Skip the doomed gch compile after header-generation failure, self-heal the placeholder from CMake

esphome/build_helpers/pch.py

@@ -165,7 +165,12 @@ def pch_cmake_consumer(target: str, sources_var: str) -> str:
         return ""
     escalation = pch_consumer_escalation()
     return f"""
-# ESPHome precompiled header (see esphome/build_helpers/pch.py)
+# ESPHome precompiled header (see esphome/build_helpers/pch.py).
+# The touch keeps OBJECT_DEPENDS satisfiable when the build system itself
+# wiped the build dir after the header was written (west --pristine)
+if(NOT EXISTS "${{CMAKE_BINARY_DIR}}/{PCH_HEADER_NAME}")
+  file(TOUCH "${{CMAKE_BINARY_DIR}}/{PCH_HEADER_NAME}")
+endif()
 target_compile_options({target} PRIVATE
     "$<$<COMPILE_LANGUAGE:CXX>:-Winvalid-pch>"
     "$<$<COMPILE_LANGUAGE:CXX>:{escalation}>"

esphome/components/nrf52/__init__.py

@@ -955,6 +955,7 @@ def run_compile(args, config: ConfigType) -> bool:
         # flags. Only when the app DB is missing: input changes wipe the
         # build dir, so an existing DB is settled and --cmake-only would
         # reconfigure for nothing.
+        prepare = True
         app_dir = _app_build_dir(build_dir)
         if not (app_dir / "compile_commands.json").is_file():
             if not run_command_ok(
@@ -980,16 +981,21 @@ def run_compile(args, config: ConfigType) -> bool:
                 stream_output=True,
                 cwd=str(paths["framework_path"]),
             ):
-                # A pch-only prerequisite: degrade, let the real build report
+                # A pch-only prerequisite: degrade, let the real build report.
+                # Also skip the .gch compile: it would fail on the missing
+                # headers and latch .gch.failed until an identity change
                 _LOGGER.warning(
                     "Zephyr header generation failed; compiling without the pch"
                 )
                 pch.discard_pch(app_dir)
                 pch.pch_degraded("zephyr_generated_headers failed")
+                prepare = False
     else:
+        prepare = True
         app_dir = _app_build_dir(build_dir)
 
-    pch.guarded_prepare(app_dir, partial(_prepare_pch, app_dir))
+    if prepare:
+        pch.guarded_prepare(app_dir, partial(_prepare_pch, app_dir))
 
     if not run_command_ok(
         west_cmd,

tests/unit_tests/build_helpers/test_pch.py

@@ -244,6 +244,8 @@ def test_pch_cmake_consumer_substitutes_target_and_sources(
     assert '"$<$<COMPILE_LANGUAGE:CXX>:esphome_pch.h>"' in block
     assert "set_source_files_properties(${APP_SOURCES} PROPERTIES" in block
     assert 'OBJECT_DEPENDS "${CMAKE_BINARY_DIR}/esphome_pch.h"' in block
+    # Placeholder guard: survives a build-system-side pristine wipe
+    assert 'file(TOUCH "${CMAKE_BINARY_DIR}/esphome_pch.h")' in block
 
 
 def test_pch_cmake_consumer_strict_escalates(

tests/unit_tests/components/nrf52/test_pch.py

@@ -230,7 +230,8 @@ class TestRunCompilePhases:
         with pytest.raises(EsphomeError, match="nRF52 native build failed"):
             self._run()
         assert "Zephyr header generation failed" in caplog.text
-        assert prepare.called
+        # The doomed .gch compile is skipped: it would latch .gch.failed
+        assert not prepare.called
 
     def test_generated_headers_failure_strict_raises(
         self, monkeypatch: pytest.MonkeyPatch, compile_ctx: CompileCtx