another attempt to avoid LED glitches during file writing

* mark synchronization variables volatile (thread-safe)
* additional logic in closeFile() to avoid file write while leds are pushed out by NPB driver
* some comments and analysis
* replace "yield()" after file ops with "delay(0)" before operations.

wled00/file.cpp

@@ -8,6 +8,7 @@
 #if WLED_FS != LITTLEFS && ESP_IDF_VERSION_MAJOR < 4
   #include "esp_spiffs.h"
 #endif
+//#define yield() {delay(0);}  // WLEDMM yield() is completely unnecessary on esp32, but delay(0) can reduce task contention
 #endif
 
 //WLEDMM seems that 256 is indeed the optimal buffer length
@@ -37,7 +38,20 @@ static File f; // don't export to other cpp files
 void closeFile() {
   #ifdef ARDUINO_ARCH_ESP32
   // WLEDMM: file.close() triggers flash writing. While flash is writing, the NPB RMT driver cannot fill its buffer which may create glitches.
+  // WLEDMM more precisely (thanks to a web research done by AI):
+  //        the RMT peripheral itself doesn’t stall, but the refill path often does. In Arduino-ESP32/WLED 
+  //        typical builds, close() that commits flash writes frequently causes enough blocking that the LED pipeline under-runs, resulting in visible glitches. 
+  //        So the assumption is practically correct for this project context.
+  //    --> with neopixelBus 2.7.5, the practical ISR stall budget is about 0.08–0.12 ms — far less than LittleFS flash commit times.
+  //        typical flash write "commit" times are between 0.5ms and 10ms, but they can be a few 100ms in worst case
+  //    --> file reads rarely cause refill stalls compared to writes, but large/fragmented reads can still exceed the ~0.08–0.12 ms budget.
+  //        esp32 recommendations: use f.setBufferSize() (512–1024 for reads is reasonable); use delay(0) after file reads, to reduce task contention
+
+  if (!f) {doCloseFile = false; return;} // WLEDMM only do all this hick-hack when f is an open file
+
   unsigned long t_wait = millis();
+  bool oldLock = suspendStripService;
+  if (strip.isUpdating()) suspendStripService = true;             // WLEDMM schedule short pause to prevent LEDs glitching during flash write
   while(strip.isUpdating() && (millis() - t_wait < 72)) delay(1); // WLEDMM try to catch a moment when strip is idle
   while(strip.isUpdating() && (millis() - t_wait < 96)) delay(0); //        try harder
   //if (strip.isUpdating()) USER_PRINTLN("closeFile: strip still updating.");
@@ -47,7 +61,12 @@ void closeFile() {
     DEBUGFS_PRINT(F("Close -> "));
     uint32_t s = millis();
   #endif
+  if ((suspendStripService == false) && (oldLock == true)) oldLock = false; // update in case of parallel lock release by another task
   f.close();
+  #ifdef ARDUINO_ARCH_ESP32
+  delay(1); // might help
+  #endif
+  suspendStripService = oldLock; // restore previous lock
   DEBUGFS_PRINTF("took %d ms\n", millis() - s);
   doCloseFile = false;
 }
@@ -61,7 +80,7 @@ static bool bufferedFind(const char *target, bool fromStart = true) {
     uint32_t s = millis();
   #endif
 
-  if (!f || !f.size()) return false;
+  if (!f || !f.size()) return false; // fast return when current file closed, or file size is zero
   size_t targetLen = strlen(target);
 
   size_t index = 0;
@@ -466,11 +485,11 @@ static const uint8_t *getPresetCache(size_t &size) {
 #endif
 
 // WLEDMM
-static bool haveLedmapFile = true;
+static volatile bool haveLedmapFile = true;
-static bool haveIndexFile = true;
+static volatile bool haveIndexFile = true;
-static bool haveSkinFile = true;
+static volatile bool haveSkinFile = true;
-static bool haveICOFile = true;
+static volatile bool haveICOFile = true;
-static bool haveCpalFile = true;
+static volatile bool haveCpalFile = true;
 void invalidateFileNameCache() { // reset "file not found" cache
   haveLedmapFile = true;
   haveIndexFile = true;

wled00/wled.cpp

@@ -99,7 +99,11 @@ void WLED::reset()
   #endif
   long dly = millis();
   while (millis() - dly < 450) {
+#ifdef ARDUINO_ARCH_ESP32
+    delay(2);       // yield => BS
+#else
     yield();        // enough time to send response to client
+#endif
   }
   applyBri();
   USER_PRINTLN(F("\nWLED RESTART\n"));
@@ -191,6 +195,9 @@ void WLED::loop()
     reset();
 
   if (doCloseFile) {
+#ifdef ARDUINO_ARCH_ESP32
+    delay(0);       // yield => BS
+#endif
     closeFile();
     yield();
   }
@@ -203,7 +210,11 @@ void WLED::loop()
     #endif
     handleNightlight();
     handlePlaylist();
+#ifdef ARDUINO_ARCH_ESP32
+    delay(0);       // yield => BS
+#else
     yield();
+#endif
 
     #ifndef WLED_DISABLE_HUESYNC
     handleHue();
@@ -211,7 +222,11 @@ void WLED::loop()
     #endif
 
     handlePresets();
+#ifdef ARDUINO_ARCH_ESP32
+    delay(0);       // yield => BS
+#else
     yield();
+#endif
 
 #if defined(_MoonModules_WLED_) && defined(WLEDMM_FASTPATH)
     #ifdef WLED_DEBUG

wled00/wled.h

@@ -714,9 +714,9 @@ WLED_GLOBAL uint16_t olen _INIT(0);
 // General filesystem
 WLED_GLOBAL size_t fsBytesUsed _INIT(0);
 WLED_GLOBAL size_t fsBytesTotal _INIT(0);
-WLED_GLOBAL unsigned long presetsModifiedTime _INIT(0L);
+WLED_GLOBAL volatile unsigned long presetsModifiedTime _INIT(0L);
 WLED_GLOBAL JsonDocument* fileDoc;
-WLED_GLOBAL bool doCloseFile _INIT(false);
+WLED_GLOBAL volatile bool doCloseFile _INIT(false);
 
 // presets
 WLED_GLOBAL byte currentPreset _INIT(0);