repo: add basic cache_check_missing

dvc/repo/__init__.py

@@ -93,6 +93,7 @@ class Repo:
     from dvc.repo.status import status  # type: ignore[misc]
     from dvc.repo.update import update  # type: ignore[misc]
 
+    from .cache import check_missing as cache_check_missing  # type: ignore[misc]
     from .data import status as data_status  # type: ignore[misc]
 
     ls = staticmethod(_ls)

dvc/repo/cache.py

@@ -0,0 +1,25 @@
+import os
+
+
+def check_missing(repo, rev=None):
+    from dvc_data.index import StorageKeyError
+
+    with repo.switch(rev or "workspace"):
+        index = repo.index.data["repo"]
+
+    def onerror(_entry, _exc):
+        pass
+
+    index.onerror = onerror
+
+    ret = []
+    for _, entry in index.iteritems():
+        try:
+            fs, path = index.storage_map.get_cache(entry)
+        except (StorageKeyError, ValueError):
+            continue
+
+        if not fs.exists(path):
+            ret.append(os.path.join(*entry.key))
+
+    return ret