Demote invalid-pch on PlatformIO too, pin the compile locale, document the .sum invariant

esphome/build_gen/arduino8266.py

@@ -1271,6 +1271,9 @@ def write_project(paths: InstalledPaths, ccache: str | None) -> bool:
             )
             write_file_if_changed(pch_header, pch_text)
             if checksum is not None:
+                # Valid only for a ninja run started by write_project: a
+                # direct ninja invocation can rebuild the .gch via its
+                # depfile while this generate-time .sum lags behind
                 write_file_if_changed(
                     build_dir / f"{PCH_HEADER_NAME}.gch.sum", checksum + "\n"
                 )

esphome/platformio/pch.py.script

@@ -95,6 +95,8 @@ def _compile_gch(cxx, flags, header: Path, gch: Path, proj_dir: Path):
     result = subprocess.run(  # noqa: PLW1510
         [cxx, "-x", "c++-header", *flags, "-c", str(header), "-o", str(gch)],
         cwd=proj_dir,
+        # C locale keeps diagnostics matchable by _TRANSIENT_ERRORS
+        env={**os.environ, "LC_ALL": "C"},
         capture_output=True,
         text=True,
     )
@@ -318,7 +320,11 @@ def _setup_pch() -> None:
     # entries: GCC only uses a .gch while no other tokens have been seen.
     # The relative name also reaches "pio run -t idedata" output; external
     # consumers replaying cxx_flags must run from the project dir.
-    projenv.Prepend(CXXFLAGS=["-Winvalid-pch", "-include", header.name])  # noqa: F821
+    projenv.Prepend(  # noqa: F821
+        # -Wno-error: the probe is per-process, so a later cc1plus can still
+        # reject the .gch; that must stay a warning under user -Werror
+        CXXFLAGS=["-Winvalid-pch", "-Wno-error=invalid-pch", "-include", header.name]
+    )
     print("ESPHome: Compiling with precompiled header")
 
 

tests/unit_tests/test_platformio_pch_script.py

@@ -156,7 +156,12 @@ def test_pch_script_builds_and_prepends_relative_include(tmp_path: Path) -> None
     assert (proj / "esphome_pch.h.gch").is_file()
     assert len((proj / "esphome_pch.h.gch.sum").read_text().strip()) == 64
     # Relative include: an absolute path would poison ccache keys
-    assert scons_env.prepended == ["-Winvalid-pch", "-include", "esphome_pch.h"]
+    assert scons_env.prepended == [
+        "-Winvalid-pch",
+        "-Wno-error=invalid-pch",
+        "-include",
+        "esphome_pch.h",
+    ]
     # In production projenv["ENV"] aliases os.environ; only the -include
     # flags are genuinely scoped to projenv (src compiles)
     assert scons_env["ENV"]["CCACHE_SLOPPINESS"] == "pch_defines,time_macros"