[wifi] Test that EAP strings stay plain literals off ESP8266

Keep the CompactString constructor as on dev and add an ESP8266-only
CompactString::from_progmem helper instead, which is smaller. Drop the
static_assert and the CompactString rewrite tests; keep the runtime
setter test.

esphome/components/wifi/__init__.py

@@ -784,6 +784,11 @@ CONFIG_SCHEMA = cv.All(
 )
 
 
+def _eap_string(value: str | None) -> cg.Expression | str:
+    """A std::string field whose literal stays in flash on ESP8266."""
+    return cg.progmem_string(value) if value else ""
+
+
 def eap_auth(config):
     if config is None:
         return None
@@ -798,9 +803,9 @@ def eap_auth(config):
         key = wpa2_eap.read_relative_config_path(config[CONF_KEY])
     return cg.StructInitializer(
         EAPAuth,
-        ("identity", config.get(CONF_IDENTITY, "")),
-        ("username", config.get(CONF_USERNAME, "")),
-        ("password", config.get(CONF_PASSWORD, "")),
+        ("identity", _eap_string(config.get(CONF_IDENTITY))),
+        ("username", _eap_string(config.get(CONF_USERNAME))),
+        ("password", _eap_string(config.get(CONF_PASSWORD))),
         ("ca_cert", ca_cert),
         ("client_cert", client_cert),
         ("client_key", key),
@@ -822,8 +827,11 @@ def manual_ip_entry(config: ConfigType) -> bytes:
 
 
 def wifi_network(config, ap, manual_ip_index: int | None):
-    cg.add_optional(ap.set_ssid, config.get(CONF_SSID))
-    cg.add_optional(ap.set_password, config.get(CONF_PASSWORD))
+    # The setters copy the text, so the literal can stay in flash on ESP8266.
+    if (ssid := config.get(CONF_SSID)) is not None:
+        cg.add(ap.set_ssid(cg.FlashStringLiteral(ssid)))
+    if (password := config.get(CONF_PASSWORD)) is not None:
+        cg.add(ap.set_password(cg.FlashStringLiteral(password)))
     if CONF_EAP in config:
         cg.add(ap.set_eap(eap_auth(config[CONF_EAP])))
         cg.add_define("USE_WIFI_WPA2_EAP")

esphome/components/wifi/wifi_component.cpp

@@ -78,6 +78,15 @@ CompactString::CompactString(const char *str, size_t len) {
   }
 }
 
+#ifdef USE_ESP8266
+CompactString CompactString::from_progmem(ProgmemStr str) {
+  char buf[MAX_LENGTH + 1];
+  ESPHOME_strncpy_P(buf, reinterpret_cast<const char *>(str), MAX_LENGTH);
+  buf[MAX_LENGTH] = '\0';
+  return CompactString(buf, strlen(buf));
+}
+#endif
+
 CompactString::CompactString(const CompactString &other) : CompactString(other.data(), other.size()) {}
 
 CompactString &CompactString::operator=(const CompactString &other) {

esphome/components/wifi/wifi_component.h

@@ -6,6 +6,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 #include "esphome/core/helpers.h"
+#include "esphome/core/progmem.h"
 #ifdef USE_ESP32
 #include "esphome/core/lock_free_queue.h"
 #endif
@@ -241,6 +242,10 @@ class CompactString {
 
   CompactString() : length_(0), is_heap_(0) { this->storage_[0] = '\0'; }
   CompactString(const char *str, size_t len);
+#ifdef USE_ESP8266
+  /// Copy a string kept in flash with ESPHOME_F.
+  static CompactString from_progmem(ProgmemStr str);
+#endif
   CompactString(const CompactString &other);
   CompactString(CompactString &&other) noexcept;
   CompactString &operator=(const CompactString &other);
@@ -296,11 +301,17 @@ class WiFiAP {
   void set_ssid(const std::string &ssid) { this->ssid_ = CompactString(ssid.c_str(), ssid.size()); }
   void set_ssid(const char *ssid) { this->set_ssid(StringRef(ssid)); }
   void set_ssid(StringRef ssid) { this->ssid_ = CompactString(ssid.c_str(), ssid.size()); }
+#ifdef USE_ESP8266
+  void set_ssid(ProgmemStr ssid) { this->ssid_ = CompactString::from_progmem(ssid); }
+#endif
   void set_bssid(const bssid_t &bssid) { this->bssid_ = bssid; }
   void clear_bssid() { this->bssid_ = {}; }
   void set_password(const std::string &password) { this->password_ = CompactString(password.c_str(), password.size()); }
   void set_password(const char *password) { this->set_password(StringRef(password)); }
   void set_password(StringRef password) { this->password_ = CompactString(password.c_str(), password.size()); }
+#ifdef USE_ESP8266
+  void set_password(ProgmemStr password) { this->password_ = CompactString::from_progmem(password); }
+#endif
 #ifdef USE_WIFI_WPA2_EAP
   void set_eap(optional<EAPAuth> eap_auth) { this->eap_ = std::move(eap_auth); }
 #endif  // USE_WIFI_WPA2_EAP

tests/component_tests/wifi/config/eap.yaml

@@ -0,0 +1,13 @@
+esphome:
+  name: test
+
+esp8266:
+  board: d1_mini
+
+wifi:
+  networks:
+    - ssid: enterprise
+      eap:
+        identity: eap_identity
+        username: eap_username
+        password: eap_password

tests/component_tests/wifi/config/eap_esp32.yaml

@@ -0,0 +1,15 @@
+esphome:
+  name: test
+
+esp32:
+  board: esp32dev
+  framework:
+    type: esp-idf
+
+wifi:
+  networks:
+    - ssid: enterprise
+      eap:
+        identity: eap_identity
+        username: eap_username
+        password: eap_password

tests/component_tests/wifi/test_credentials_progmem.py

@@ -0,0 +1,45 @@
+"""Tests that wifi codegen keeps the configured credentials in flash on ESP8266."""
+
+from collections.abc import Callable
+from pathlib import Path
+
+import pytest
+
+
+@pytest.mark.parametrize("config_file", ["bare.yaml", "bare_ln882x.yaml"])
+def test_ssid_and_password_use_flash_literals(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+    config_file: str,
+) -> None:
+    """Codegen emits ESPHOME_F for station and AP credentials; the macro decides placement."""
+    main_cpp = generate_main(component_config_path(config_file))
+
+    assert 'set_ssid(ESPHOME_F("test"));' in main_cpp
+    assert 'set_password(ESPHOME_F("testtest"));' in main_cpp
+    assert 'set_ssid(ESPHOME_F("fallback"));' in main_cpp
+    assert 'set_ssid("' not in main_cpp
+    assert 'set_password("' not in main_cpp
+
+
+def test_eap_strings_use_flash_literals(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    """EAP identity, username and password are built from flash on ESP8266."""
+    main_cpp = generate_main(component_config_path("eap.yaml"))
+
+    for value in ("eap_identity", "eap_username", "eap_password"):
+        assert f'progmem_string(ESPHOME_F("{value}"))' in main_cpp
+
+
+def test_eap_strings_plain_off_esp8266(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    """Off ESP8266 the EAP strings stay plain literals, with no copy from flash."""
+    main_cpp = generate_main(component_config_path("eap_esp32.yaml"))
+
+    assert "progmem_string(" not in main_cpp
+    for value in ("eap_identity", "eap_username", "eap_password"):
+        assert f'"{value}"' in main_cpp

tests/components/wifi/common.yaml

@@ -33,6 +33,16 @@ esphome:
           then:
             - logger.log: "WiFi is enabled!"
       - wifi.roam
+      # Runtime credentials keep resolving to the RAM setters, never the flash one
+      - lambda: |-
+          wifi::WiFiAP sta;
+          std::string ssid = "RuntimeSSID";
+          sta.set_ssid(ssid);
+          char password[16] = "runtime_pass";
+          sta.set_password(password);
+          const char *ssid_ptr = ssid.c_str();
+          sta.set_ssid(ssid_ptr);
+          sta.set_password(StringRef(password));
 
 wifi:
   networks:

tests/components/wifi/wifi_ap_setters_test.cpp

@@ -0,0 +1,28 @@
+#include <gtest/gtest.h>
+
+#include <string>
+
+#include "esphome/components/wifi/wifi_component.h"
+
+namespace esphome::wifi::testing {
+
+TEST(WiFiAP, RuntimeSetters) {
+  // Credentials from improv, captive portal, wifi.configure and lambdas arrive in RAM and are copied.
+  WiFiAP ap;
+  std::string ssid = "runtime_ssid";
+  ap.set_ssid(ssid);
+  char password[] = "runtime_pass";
+  ap.set_password(password);
+  ssid[0] = 'X';
+  password[0] = 'X';
+  EXPECT_EQ(ap.get_ssid(), "runtime_ssid");
+  EXPECT_EQ(ap.get_password(), "runtime_pass");
+
+  const char *ssid_ptr = ssid.c_str();
+  ap.set_ssid(ssid_ptr);
+  ap.set_password(StringRef(password, 4));
+  EXPECT_EQ(ap.get_ssid(), "Xuntime_ssid");
+  EXPECT_EQ(ap.get_password(), "Xunt");
+}
+
+}  // namespace esphome::wifi::testing