[logger] Stub out ROM ets_putc on ESP8266 when serial logging is disabled (#17970)

esphome/components/logger/__init__.py

@@ -410,10 +410,16 @@ async def _late_logger_init(config: ConfigType) -> None:
         from esphome.components.esp8266.const import enable_serial, enable_serial1
 
         hw_uart = config.get(CONF_HARDWARE_UART, UART0)
-        if has_serial_logging and hw_uart in (UART0, UART0_SWAP):
+        if not has_serial_logging:
+            # No serial logging: stub out ROM ets_putc so stray output (newlib
+            # stdout, lwIP diagnostics) cannot block on a slow or shared UART0.
+            # ets_putc always writes to the physical UART and cannot be disabled
+            # through uart_set_debug(); see __wrap_ets_putc in logger_esp8266.cpp.
+            cg.add_build_flag("-Wl,--wrap=ets_putc")
+        elif hw_uart in (UART0, UART0_SWAP):
             cg.add_define("USE_ESP8266_LOGGER_SERIAL")
             enable_serial()
-        elif has_serial_logging and hw_uart == UART1:
+        elif hw_uart == UART1:
             cg.add_define("USE_ESP8266_LOGGER_SERIAL1")
             enable_serial1()
 

esphome/components/logger/logger_esp8266.cpp

@@ -49,4 +49,17 @@ const LogString *Logger::get_uart_selection_() {
 }
 
 }  // namespace esphome::logger
+
+#if !defined(USE_ESP8266_LOGGER_SERIAL) && !defined(USE_ESP8266_LOGGER_SERIAL1)
+// With serial logging disabled, ROM ets_putc still writes to the physical UART0
+// at whatever baud rate a uart bus configured there; uart_set_debug(UART_NO)
+// only silences the installable putc1 hook, not ets_putc itself. Blocking
+// writes at a low baud rate (for example 4800 for a power monitoring chip) can
+// starve the soft watchdog. All linked callers (newlib stdout, lwIP
+// diagnostics, postmortem dumps) are redirected here by -Wl,--wrap=ets_putc.
+// IRAM_ATTR because the ROM original is callable with the flash cache
+// disabled (for example from newlib's _write_r, which is placed in IRAM).
+extern "C" void IRAM_ATTR __wrap_ets_putc(char) {}
+#endif
+
 #endif

tests/components/logger/test-uart0_no_logging.esp8266-ard.yaml

@@ -0,0 +1 @@
+<<: !include common-uart0_no_logging.yaml