terrain_world.cpp

@@ -34,6 +34,7 @@
 #include "core/containers/hash_set.h"
 
 #include "core/object/message_queue.h"
+#include "core/os/main_loop.h"
 #include "terrain_chunk.h"
 #include "terrain_structure.h"
 
@@ -1429,6 +1430,11 @@ void TerrainWorld::_notification(int p_what) {
 			}
 			break;
 		}
+		case MainLoop::NOTIFICATION_QUITTING: {
+			if (_library.is_valid()) {
+				_library->notification(MainLoop::NOTIFICATION_QUITTING);
+			}
+		} break;
 	}
 }