Fail fast on empty compile databases and unsupported analyze-memory toolchains; degrade a missing size tool

ninja -t compdb exits 0 with an empty list when the rule names drift, so
the compile database is now parsed and an empty result fails the build
and drops the stale file. analyze-memory refuses an unsupported
toolchain before paying for the compile. A missing size binary warns
instead of discarding an already-linked build, _parse_app_size returns
explicitly from its not-found branch, and the CI cache action asserts
the caller's install prefix matches the cached path. The esp8266
run_compile dispatch hook and copy_files early-return gain direct
tests.

tests/unit_tests/components/esp8266/test_toolchain_validation.py

@@ -24,7 +24,7 @@ from esphome.const import (
     CONF_VERSION,
     Toolchain,
 )
-from esphome.core import CORE
+from esphome.core import CORE, EsphomeError
 from esphome.types import ConfigType
 
 
@@ -163,3 +163,29 @@ def test_resolve_toolchain_rejects_unsupported() -> None:
     CORE.toolchain = Toolchain.SDK_NRF
     with pytest.raises(cv.Invalid, match="Unsupported toolchain 'sdk-nrf'"):
         _resolve_toolchain({})
+
+
+def test_run_compile_platformio_falls_through() -> None:
+    """Under toolchain: platformio the hook returns False without touching
+    the native backend; this is what keeps existing users on PlatformIO."""
+    CORE.toolchain = Toolchain.PLATFORMIO
+    with patch("esphome.arduino8266.toolchain.run_compile") as mock_native:
+        assert esp8266.run_compile(SimpleNamespace(), {}) is False
+    mock_native.assert_not_called()
+
+
+def test_run_compile_arduino_failure_raises() -> None:
+    """A non-zero native build fails by name instead of returning success."""
+    CORE.verbose = False
+    with (
+        patch("esphome.arduino8266.toolchain.run_compile", return_value=1),
+        pytest.raises(EsphomeError, match="native build failed"),
+    ):
+        esp8266.run_compile(SimpleNamespace(), {})
+
+
+def test_copy_files_native_skips_platformio_scripts(tmp_path: Path) -> None:
+    """The native build writes no PlatformIO extra scripts."""
+    CORE.build_path = tmp_path
+    esp8266.copy_files()
+    assert list(tmp_path.iterdir()) == []

tests/unit_tests/test_arduino8266_toolchain.py

@@ -120,13 +120,33 @@ def test_run_compile_warns_when_idedata_fails(
 def test_write_compile_commands(tmp_path: Path) -> None:
     build_dir = tmp_path / "build"
     build_dir.mkdir()
+    entries = '[{"file": "a.cpp", "command": "cc"}]\n'
     with patch.object(
         toolchain.subprocess,
         "run",
-        return_value=MagicMock(returncode=0, stdout="[]\n"),
+        return_value=MagicMock(returncode=0, stdout=entries),
     ):
         toolchain._write_compile_commands(tmp_path / "ninja", build_dir, {})
-    assert (build_dir / "compile_commands.json").read_text() == "[]\n"
+    assert (build_dir / "compile_commands.json").read_text() == entries
+
+
+@pytest.mark.parametrize("stdout", ["[]\n", "not json"])
+def test_write_compile_commands_empty_db_raises(tmp_path: Path, stdout: str) -> None:
+    """An empty compile database (compdb exits 0 with [] for unknown rule
+    names) must fail the build and drop any stale database."""
+    build_dir = tmp_path / "build"
+    build_dir.mkdir()
+    (build_dir / "compile_commands.json").write_text("[stale]")
+    with (
+        patch.object(
+            toolchain.subprocess,
+            "run",
+            return_value=MagicMock(returncode=0, stdout=stdout),
+        ),
+        pytest.raises(EsphomeError, match="empty compile database"),
+    ):
+        toolchain._write_compile_commands(tmp_path / "ninja", build_dir, {})
+    assert not (build_dir / "compile_commands.json").exists()
 
 
 def test_write_compile_commands_failure_removes_stale_db(tmp_path: Path) -> None:
@@ -183,6 +203,18 @@ def test_print_size_summary(tmp_path: Path, capsys: pytest.CaptureFixture[str])
     assert "Flash: [====      ]  35.9% (used 375301 bytes from 1044464 bytes)" in out
 
 
+def test_print_size_summary_missing_size_tool_warns(
+    tmp_path: Path, caplog: pytest.LogCaptureFixture
+) -> None:
+    """A missing size binary degrades to a warning; the firmware already
+    linked and must not be discarded."""
+    with patch.object(
+        toolchain.subprocess, "run", side_effect=FileNotFoundError("no size")
+    ):
+        toolchain._print_size_summary(tmp_path, _paths(tmp_path))
+    assert "Could not summarize firmware size" in caplog.text
+
+
 def test_print_size_summary_no_app_size(
     tmp_path: Path, capsys: pytest.CaptureFixture[str]
 ) -> None: