Reject relative CCACHE_BASEDIR values too

esphome/build_helpers/ccache.py

@@ -99,10 +99,7 @@ def effective_ccache_basedir() -> str:
     from esphome.core import CORE
 
     raw = os.environ.get("CCACHE_BASEDIR")
-    if raw is not None:
-        # A degenerate value ("", "/", relative) must not be used for
-        # substring stripping; fall back to the resolved build path
-        if len(Path(raw).parts) > 1:
-            return raw
-        return str(Path(CORE.build_path).resolve())
+    if raw is not None and Path(raw).is_absolute() and len(Path(raw).parts) > 1:
+        return raw
+    # Unset or degenerate ("", "/", relative): fall back to the build path
     return str(Path(CORE.build_path).resolve())

tests/unit_tests/build_helpers/test_ccache.py

@@ -130,6 +130,6 @@ def test_effective_ccache_basedir_prefers_user_value(tmp_path: Path) -> None:
     with patch.dict(os.environ, {}, clear=True):
         assert ccache.effective_ccache_basedir() == str(tmp_path.resolve())
     # Degenerate values would strip substrings ccache never rewrites
-    for bad in ("", "/"):
+    for bad in ("", "/", "a/b"):
         with patch.dict(os.environ, {"CCACHE_BASEDIR": bad}, clear=True):
             assert ccache.effective_ccache_basedir() == str(tmp_path.resolve())