[diffusion] Honor XDG cache for model overlays (#36019)

python/sglang/multimodal_gen/runtime/utils/model_overlay.py

@@ -17,6 +17,7 @@ from huggingface_hub.errors import (
 from requests.exceptions import ConnectionError as RequestsConnectionError
 from requests.exceptions import RequestException
 
+from sglang.multimodal_gen import envs
 from sglang.multimodal_gen.runtime.loader.weight_utils import get_lock
 from sglang.multimodal_gen.runtime.utils.logging_utils import init_logger
 from sglang.utils import load_diffusion_overlay_registry_from_env
@@ -106,9 +107,7 @@ def _resolve_bundled_overlay_dir(overlay_spec: dict[str, Any]) -> str | None:
 
 
 def get_diffusion_cache_root() -> str:
-    return os.path.expanduser(
+    return envs.SGLANG_DIFFUSION_CACHE_ROOT
-        os.getenv("SGLANG_DIFFUSION_CACHE_ROOT", "~/.cache/sgl_diffusion")
-    )
 
 
 def clear_model_overlay_registry_cache() -> None:

python/sglang/multimodal_gen/test/unit/test_model_overlay.py

@@ -0,0 +1,19 @@
+"""Unit tests for diffusion model-overlay cache paths."""
+
+from sglang.multimodal_gen.runtime.utils.model_overlay import (
+    get_diffusion_cache_root,
+)
+
+
+def test_uses_xdg_cache_home_by_default(monkeypatch):
+    monkeypatch.setenv("XDG_CACHE_HOME", "/tmp/sglang-xdg-cache")
+    monkeypatch.delenv("SGLANG_DIFFUSION_CACHE_ROOT", raising=False)
+
+    assert get_diffusion_cache_root() == "/tmp/sglang-xdg-cache/sgl_diffusion"
+
+
+def test_explicit_diffusion_cache_root_takes_precedence(monkeypatch):
+    monkeypatch.setenv("SGLANG_DIFFUSION_CACHE_ROOT", "/tmp/sglang-diffusion-cache")
+    monkeypatch.setenv("XDG_CACHE_HOME", "/tmp/sglang-xdg-cache")
+
+    assert get_diffusion_cache_root() == "/tmp/sglang-diffusion-cache"