[build] Skip target-platform deps when populating host unit-test config (#17039)

script/build_helpers.py

@@ -70,12 +70,15 @@ def populate_dependency_config(
 
     * ``domain.platform`` form (e.g. ``sensor.gpio``) appends
       ``{platform: <name>}`` to ``config[domain]``, creating the list if needed.
-    * Bare components are looked up via ``get_component_fn``. Platform
-      components (``IS_PLATFORM_COMPONENT``) and ``MULTI_CONF`` components are
-      initialised as ``[]`` so the sibling ``domain.platform`` branch can
-      ``append`` into them. Everything else is populated by running the
-      component's schema with ``{}`` so defaults exist; if the schema requires
-      explicit input, an empty ``{}`` is used as a fallback.
+    * Bare components are looked up via ``get_component_fn``. Target-platform
+      components (``is_target_platform``, e.g. ``esp32``) are skipped entirely:
+      a host build targets ``host``, so a foreign target platform's sources are
+      guarded out and its schema must not run here (it would mutate global CORE
+      state as a side effect). Platform components (``IS_PLATFORM_COMPONENT``)
+      and ``MULTI_CONF`` components are initialised as ``[]`` so the sibling
+      ``domain.platform`` branch can ``append`` into them. Everything else is
+      populated by running the component's schema with ``{}`` so defaults exist;
+      if the schema requires explicit input, an empty ``{}`` is used as a fallback.
 
     Platform components must always be a list here even when no
     ``domain.platform`` entry follows, because the ``domain.platform`` branch
@@ -96,6 +99,12 @@ def populate_dependency_config(
         component = get_component_fn(component_name)
         if component is None:
             continue
+        # Skip target platforms (e.g. esp32): a host build targets `host`, so a
+        # foreign target's sources are guarded out, and running its schema with
+        # {} leaks global CORE state (esp32 pins CORE.toolchain to ESP-IDF),
+        # crashing the host compile. See #17035.
+        if component.is_target_platform:
+            continue
         if component.multi_conf or component.is_platform_component:
             config.setdefault(component_name, [])
         elif component_name not in config:

tests/script/test_build_helpers.py

@@ -0,0 +1,76 @@
+"""Unit tests for script/build_helpers.py."""
+
+from pathlib import Path
+import sys
+
+import pytest
+
+# Add the script directory to the path so we can import build_helpers.
+sys.path.insert(0, str(Path(__file__).parent.parent.parent / "script"))
+
+import build_helpers  # noqa: E402
+
+from esphome.core import CORE  # noqa: E402
+
+
+class _FakeComponent:
+    def __init__(self, config_schema, *, is_target_platform=False):
+        self.multi_conf = False
+        self.is_platform_component = False
+        self.is_target_platform = is_target_platform
+        self.config_schema = config_schema
+
+
+@pytest.fixture(autouse=True)
+def _restore_core_toolchain():
+    """Keep CORE.toolchain changes from leaking between tests."""
+    saved = CORE.toolchain
+    try:
+        yield
+    finally:
+        CORE.toolchain = saved
+
+
+def test_populate_dependency_config_skips_target_platforms() -> None:
+    """Target-platform deps must be skipped, not config-populated, in a host build.
+
+    Regression test for #17035: esp32 (a target platform) appears only as a
+    transitive dependency of a host C++ unit test. Running its schema with {}
+    set ``CORE.toolchain = ESP_IDF`` as a side effect before failing validation,
+    which crashed the host compile with KeyError('esp32'). The fix skips
+    target-platform components entirely so their schema never runs.
+    """
+    CORE.toolchain = None  # the state a host build starts from
+    schema_calls = []
+
+    def leaky_schema(value):
+        # If this ever runs for a target platform, the bug is back.
+        schema_calls.append(value)
+        CORE.toolchain = "esp-idf-leak"
+        raise ValueError("no board or variant")
+
+    config: dict = {}
+    build_helpers.populate_dependency_config(
+        config,
+        ["esp32"],
+        get_component_fn=lambda name: _FakeComponent(
+            leaky_schema, is_target_platform=True
+        ),
+        register_platform_fn=lambda domain: None,
+    )
+
+    assert "esp32" not in config  # skipped: no synthesized entry
+    assert schema_calls == []  # schema never run
+    assert CORE.toolchain is None  # no global side effect leaked
+
+
+def test_populate_dependency_config_populates_defaults() -> None:
+    """A non-target-platform dep still has its schema defaults harvested."""
+    config: dict = {}
+    build_helpers.populate_dependency_config(
+        config,
+        ["ok"],
+        get_component_fn=lambda name: _FakeComponent(lambda value: {"default": 1}),
+        register_platform_fn=lambda domain: None,
+    )
+    assert config["ok"] == {"default": 1}

tests/script/test_test_helpers.py

@@ -266,11 +266,13 @@ def _make_component_stub(
     *,
     multi_conf: bool = False,
     is_platform_component: bool = False,
+    is_target_platform: bool = False,
     config_schema=None,
 ) -> MagicMock:
     stub = MagicMock()
     stub.multi_conf = multi_conf
     stub.is_platform_component = is_platform_component
+    stub.is_target_platform = is_target_platform
     stub.config_schema = config_schema
     return stub