Fix Text effect font-cache race to allow deterministic renders (#6959)

Author: derwin12

README.txt

@@ -12,6 +12,7 @@ XLIGHTS/NUTCRACKER RELEASE NOTES
 ---------------------------------
 2026.16  August ??, 2026
 
+    -bug (derwin12)              Fix Text effect rendering nondeterministically (and occasionally crashing)
     -bug (dkulp)                 FPP Connect: uploading UDP outputs no longer resets FPP10's E1.31 Pacing
                                  and Sending mode - every setting on the universes output that xLights
                                  does not own is now carried forward unchanged

src-core/render/FontManager.cpp

@@ -63,12 +63,20 @@ FontManager::FontManager() {
 std::vector<std::unique_ptr<xlFont>> FontManager::fonts;
 bool FontManager::initialized = false;
 std::vector<std::string> FontManager::names;
+std::once_flag FontManager::initOnceFlag;
+std::once_flag FontManager::namesOnceFlag;
 
 FontManager::~FontManager() {
 }
 
 void FontManager::init() {
-    if (!initialized) {
+    // TextEffect::RenderXLText calls this from frame-parallel render worker
+    // threads, so the one-time population of the static fonts/names vectors
+    // below must be serialized rather than gated by a plain bool check -
+    // concurrent first calls used to race on the same push_back'd vectors,
+    // corrupting them (xLightsSequencer/xLights headless-render nondeterminism
+    // investigation, ACCESS_VIOLATION inside FontManager::init under load).
+    std::call_once(initOnceFlag, []() {
         get_font_names(); // ensure names are populated
 
         fonts.push_back(std::make_unique<xlFont>(font_5_5x5_thin_system_png, sizeof(font_5_5x5_thin_system_png)));
@@ -97,11 +105,11 @@ void FontManager::init() {
         }
 
         initialized = true;
-    }
+    });
 }
 
 const std::vector<std::string>& FontManager::get_font_names() {
-    if (names.empty()) {
+    std::call_once(namesOnceFlag, []() {
         names.push_back("5-5x5 Thin");
         names.push_back("5-5x5 Mono");
         names.push_back("6-5x6 Thin");
@@ -119,7 +127,7 @@ const std::vector<std::string>& FontManager::get_font_names() {
         names.push_back("10-12x12 Thin Vertical");
         names.push_back("12-15x15 Bold");
         names.push_back("12-15x15 Bold Vertical");
-    }
+    });
 
     return names;
 }

src-core/render/FontManager.h

@@ -11,6 +11,7 @@
  **************************************************************/
 
 #include <memory>
+#include <mutex>
 #include <string>
 #include <vector>
 
@@ -67,4 +68,6 @@ class FontManager {
     static std::vector<std::unique_ptr<xlFont>> fonts;
     static bool initialized;
     static std::vector<std::string> names;
+    static std::once_flag initOnceFlag;
+    static std::once_flag namesOnceFlag;
 };