Neutralize lwIP SNTP client to prevent heap drain on WiFi reconnects (#24593)

The ESP8266 SDK automatically invokes sntp_init() via
netif_sta_status_callback on every WiFi reconnect. This results in repeated
allocations of UDP PCBs (udp_new()), leading to heap exhaustion and
potential lwIP timeout list corruption due to unmatched sntp_stop() calls.

Tasmota does not use the lwIP SNTP client and relies solely on WifiGetNtp()
for time synchronization. To avoid unnecessary resource usage, wrap and
neutralize sntp_init() and sntp_stop() with no-op implementations.

This prevents UDP PCB allocations and avoids issues caused by repeated
initialization during reconnect cycles.

Additionally, a PlatformIO post-build script has been added to validate
that the linker wraps are correctly applied. The script runs after ELF
generation and uses xtensa-lx106-elf-nm to inspect symbols, ensuring that
the __wrap_sntp_init and __wrap_sntp_stop functions are present in the
final firmware. If the wrappers are missing, the build fails.

Confirmed via firmware disassembly:
- Single call site in netif_sta_status_callback
- Requires linker wraps: --wrap=sntp_init,--wrap=sntp_stop

Fixes: #24566

CHANGELOG.md

@@ -11,6 +11,7 @@ All notable changes to this project will be documented in this file.
 
 ### Changed
 - ESP8266 platform update from 2026.02.00 to 2026.03.00 (#24547)
+- ESP8266 use wrapped symbols for sntp_init and sntp_stop (#24566)
 - ESP32 Platform from 2025.02.30 to 2026.03.30, Framework (Arduino Core) from v3.1.9 to v3.1.10 and IDF from v5.3.4.251226 to v5.3.4.260127 (#24547)
 - Matter don't advertize IPv6 global address, only link-local (#24563)
 - ESP32-C5/C6/P4 Platform from 2025.03.30 to 2026.03.50, Framework (Arduino Core) from v3.1.10 to v3.3.7 and IDF from v5.3.4.260127 to v5.5.3+ (#24567)
@@ -18,7 +19,7 @@ All notable changes to this project will be documented in this file.
 
 ### Fixed
 - Athom esp32 2-3-4 gang change led behaviour after firmware update (#24509)
-- ESP8266 heap drain and exception 29 when DHCP provides NTP server (#24515)
+- ESP8266 heap drain and exception 29 when DHCP provides NTP server (#24515,#24566)
 - NeoPool possible IntegerDivideByZero (#24578)
 
 ### Removed

pio-tools/check-wrapped-lwip-sntp-calls.py

@@ -0,0 +1,94 @@
+Import("env")
+
+import subprocess
+import sys
+import os
+
+
+def parse_nm_symbols(nm_output):
+    symbols = set()
+    for line in nm_output.splitlines():
+        parts = line.strip().split()
+        if len(parts) >= 3:
+            symbols.add(parts[-1])
+    return symbols
+
+
+def check_sntp_wrap(target, source, env):
+    print("SNTP WRAP SYMBOL CHECK: Checking firmware")
+
+    board = env.BoardConfig()
+    mcu = board.get("build.mcu", "esp32").lower()
+
+    if mcu != "esp8266":
+        print("SNTP WRAP SYMBOL CHECK: Skipped for MCU =", mcu)
+        return
+
+    firmware = str(target[0])
+
+    if not os.path.exists(firmware):
+        print("ERROR: Firmware ELF not found:", firmware)
+        sys.exit(1)
+    else:
+        print("Firmware ELF found:", firmware)
+
+    nm = "xtensa-lx106-elf-nm"
+
+    result = subprocess.run(
+        [nm, "-n", firmware],
+        capture_output=True,
+        text=True
+    )
+
+    if result.returncode != 0:
+        print("ERROR: nm failed to execute")
+        print(result.stderr)
+        sys.exit(1)
+
+    symbols = parse_nm_symbols(result.stdout)
+
+    required = [
+        "__wrap_sntp_init",
+        "__wrap_sntp_stop",
+    ]
+
+    forbidden = [
+        "sntp_init",
+        "sntp_stop",
+    ]
+
+    found_required = [s for s in required if s in symbols]
+    missing_required = [s for s in required if s not in symbols]
+    found_forbidden = [s for s in forbidden if s in symbols]
+
+    print("Required wrap symbols:")
+    for s in required:
+        status = "OK" if s in symbols else "MISSING"
+        print(" ", status + ":", s)
+
+    print("Forbidden original symbols:")
+    if found_forbidden:
+        for s in found_forbidden:
+            print("  FORBIDDEN:", s)
+    else:
+        print("  None")
+
+    if missing_required or found_forbidden:
+        print("FAILED: lwIP SNTP wrap symbol check failed.")
+
+        if missing_required:
+            print("Missing required wrap symbols:")
+            for s in missing_required:
+                print("  MISSING:", s)
+
+        if found_forbidden:
+            print("Forbidden original symbols detected:")
+            for s in found_forbidden:
+                print("  FORBIDDEN:", s)
+
+        sys.exit(1)
+
+    print("PASSED: All lwIP SNTP wrap symbols are present and original symbols are not found.")
+
+
+env.AddPostAction("$BUILD_DIR/${PROGNAME}.elf", check_sntp_wrap)

platformio.ini

@@ -84,6 +84,7 @@ extra_scripts               = post:pio-tools/name-firmware.py
                               post:pio-tools/metrics-firmware.py
                               pre:pio-tools/custom_target.py
 ;                              post:pio-tools/obj-dump.py
+                              post:pio-tools/check-wrapped-lwip-sntp-calls.py
                               ${scripts_defaults.extra_scripts}
 ; *** remove undesired all warnings
 build_unflags               = ${tasmota.build_unflags}
@@ -123,6 +124,9 @@ build_flags                 = ${esp_defaults.build_flags}
                               -DMIMETYPE_MINIMAL
                               ; uncomment the following to enable TLS with 4096 RSA certificates
                               ;-DUSE_4K_RSA
+                              ; Prevent lwIP SNTP client from heap drain in case of repeated calls to sntp_init() and sntp_stop() from SDK/lwIP
+                              -Wl,--wrap=sntp_init
+                              -Wl,--wrap=sntp_stop
 lib_ignore                  = ESP8266Audio
                               ESP8266SAM
                               ESP8266LLMNR

tasmota/tasmota_support/support_esp8266.ino

@@ -290,4 +290,26 @@ uint32_t HwRandom(void) {
   #undef _RAND_ADDR
 }
 
+/*********************************************************************************************\
+ * Neutralize lwIP SNTP client to prevent heap drain on WiFi reconnects
+ *
+ * The ESP8266 SDK autonomously calls sntp_init() via netif_sta_status_callback
+ * on every WiFi reconnect, allocating a UDP PCB via udp_new() each time.
+ * Tasmota manages NTP entirely via WifiGetNtp() and has no use for the lwIP
+ * SNTP client. These wrappers replace both functions with NOPs, preventing
+ * UDP PCB heap allocation and lwIP timeout list corruption from unpaired
+ * sntp_stop() calls.
+ *
+ * Confirmed by disassembly of firmware.elf - single call site at netif_sta_status_callback
+ * Source: liblwip2-1460.a(sntp.o) - requires -Wl,--wrap=sntp_init,--wrap=sntp_stop
+\*********************************************************************************************/
+
+extern "C" void __wrap_sntp_init(void) {
+  // Prevent lwIP SNTP client from starting on WiFi reconnects and causing heap drain and timeout list corruption
+}
+
+extern "C" void __wrap_sntp_stop(void) {
+  // Prevent lwIP SNTP client from stopping on WiFi disconnects and causing timeout list corruption
+}
+
 #endif  // ESP8266