[i2c] Let host unit tests with I2C build on macOS (#20738)

esphome/components/i2c/__init__.py

@@ -144,10 +144,15 @@ def validate_config(config: ConfigType) -> ConfigType:
     return config
 
 
+def _host_is_linux() -> bool:
+    """The host I2C bus needs Linux i2c-dev."""
+    return sys.platform.lower().startswith("linux")
+
+
 def validate_host_config(config: ConfigType) -> ConfigType:
     if CORE.is_host:
         # Host I2C is currently only supported on Linux
-        if not sys.platform.lower().startswith("linux"):
+        if not _host_is_linux():
             raise cv.Invalid(
                 "I2C is only supported on Linux for the host platform. "
                 f"Current platform: {sys.platform}"
@@ -446,7 +451,7 @@ def final_validate_device_schema(
     )
 
 
-FILTER_SOURCE_FILES = filter_source_files_from_platform(
+_platform_filter = filter_source_files_from_platform(
     {
         "i2c_bus_arduino.cpp": {
             PlatformFramework.ESP8266_ARDUINO,
@@ -463,3 +468,15 @@ FILTER_SOURCE_FILES = filter_source_files_from_platform(
         "i2c_bus_host.cpp": {PlatformFramework.HOST_NATIVE},
     }
 )
+
+
+def FILTER_SOURCE_FILES() -> list[str]:
+    excluded = _platform_filter()
+    # An empty list means the target isn't known yet. Otherwise the host bus is
+    # only kept for the host platform, and it needs Linux i2c-dev
+    # (validate_host_config rejects host I2C elsewhere). Leaving it out lets host
+    # builds without a bus, like the C++ unit tests with their fake bus, build on
+    # macOS too.
+    if excluded and "i2c_bus_host.cpp" not in excluded and not _host_is_linux():
+        excluded.append("i2c_bus_host.cpp")
+    return excluded

tests/unit_tests/components/test_i2c.py

@@ -0,0 +1,51 @@
+"""I2C backend source selection."""
+
+from unittest.mock import patch
+
+import pytest
+
+from esphome.components import i2c
+from esphome.const import KEY_CORE, KEY_TARGET_FRAMEWORK, KEY_TARGET_PLATFORM
+
+
+@pytest.mark.parametrize(
+    "build_os, host_bus_built",
+    [
+        ("linux", True),
+        ("darwin", False),
+    ],
+)
+def test_host_bus_is_only_built_on_linux(build_os: str, host_bus_built: bool) -> None:
+    core_data = {KEY_CORE: {KEY_TARGET_PLATFORM: "host", KEY_TARGET_FRAMEWORK: "host"}}
+    with (
+        patch("esphome.config_helpers.CORE.data", core_data),
+        patch.object(i2c.sys, "platform", build_os),
+    ):
+        excluded = i2c.FILTER_SOURCE_FILES()
+    assert ("i2c_bus_host.cpp" not in excluded) is host_bus_built
+    assert {"i2c_bus_arduino.cpp", "i2c_bus_esp_idf.cpp", "i2c_bus_zephyr.cpp"} <= set(
+        excluded
+    )
+
+
+@pytest.mark.parametrize("build_os", ["linux", "darwin"])
+def test_nothing_excluded_before_the_target_is_known(build_os: str) -> None:
+    with (
+        patch("esphome.config_helpers.CORE.data", {}),
+        patch.object(i2c.sys, "platform", build_os),
+    ):
+        assert i2c.FILTER_SOURCE_FILES() == []
+
+
+@pytest.mark.parametrize("build_os", ["linux", "darwin"])
+def test_host_bus_left_out_for_other_targets(build_os: str) -> None:
+    core_data = {
+        KEY_CORE: {KEY_TARGET_PLATFORM: "esp32", KEY_TARGET_FRAMEWORK: "esp-idf"}
+    }
+    with (
+        patch("esphome.config_helpers.CORE.data", core_data),
+        patch.object(i2c.sys, "platform", build_os),
+    ):
+        excluded = i2c.FILTER_SOURCE_FILES()
+    assert excluded.count("i2c_bus_host.cpp") == 1
+    assert "i2c_bus_esp_idf.cpp" not in excluded