Fix race condition in FontFile data access during threaded imports

Add mutex protection to FontFile data access methods to prevent
race conditions when multiple import threads access the same
cached FontFile simultaneously.

Use data_external flag to track whether data came from set_data_ptr()
vs set_data() to avoid unnecessary PackedByteArray COW copies.

Author: Danielle Jenkins

scene/resources/font.cpp

@@ -591,6 +591,8 @@ _FORCE_INLINE_ void FontFile::_ensure_rid(int p_cache_index, int p_make_linked_f
 		if (p_make_linked_from >= 0 && p_make_linked_from != p_cache_index && p_make_linked_from < cache.size()) {
 			cache.write[p_cache_index] = TS->create_font_linked_variation(cache[p_make_linked_from]);
 		} else {
+			MutexLock lock(data_mutex);
+
 			cache.write[p_cache_index] = TS->create_font();
 			TS->font_set_data_ptr(cache[p_cache_index], data_ptr, data_size);
 			TS->font_set_antialiasing(cache[p_cache_index], antialiasing);
@@ -1408,9 +1410,13 @@ void FontFile::_get_property_list(List<PropertyInfo> *p_list) const {
 
 void FontFile::reset_state() {
 	_clear_cache();
-	data.clear();
-	data_ptr = nullptr;
-	data_size = 0;
+	{
+		MutexLock lock(data_mutex);
+		data = PackedByteArray();
+		data_ptr = nullptr;
+		data_size = 0;
+		data_external = false;
+	}
 	cache.clear();
 
 	antialiasing = TextServer::FONT_ANTIALIASING_GRAY;
@@ -2070,9 +2076,12 @@ Error FontFile::load_dynamic_font(const String &p_path) {
 }
 
 void FontFile::set_data_ptr(const uint8_t *p_data, size_t p_size) {
-	data.clear();
+	MutexLock lock(data_mutex);
+
+	data = PackedByteArray();
 	data_ptr = p_data;
 	data_size = p_size;
+	data_external = true;
 
 	for (int i = 0; i < cache.size(); i++) {
 		if (cache[i].is_valid()) {
@@ -2082,9 +2091,12 @@ void FontFile::set_data_ptr(const uint8_t *p_data, size_t p_size) {
 }
 
 void FontFile::set_data(const PackedByteArray &p_data) {
+	MutexLock lock(data_mutex);
+
 	data = p_data;
 	data_ptr = data.ptr();
 	data_size = data.size();
+	data_external = false;
 
 	for (int i = 0; i < cache.size(); i++) {
 		if (cache[i].is_valid()) {
@@ -2094,7 +2106,9 @@ void FontFile::set_data(const PackedByteArray &p_data) {
 }
 
 PackedByteArray FontFile::get_data() const {
-	if (unlikely((size_t)data.size() != data_size)) {
+	MutexLock lock(data_mutex);
+
+	if (unlikely(data_external && data.is_empty() && data_ptr && data_size > 0)) {
 		data.resize(data_size);
 		memcpy(data.ptrw(), data_ptr, data_size);
 	}

scene/resources/font.h

@@ -31,6 +31,7 @@
 #pragma once
 
 #include "core/io/resource.h"
+#include "core/os/mutex.h"
 #include "core/templates/lru.h"
 #include "scene/resources/texture.h"
 #include "servers/text_server.h"
@@ -185,8 +186,10 @@ class FontFile : public Font {
 	RES_BASE_EXTENSION("fontdata");
 
 	// Font source data.
+	mutable Mutex data_mutex;
 	const uint8_t *data_ptr = nullptr;
 	size_t data_size = 0;
+	bool data_external = false;
 	mutable PackedByteArray data;
 
 	TextServer::FontAntialiasing antialiasing = TextServer::FONT_ANTIALIASING_GRAY;