[esp8266] Strip lwIP glue dhcp stub message strings from DRAM (#17395)

esphome/components/esp8266/__init__.py

@@ -332,6 +332,12 @@ async def to_code(config):
         for symbol in ("vprintf", "printf", "fprintf"):
             cg.add_build_flag(f"-Wl,--wrap={symbol}")
 
+    # Wrap the lwIP2 glue's do-nothing dhcp_cleanup()/dhcp_release() stubs so the
+    # linker can drop their "STUB: ..." message strings from DRAM.
+    # See lwip_glue_stubs.cpp for implementation.
+    for symbol in ("dhcp_cleanup", "dhcp_release"):
+        cg.add_build_flag(f"-Wl,--wrap={symbol}")
+
     # Wrap Arduino's millis() so all callers (including Arduino libraries and ISR
     # handlers) use our fast accumulator instead of the expensive 4x 64-bit multiply
     # implementation in the Arduino ESP8266 core.

esphome/components/esp8266/lwip_glue_stubs.cpp

@@ -0,0 +1,35 @@
+/*
+ * Linker wrap stubs for the lwIP2 glue's dead DHCP entry points.
+ *
+ * The ESP8266 SDK blobs call dhcp_cleanup() and dhcp_release() when the
+ * station leaves an access point (cnx_sta_leave, wifi_station_dhcpc_stop).
+ * In the prebuilt lwIP2 glue (liblwip2-*.a, glue-esp/lwip-esp.c) these are
+ * stubs whose only effect is printing "STUB: dhcp_cleanup" and
+ * "STUB: dhcp_release"; the real DHCP teardown happens through lwIP2's
+ * renamed dhcp_cleanup_LWIP2()/dhcp_release_LWIP2() functions.
+ *
+ * On ESP8266 .rodata lives in DRAM, so those message strings waste scarce
+ * RAM. Wrapping the stubs with silent equivalents lets the linker garbage
+ * collect the glue stub bodies together with their strings.
+ *
+ * Saves 38 bytes of RAM and removes the "STUB:" log noise on Wi-Fi
+ * disconnect. Behavior is otherwise unchanged.
+ */
+
+#if defined(USE_ESP8266)
+
+namespace esphome::esp8266 {}
+
+// NOLINTBEGIN(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
+extern "C" {
+
+// The callers are closed-source SDK blobs; the netif argument is unused.
+void __wrap_dhcp_cleanup(void * /*netif*/) {}
+
+// The glue stub returns ERR_ABRT (-8; lwIP 1.4 err_t is a signed char).
+signed char __wrap_dhcp_release(void * /*netif*/) { return -8; }
+
+}  // extern "C"
+// NOLINTEND(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
+
+#endif  // USE_ESP8266