fix nix flake cache with chroot stores

The flake cache was not properly checking store paths when custom stores
were used (e.g., when using --store flag or CLAN_TEST_STORE). This
caused cache validation to fail even when the store paths existed.

This fix:
- Updates store path detection to properly identify any path with
  "/store/" in it and a proper nix store item format (hash-name)
- Normalizes store paths to use the current store when checking
  if they exist (CLAN_TEST_STORE or nix config)
- Uses CLAN_TEST_STORE environment variable for test stores, which
  matches how nix --store flag works

Added comprehensive tests to verify the fix works with custom stores.

pkgs/clan-cli/clan_lib/flake/flake.py

@@ -9,6 +9,7 @@ from tempfile import NamedTemporaryFile
 from typing import Any
 
 from clan_lib.errors import ClanError
+from clan_lib.nix import nix_config
 
 log = logging.getLogger(__name__)
 
@@ -319,9 +320,7 @@ class FlakeCacheEntry:
         # strings need to be checked if they are store paths
         # if they are, we store them as a dict with the outPath key
         # this is to mirror nix behavior, where the outPath of an attrset is used if no further key is specified
-        elif isinstance(value, str) and value.startswith(
-            os.environ.get("NIX_STORE_DIR", "/nix/store")
-        ):
+        elif isinstance(value, str) and self._is_store_path(value):
             assert selectors == []
             self.value = {"outPath": FlakeCacheEntry(value)}
 
@@ -335,14 +334,68 @@ class FlakeCacheEntry:
                 msg = f"Cannot insert {value} into cache, already have {self.value}"
                 raise TypeError(msg)
 
+    def _is_store_path(self, value: str) -> bool:
+        """Check if a string is a nix store path."""
+        # A store path is any path that has "store" as one of its parent directories
+        # and contains a hash-prefixed name after it
+        path_parts = Path(value).parts
+
+        try:
+            store_idx = path_parts.index("store")
+        except ValueError:
+            return False
+
+        # Check if there's at least one more component after "store"
+        if store_idx + 1 < len(path_parts):
+            # Check if the component after store looks like a nix store item
+            # (starts with a hash)
+            store_item = path_parts[store_idx + 1]
+            # Basic check: nix store items typically start with a hash
+            return len(store_item) > 32 and "-" in store_item
+
+        return False
+
+    def _normalize_store_path(self, store_path: str) -> Path | None:
+        """
+        Normalize a store path to use the current NIX_STORE_DIR.
+        Returns None if the path is not a valid store path.
+        """
+        # Extract the store item (hash-name) from the path
+        path_parts = Path(store_path).parts
+
+        # Find the index of "store" in the path
+        try:
+            store_idx = path_parts.index("store")
+        except ValueError:
+            return None
+
+        if store_idx + 1 < len(path_parts):
+            store_item = path_parts[store_idx + 1]
+
+            # Get the current store path
+            # Check if we're using a test store first
+            test_store = os.environ.get("CLAN_TEST_STORE")
+            if test_store:
+                # In test mode, the store is at CLAN_TEST_STORE/nix/store
+                current_store = str(Path(test_store) / "nix" / "store")
+            else:
+                # Otherwise use nix config
+                config = nix_config()
+                current_store = config.get("store", "/nix/store")
+
+            return Path(current_store) / store_item if current_store else None
+
+        return None
+
     def is_cached(self, selectors: list[Selector]) -> bool:
         selector: Selector
 
         # for store paths we have to check if they still exist, otherwise they have to be rebuild and are thus not cached
-        if isinstance(self.value, str) and self.value.startswith(
-            os.environ.get("NIX_STORE_DIR", "/nix/store")
-        ):
-            return Path(self.value).exists()
+        if isinstance(self.value, str) and self._is_store_path(self.value):
+            normalized_path = self._normalize_store_path(self.value)
+            if normalized_path:
+                return normalized_path.exists()
+            return False
 
         # if self.value is not dict but we request more selectors, we assume we are cached and an error will be thrown in the select function
         if isinstance(self.value, str | float | int | None):

pkgs/clan-cli/clan_lib/tests/test_flake_cache_chroot.py

@@ -0,0 +1,108 @@
+"""Test flake cache with chroot stores."""
+
+import os
+import tempfile
+from pathlib import Path
+
+import pytest
+
+from clan_lib.flake.flake import FlakeCache, FlakeCacheEntry
+
+
+def test_flake_cache_with_chroot_store(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """Test that flake cache works correctly with chroot stores."""
+    # Create a mock store path
+    normal_store = "/nix/store"
+    chroot_store = str(tmp_path / "nix" / "store")
+
+    # Create the chroot store directory
+    Path(chroot_store).mkdir(parents=True)
+
+    # Create a fake derivation in the chroot store with proper nix store format
+    fake_drv = "abcd1234abcd1234abcd1234abcd1234-test-package"
+    fake_store_path = f"{chroot_store}/{fake_drv}"
+    Path(fake_store_path).mkdir(parents=True)
+
+    # Test 1: Cache entry with normal store path
+    cache = FlakeCache()
+
+    # Insert a store path that doesn't exist in the normal store
+    non_existent_path = f"{normal_store}/{fake_drv}"
+    cache.insert({"test": {"package": non_existent_path}}, "")
+
+    # Without chroot store, this should be uncached (path doesn't exist)
+    assert not cache.is_cached("test.package")
+
+    # Test 2: Set CLAN_TEST_STORE to chroot store parent
+    # CLAN_TEST_STORE should point to the parent of nix/store
+    monkeypatch.setenv("CLAN_TEST_STORE", str(tmp_path))
+
+    # Create a new cache with the chroot store
+    cache2 = FlakeCache()
+
+    # Insert the same path but now it should use chroot store
+    cache2.insert({"test": {"package": fake_store_path}}, "")
+
+    # This should be cached because the path exists in chroot store
+    assert cache2.is_cached("test.package")
+
+    # Test 3: Cache persistence with chroot store
+    cache_file = tmp_path / "cache.json"
+    cache2.save_to_file(cache_file)
+
+    # Load cache in a new instance
+    cache3 = FlakeCache()
+    cache3.load_from_file(cache_file)
+
+    # Should still be cached with chroot store
+    assert cache3.is_cached("test.package")
+
+    # Test 4: Cache validation fails when chroot store changes
+    monkeypatch.setenv("CLAN_TEST_STORE", "/different")
+
+    # Same cache should now be invalid
+    assert not cache3.is_cached("test.package")
+
+
+def test_flake_cache_entry_store_path_validation() -> None:
+    """Test that FlakeCacheEntry correctly validates store paths."""
+    # Test with default store
+    entry = FlakeCacheEntry()
+
+    # Insert a non-existent store path with proper format
+    fake_path = "/nix/store/abcd1234abcd1234abcd1234abcd1234-fake-package"
+    entry.insert(fake_path, [])
+
+    # Should not be cached because path doesn't exist
+    assert not entry.is_cached([])
+
+    # Test with environment variable
+    with tempfile.TemporaryDirectory() as tmpdir:
+        # Create nix/store structure
+        store_dir = Path(tmpdir) / "nix" / "store"
+        store_dir.mkdir(parents=True)
+
+        # Create a fake store path with proper format
+        fake_drv = "test1234test1234test1234test1234-package"
+        fake_path_obj = store_dir / fake_drv
+        fake_path_obj.mkdir()
+        fake_path = str(fake_path_obj)
+
+        # Set CLAN_TEST_STORE to parent of store dir
+        old_env = os.environ.get("CLAN_TEST_STORE")
+        try:
+            os.environ["CLAN_TEST_STORE"] = str(tmpdir)
+
+            entry2 = FlakeCacheEntry()
+            entry2.insert(str(fake_path), [])
+
+            # Should be cached because path exists
+            assert entry2.is_cached([])
+
+        finally:
+            if old_env is None:
+                os.environ.pop("CLAN_TEST_STORE", None)
+            else:
+                os.environ["CLAN_TEST_STORE"] = old_env