[socket] Add an IPv4 allow list (#20025)

Co-authored-by: J. Nick Koston <nick@home-assistant.io>

tests/unit_tests/components/socket/test_socket_ipv4_allow.py

@@ -0,0 +1,46 @@
+"""Tests for the socket component's IPv4 allow list codegen helper."""
+
+from ipaddress import IPv4Address, IPv4Network
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from esphome.components import socket
+import esphome.config_validation as cv
+
+
+def test_network_order_swaps_to_sockaddr_value() -> None:
+    """The emitted uint32 must equal s_addr on the little endian targets."""
+    assert socket._network_order(IPv4Address("192.168.175.20")) == 0x14AFA8C0
+    assert socket._network_order(IPv4Address("255.255.255.0")) == 0x00FFFFFF
+    assert socket._network_order(IPv4Address("0.0.0.0")) == 0
+
+
+def test_add_ipv4_allow_emits_nothing_for_an_empty_list() -> None:
+    setter = MagicMock()
+    with patch.object(socket.cg, "add") as add:
+        socket.add_ipv4_allow(setter, [], "bridge")
+    add.assert_not_called()
+    setter.assert_not_called()
+
+
+def test_add_ipv4_allow_wires_the_setter_with_cleared_host_bits() -> None:
+    setter = MagicMock()
+    networks = [IPv4Network("192.168.175.33/24", strict=False)]
+    with (
+        patch.object(socket.cg, "add") as add,
+        patch.object(socket.cg, "progmem_array") as array,
+    ):
+        socket.add_ipv4_allow(setter, networks, "bridge")
+    rendered = str(array.call_args.args[1])
+    assert str(socket._network_order(IPv4Address("192.168.175.0"))) in rendered
+    assert str(socket._network_order(IPv4Address("255.255.255.0"))) in rendered
+    setter.assert_called_once_with(array.return_value, 1)
+    add.assert_called_once()
+
+
+def test_schema_caps_the_list_length() -> None:
+    """The sanity cap rejects a list past 255 entries."""
+    assert len(socket.IPV4_ALLOW_SCHEMA([f"10.0.{i}.0/24" for i in range(255)])) == 255
+    with pytest.raises(cv.Invalid):
+        socket.IPV4_ALLOW_SCHEMA([f"10.0.{i}.0/24" for i in range(256)])