[core] Read the mqtt IP discovery flag without importing the mqtt component (#18066)

esphome/__main__.py

@@ -29,6 +29,7 @@ from esphome.const import (
     CONF_BROKER,
     CONF_DEASSERT_RTS_DTR,
     CONF_DISABLED,
+    CONF_DISCOVER_IP,
     CONF_ESPHOME,
     CONF_LEVEL,
     CONF_LOG_TOPIC,
@@ -486,8 +487,6 @@ def has_web_server_ota() -> bool:
 
 def has_mqtt_ip_lookup() -> bool:
     """Check if MQTT is available and IP lookup is supported."""
-    from esphome.components.mqtt import CONF_DISCOVER_IP
-
     if CONF_MQTT not in CORE.config:
         return False
     # Default Enabled

esphome/components/mqtt/__init__.py

@@ -21,6 +21,7 @@ from esphome.const import (
     CONF_CLIENT_ID,
     CONF_COMMAND_RETAIN,
     CONF_COMMAND_TOPIC,
+    CONF_DISCOVER_IP,
     CONF_DISCOVERY,
     CONF_DISCOVERY_OBJECT_ID_GENERATOR,
     CONF_DISCOVERY_PREFIX,
@@ -74,7 +75,6 @@ def AUTO_LOAD():
     return ["json"]
 
 
-CONF_DISCOVER_IP = "discover_ip"
 CONF_IDF_SEND_ASYNC = "idf_send_async"
 CONF_WAIT_FOR_CONNECTION = "wait_for_connection"
 

esphome/const.py

@@ -347,6 +347,7 @@ CONF_DISABLE_CRC = "disable_crc"
 CONF_DISABLED = "disabled"
 CONF_DISABLED_BY_DEFAULT = "disabled_by_default"
 CONF_DISCONNECT_DELAY = "disconnect_delay"
+CONF_DISCOVER_IP = "discover_ip"
 CONF_DISCOVERY = "discovery"
 CONF_DISCOVERY_OBJECT_ID_GENERATOR = "discovery_object_id_generator"
 CONF_DISCOVERY_PREFIX = "discovery_prefix"

script/ci-custom.py

@@ -557,7 +557,7 @@ def lint_constants_usage():
 # Maximum allowed CONF_ constants in esphome/const.py.
 # This file is frozen — new constants go in esphome/components/const/__init__.py.
 # Decrease this number when constants are moved out of const.py.
-CONST_PY_MAX_CONF = 1016
+CONST_PY_MAX_CONF = 1017
 
 
 @lint_content_check(include=["esphome/const.py"])

tests/unit_tests/test_lazy_imports.py

@@ -158,3 +158,37 @@ def test_espidf_toolchain_does_not_import_heavy_modules() -> None:
         "The upload fast path skips validation; importing the validation "
         "stack anyway defeats the validated-config cache."
     )
+
+
+def test_has_mqtt_ip_lookup_does_not_import_mqtt() -> None:
+    """``has_mqtt_ip_lookup`` runs on the upload/logs fast path for mqtt
+    configs; reading ``CONF_DISCOVER_IP`` must not drag in the mqtt
+    component and, with it, the validation stack.
+
+    Runs in a subprocess because this session's other tests import the
+    mqtt component; the fast path itself must not.
+    """
+    check = (
+        "import sys; from esphome.__main__ import has_mqtt_ip_lookup; "
+        "from esphome.core import CORE; from esphome.const import CONF_MQTT; "
+        "CORE.config = {CONF_MQTT: {}}; "
+        "assert has_mqtt_ip_lookup() is True, 'mqtt IP lookup default broke'; "
+        f"leaked = [m for m in {HEAVY_MODULES!r} if m in sys.modules]; "
+        "leaked += [m for m in sys.modules if m.startswith('esphome.components.')]; "
+        "print(','.join(leaked))"
+    )
+    # check=False keeps the child's stderr (its assertion message or an
+    # import traceback) visible on failure.
+    result = subprocess.run(
+        [sys.executable, "-c", check],
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    assert result.returncode == 0, result.stderr
+    leaked = result.stdout.strip()
+    assert not leaked, (
+        f"has_mqtt_ip_lookup pulls in heavy modules: {leaked}. "
+        "The upload/logs fast path skips validation; importing the "
+        "validation stack anyway defeats the validated-config cache."
+    )

tests/unit_tests/test_main.py

@@ -3257,6 +3257,14 @@ def test_get_port_type() -> None:
     assert get_port_type("BOOTSEL") == "BOOTSEL"
 
 
+def test_mqtt_reexports_discover_ip() -> None:
+    """The old import path must keep working for external code."""
+    from esphome.components import mqtt
+    from esphome.const import CONF_DISCOVER_IP
+
+    assert mqtt.CONF_DISCOVER_IP is CONF_DISCOVER_IP
+
+
 def test_has_mqtt_ip_lookup() -> None:
     """Test has_mqtt_ip_lookup function."""