[ethernet][network][wifi] Arbitrate the default route from the network priority list (#17797)

tests/component_tests/network/config/priority_arduino.yaml

@@ -0,0 +1,26 @@
+esphome:
+  name: test
+
+esp32:
+  board: esp32dev
+  framework:
+    type: arduino
+
+wifi:
+  ssid: "test_ssid"
+  password: "test_password"
+
+ethernet:
+  type: W5500
+  clk_pin: 19
+  mosi_pin: 21
+  miso_pin: 23
+  cs_pin: 18
+  interrupt_pin: 36
+  reset_pin: 22
+  clock_speed: 10Mhz
+
+network:
+  priority:
+    - ethernet
+    - wifi

tests/component_tests/network/config/priority_rp2040.yaml

@@ -0,0 +1,24 @@
+esphome:
+  name: test
+
+rp2:
+  board: rpipicow
+
+wifi:
+  ssid: "test_ssid"
+  password: "test_password"
+
+ethernet:
+  type: W5500
+  clk_pin: 18
+  mosi_pin: 19
+  miso_pin: 16
+  cs_pin: 17
+  interrupt_pin: 21
+  reset_pin: 20
+  mac_address: "02:AA:BB:CC:DD:01"
+
+network:
+  priority:
+    - ethernet
+    - wifi

tests/component_tests/network/config/priority_single.yaml

@@ -0,0 +1,15 @@
+esphome:
+  name: test
+
+esp32:
+  board: esp32dev
+  framework:
+    type: esp-idf
+
+wifi:
+  ssid: "test_ssid"
+  password: "test_password"
+
+network:
+  priority:
+    - wifi

tests/component_tests/network/test_priority.py

@@ -16,9 +16,10 @@ from esphome.components.network import (
     _validate_priority_list,
     get_network_priority,
 )
-from esphome.const import CONF_PRIORITY
+from esphome.const import CONF_PRIORITY, PlatformFramework
 from esphome.core import CORE
 import esphome.final_validate as fv
+from tests.component_tests.types import SetCoreConfigCallable
 
 
 @pytest.fixture(autouse=True)
@@ -138,6 +139,22 @@ def test_final_validate_noop_without_priority_list() -> None:
     _final_validate({})  # must not raise
 
 
+def test_final_validate_rejects_unsupported_arbitration_interface(
+    set_core_config: SetCoreConfigCallable,
+) -> None:
+    """The ethernet/wifi-only arbitration tripwire fails as a clean config error.
+
+    Unreachable through the public schema today (VALID_NETWORK_TYPES gates the
+    list), so the config is hand-built to simulate a future interface type that
+    was added to the schema without extending NetworkComponent::loop().
+    """
+    set_core_config(PlatformFramework.ESP32_IDF)
+    fv.full_config.set({"openthread": {}, "wifi": {}})
+    config = {CONF_PRIORITY: [{"interface": "openthread"}, {"interface": "wifi"}]}
+    with pytest.raises(Invalid, match="arbitration does not support: openthread"):
+        _final_validate(config)
+
+
 def _cpp_setup_priority(name: str) -> float:
     """Read a setup_priority constant straight from esphome/core/component.h."""
     header = Path(__file__).parents[3] / "esphome" / "core" / "component.h"
@@ -199,3 +216,55 @@ def test_no_primary_interface_define_without_priority(
     assert not any(
         d.name.startswith("USE_NETWORK_PRIMARY_INTERFACE_") for d in CORE.defines
     )
+
+
+def _dns_per_default_netif_option() -> bool | None:
+    from esphome.components.esp32.const import KEY_ESP32, KEY_SDKCONFIG_OPTIONS
+
+    if KEY_ESP32 not in CORE.data:  # non-ESP32 configs have no sdkconfig at all
+        return None
+    return CORE.data[KEY_ESP32][KEY_SDKCONFIG_OPTIONS].get(
+        "CONFIG_ESP_NETIF_SET_DNS_PER_DEFAULT_NETIF"
+    )
+
+
+@pytest.mark.parametrize(
+    "config_file",
+    [
+        "priority_wifi_first.yaml",
+        "priority_ethernet_first.yaml",
+        "priority_arduino.yaml",
+    ],
+)
+def test_multi_interface_priority_enables_default_route_arbitration(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+    config_file: str,
+) -> None:
+    """More than one interface in 'priority' enables default-route arbitration."""
+    generate_main(component_config_path(config_file))
+    assert "USE_NETWORK_DEFAULT_ROUTE" in {d.name for d in CORE.defines}
+    assert _dns_per_default_netif_option() is True
+
+
+@pytest.mark.parametrize(
+    "config_file",
+    [
+        # Single-entry priority list / no list at all.
+        "priority_single.yaml",
+        "wifi_only.yaml",
+        # Dual-interface on rp2040: validates, but the arbitration is ESP32-only
+        # (NetworkComponent::loop() is compiled under USE_ESP32) — emitting the
+        # define here would be a hard build break.
+        "priority_rp2040.yaml",
+    ],
+)
+def test_single_interface_has_no_default_route_arbitration(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+    config_file: str,
+) -> None:
+    """Single-interface and non-ESP32 configs must not compile in the arbitration."""
+    generate_main(component_config_path(config_file))
+    assert "USE_NETWORK_DEFAULT_ROUTE" not in {d.name for d in CORE.defines}
+    assert _dns_per_default_netif_option() is None