Pin that the native upload path imports no platform component package

tests/unit_tests/fixtures/lazy_imports/native_upload_fast_path.py

@@ -0,0 +1,65 @@
+"""Run the native-toolchain serial-upload path and report heavy modules.
+
+Executed as a subprocess by test_lazy_imports.py: heavy module names come
+in on argv, the ones found in sys.modules afterwards go out on stdout.
+``upload_using_esptool`` dispatches native toolchains through a
+toolchain-keyed table; if someone routes it back through the platform
+component packages (esp32 or esp8266), this reports the leak.
+"""
+
+import os
+from pathlib import Path
+import sys
+import tempfile
+from unittest.mock import patch
+
+from _leak_report import print_leaked_modules
+
+from esphome.__main__ import upload_using_esptool
+from esphome.const import (
+    CONF_ESPHOME,
+    KEY_CORE,
+    KEY_ESP32,
+    KEY_TARGET_PLATFORM,
+    KEY_VARIANT,
+    Toolchain,
+)
+from esphome.core import CORE
+
+# An ambient ESPHOME_USE_SUBPROCESS would route past the patched
+# run_external_command into run_external_process and confuse the checks.
+os.environ.pop("ESPHOME_USE_SUBPROCESS", None)
+
+config = {CONF_ESPHOME: {"platformio_options": {}}}
+
+with tempfile.TemporaryDirectory() as build_dir:
+    CORE.name = "leaktest"
+    CORE.build_path = build_dir
+
+    for platform, toolchain, backend in (
+        ("esp8266", Toolchain.ARDUINO, "esphome.arduino8266.toolchain"),
+        ("esp32", Toolchain.ESP_IDF, "esphome.espidf.toolchain"),
+    ):
+        CORE.data[KEY_CORE] = {KEY_TARGET_PLATFORM: platform}
+        if platform == "esp32":
+            CORE.data[KEY_ESP32] = {KEY_VARIANT: "ESP32S3"}
+        CORE.toolchain = toolchain
+
+        import importlib
+
+        image = importlib.import_module(backend).get_factory_firmware_path()
+        image.parent.mkdir(parents=True, exist_ok=True)
+        image.write_bytes(b"\x00")
+
+        with patch("esphome.__main__.run_external_command", return_value=0) as mock_run:
+            rc = upload_using_esptool(config, "/dev/ttyUSB0", None, None)
+
+        # Fail loudly if the upload path stopped doing its work; otherwise
+        # an empty leak list could just mean nothing ran.
+        if rc != 0:
+            sys.exit(f"upload_using_esptool({platform}) returned {rc}")
+        cmd = list(mock_run.call_args[0][1:])
+        if str(image) not in [str(Path(c)) for c in cmd]:
+            sys.exit(f"native factory image did not reach esptool: {cmd}")
+
+print_leaked_modules()

tests/unit_tests/test_lazy_imports.py

@@ -175,6 +175,27 @@ def test_esptool_upload_fast_path_does_not_import_heavy_modules(
     )
 
 
+def test_native_upload_fast_path_does_not_import_heavy_modules(
+    fixture_path: Path,
+    probe_env: dict[str, str],
+) -> None:
+    """The native-toolchain serial upload dispatches through a
+    toolchain-keyed backend table; resolving the flash image must not drag
+    in the esp32 or esp8266 component package or the validation stack.
+    """
+    leaked = _leaked_from_fixture(
+        fixture_path,
+        probe_env,
+        "native_upload_fast_path.py",
+        extra=("esphome.components.esp8266",),
+    )
+    assert not leaked, (
+        f"native upload_using_esptool pulls in heavy modules: {leaked}. "
+        "The upload fast path skips validation; importing a platform "
+        "component package executes its codegen module by design."
+    )
+
+
 def test_api_client_does_not_import_heavy_modules() -> None:
     """``esphome.api_client`` is on the logs fast path and must stay light.