Probe the built .gch on the native paths so strict CI reds on an unloadable pch

esphome/build_gen/arduino8266.py

@@ -42,6 +42,7 @@ from esphome.build_helpers.pch import (
     pch_degraded,
     pch_enabled,
     pch_header_text,
+    pch_strict,
 )
 from esphome.components.esp8266 import build_surgery
 from esphome.components.esp8266.boards import (
@@ -1290,7 +1291,27 @@ def write_project(paths: InstalledPaths, ccache: str | None) -> bool:
                 f"-Winvalid-pch -Wno-error=invalid-pch -include {PCH_HEADER_NAME}"
             ]
             lines.append(f"srccxxflags = {' '.join(cxx_parts)}")
-            src_cxx_override = ("$srccxxflags", gch)
+            pch_dep = gch
+            if pch_strict():
+                # Consumers wait on the probe stamp, so an unloadable .gch
+                # reds the build here instead of warning ~100 times
+                lines.append("rule pchprobe")
+                lines.append(
+                    "  command = $cxx $cxxflags $flags -Winvalid-pch"
+                    " -Werror=invalid-pch"
+                    f" -include {PCH_HEADER_NAME} -fsyntax-only -x c++"
+                    f" {_q(Path(os.devnull))} && $stamp"
+                )
+                lines.append("  description = PCHPROBE $out")
+                lines.append(f"build esphome_pch.probe: pchprobe {gch}")
+                if src_other:
+                    lines.append(f"  flags = {' '.join(src_other)}")
+                stamp = (
+                    "cmd /c copy /y nul $out >nul" if os.name == "nt" else "touch $out"
+                )
+                lines.append(f"  stamp = {stamp}")
+                pch_dep = f"{gch} esphome_pch.probe"
+            src_cxx_override = ("$srccxxflags", pch_dep)
             mark_pch_emitted()
     src_objs = _ninja_compile_edges(
         lines,

esphome/build_helpers/pch.py

@@ -435,6 +435,42 @@ def prepare_pch(
         os.utime(header)
         pch_degraded(f"compile failed: {error[:200]}")
         return
+    # Load probe: some toolchains build a .gch they then refuse to load
+    # (per-process ASLR); dep flags are already stripped from cmd, so no
+    # -MF is needed
+    probe_cmd = [
+        *cmd[: cmd.index("-x")],
+        "-Winvalid-pch",
+        "-include",
+        str(header),
+        "-fsyntax-only",
+        "-x",
+        "c++",
+        os.devnull,
+    ]
+    try:
+        probe = subprocess.run(
+            probe_cmd,
+            cwd=cmd_dir,
+            env={**os.environ, "LC_ALL": "C"},
+            capture_output=True,
+            text=True,
+            check=False,
+            timeout=300,
+        )
+    except (OSError, subprocess.SubprocessError) as err:
+        _LOGGER.warning("Precompiled header probe did not run: %s", err)
+        discard_pch(build_dir)
+        pch_degraded(f"probe did not run: {err}")
+        return
+    if probe.returncode != 0 or ".gch" in probe.stderr:
+        error = f"toolchain cannot load the pch: {probe.stderr.strip()[:400]}"
+        _LOGGER.warning("Precompiled header failed; compiling without it: %s", error)
+        discard_pch(build_dir)
+        failed_marker.write_text(checksum + "\n", encoding="utf-8")
+        os.utime(header)
+        pch_degraded(error)
+        return
     failed_marker.unlink(missing_ok=True)
     sum_path.write_text(checksum + "\n", encoding="utf-8")
     # Consumers depend on the header (depfiles cannot see through a .gch);

tests/unit_tests/build_gen/test_arduino8266.py

@@ -1856,3 +1856,21 @@ def test_write_project_pch_strict_raises_on_skip(
     )
     with pytest.raises(EsphomeError, match="precedes the pch"):
         _write_ninja(paths, ccache="/usr/bin/ccache")
+
+
+def test_write_project_pch_strict_emits_probe_edge(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """Strict mode gates C++ src edges on a hard-failing load probe."""
+    paths = _make_framework(tmp_path)
+    _set_flags("-DPIO_FRAMEWORK_ARDUINO_LWIP2_HIGHER_BANDWIDTH_LOW_FLASH")
+    content = _write_ninja(paths, ccache="/usr/bin/ccache")
+    assert "pchprobe" not in content
+
+    monkeypatch.setenv("ESPHOME_PCH_STRICT", "1")
+    content = _write_ninja(paths, ccache="/usr/bin/ccache")
+    assert "build esphome_pch.probe: pchprobe esphome_pch.h.gch" in content
+    assert "-Werror=invalid-pch" in content
+    for line in content.splitlines():
+        if line.startswith("build obj/src/main.cpp.o:"):
+            assert line.endswith("| esphome_pch.h.gch esphome_pch.probe")

tests/unit_tests/build_gen/test_espidf.py

@@ -550,6 +550,9 @@ def test_prepare_pch_writes_header_and_sum(tmp_path: Path) -> None:
     gch = dev / "build" / "esphome_pch.h.gch"
 
     def fake_compile(cmd, **kwargs):
+        if "-fsyntax-only" in cmd:
+            # The load probe follows a successful .gch build
+            return subprocess.CompletedProcess(cmd, 0, "", "")
         # The compile must target the header, not the stub TU
         assert cmd[-5:-3] == ["c++-header", "-c"]
         gch.write_bytes(b"gch")
@@ -1097,3 +1100,41 @@ def test_prepare_pch_strict_raises_on_missing_db(
         pytest.raises(EsphomeError, match="no usable compile command"),
     ):
         prepare_pch()
+
+
+def test_prepare_pch_probe_rejection_latches_and_degrades(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """A toolchain that cannot load its own .gch discards it, latches the
+    marker, and fails strict mode."""
+    from esphome.build_gen.espidf import prepare_pch
+    from esphome.core import EsphomeError
+
+    dev = _make_pch_device(tmp_path, "dev_p")
+    CORE.build_path = dev
+    gch = dev / "build" / "esphome_pch.h.gch"
+
+    def rejecting(cmd, **kwargs):
+        if "-fsyntax-only" in cmd:
+            return subprocess.CompletedProcess(
+                cmd, 0, "", "warning: esphome_pch.h.gch: had text segment "
+            )
+        gch.write_bytes(b"gch")
+        return subprocess.CompletedProcess(cmd, 0, "", "")
+
+    with (
+        patch.object(CORE, "name", "test"),
+        patch("esphome.build_helpers.pch.subprocess.run", side_effect=rejecting),
+    ):
+        prepare_pch()
+    assert not gch.exists()
+    assert (dev / "build" / "esphome_pch.h.gch.failed").exists()
+
+    monkeypatch.setenv("ESPHOME_PCH_STRICT", "1")
+    (dev / "build" / "esphome_pch.h.gch.failed").unlink()
+    with (
+        patch.object(CORE, "name", "test"),
+        patch("esphome.build_helpers.pch.subprocess.run", side_effect=rejecting),
+        pytest.raises(EsphomeError, match="cannot load the pch"),
+    ):
+        prepare_pch()