From 55b2e58a98697f9d65fa1dd41cd4ae8184213f43 Mon Sep 17 00:00:00 2001
From: Ignacio Etcheverry <ignalfonsore@gmail.com>
Date: Thu, 23 Jan 2020 21:23:32 +0100
Subject: [PATCH] Mono/C#: Lighten up unsafe reference checks

Because of the weird case with multi-threading and ResourceLoader, it can be the case that a resource is GCed while being referenced again in the main thread. In such cases, a new unsafe reference is created before the finalizer thread removes the previous one.
---
 modules/mono/csharp_script.cpp | 27 ++++++++++++++++++++++-----
 modules/mono/csharp_script.h   |  5 +++--
 2 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/modules/mono/csharp_script.cpp b/modules/mono/csharp_script.cpp
index 210267e6818..3d868b08393 100644
--- a/modules/mono/csharp_script.cpp
+++ b/modules/mono/csharp_script.cpp
@@ -160,7 +160,7 @@ void CSharpLanguage::finish() {
 	script_bindings.clear();
 
 #ifdef DEBUG_ENABLED
-	for (List<ObjectID>::Element *E = unsafely_referenced_objects.front(); E; E = E->next()) {
+	for (Map<ObjectID, int>::Element *E = unsafe_object_references.front(); E; E = E->next()) {
 		const ObjectID &id = E->get();
 		Object *obj = ObjectDB::get_instance(id);
 
@@ -632,18 +632,20 @@ Vector<ScriptLanguage::StackInfo> CSharpLanguage::stack_trace_get_info(MonoObjec
 
 void CSharpLanguage::post_unsafe_reference(Object *p_obj) {
 #ifdef DEBUG_ENABLED
+	SCOPED_MUTEX_LOCK(unsafe_object_references_lock);
 	ObjectID id = p_obj->get_instance_id();
-	ERR_FAIL_COND_MSG(unsafely_referenced_objects.find(id), "Multiple unsafe references for object: " + p_obj->get_class() + ":" + itos(id));
-	unsafely_referenced_objects.push_back(id);
+	unsafe_object_references[id]++;
 #endif
 }
 
 void CSharpLanguage::pre_unsafe_unreference(Object *p_obj) {
 #ifdef DEBUG_ENABLED
+	SCOPED_MUTEX_LOCK(unsafe_object_references_lock);
 	ObjectID id = p_obj->get_instance_id();
-	List<ObjectID>::Element *elem = unsafely_referenced_objects.find(id);
+	Map<ObjectID, int>::Element *elem = unsafe_object_references.find(id);
 	ERR_FAIL_NULL(elem);
-	unsafely_referenced_objects.erase(elem);
+	if (--elem->value() == 0)
+		unsafe_object_references.erase(elem);
 #endif
 }
 
@@ -1246,6 +1248,14 @@ CSharpLanguage::CSharpLanguage() {
 	language_bind_mutex = Mutex::create();
 #endif
 
+#ifdef DEBUG_ENABLED
+#ifdef NO_THREADS
+	unsafe_object_references_lock = NULL;
+#else
+	unsafe_object_references_lock = Mutex::create();
+#endif
+#endif
+
 	lang_idx = -1;
 
 	scripts_metadata_invalidated = true;
@@ -1274,6 +1284,13 @@ CSharpLanguage::~CSharpLanguage() {
 		script_gchandle_release_mutex = NULL;
 	}
 
+#ifdef DEBUG_ENABLED
+	if (unsafe_object_references_lock) {
+		memdelete(unsafe_object_references_lock);
+		unsafe_object_references_lock = NULL;
+	}
+#endif
+
 	singleton = NULL;
 }
 
diff --git a/modules/mono/csharp_script.h b/modules/mono/csharp_script.h
index 30f56e00bd9..f244bc41193 100644
--- a/modules/mono/csharp_script.h
+++ b/modules/mono/csharp_script.h
@@ -308,8 +308,9 @@ class CSharpLanguage : public ScriptLanguage {
 	Map<Object *, CSharpScriptBinding> script_bindings;
 
 #ifdef DEBUG_ENABLED
-	// List of unsafely referenced objects
-	List<ObjectID> unsafely_referenced_objects;
+	// List of unsafe object references
+	Map<ObjectID, int> unsafe_object_references;
+	Mutex *unsafe_object_references_lock;
 #endif
 
 	struct StringNameCache {