remove unnecessary assertion in tokenization (#807)

src/levanter/store/cache.py

@@ -1023,8 +1023,6 @@ def _start_copies(
     copy_refs: dict[str, ray.ObjectRef] = {}
     last_ref: ray.ObjectRef | None = None
 
-    found_one_to_copy = False
-
     for group in shard_groups:
         # first make sure it's either done this run or already done
         if write_refs.get(group) is not None:
@@ -1043,20 +1041,21 @@ def _start_copies(
 
         assert this_ledger is not None
         # see if we already copied this group, meaning all the shards are in the permanent cache
-        shards_copied = sum(1 if shard in overall_ledger.finished_shards else 0 for shard in shard_groups[group])
+        shards_copied = [shard for shard in shard_groups[group] if shard in overall_ledger.finished_shards]
 
-        if found_one_to_copy and shards_copied > 0:
-            raise RuntimeError("A previous group was copied, but this group was not. This should never happen.")
-        elif shards_copied == len(shard_groups[group]):
+        if len(shards_copied) == len(shard_groups[group]):
             assert (
                 overall_ledger.total_num_rows >= total_rows_from_caches
             ), f"{overall_ledger.total_num_rows} < {total_rows_from_caches}. {group}"
             continue  # nothing to do
-        elif shards_copied > 0:
-            # In theory we can handle this, but it's a bit tricky, so we're going to punt for now
-            raise RuntimeError("Some shards were copied but not all. This should never happen.")
+        elif len(shards_copied) > 0:
+            # In theory, we can handle this, but it's a bit tricky, so we're going to punt for now
+            raise RuntimeError(
+                "Some shards were copied but not all. This should never happen."
+                f"Specifically the following shards were copied: {shards_copied}"
+                f"And the following shards were not: {set(shard_groups[group]) - set(shards_copied)}"
+            )
 
-        found_one_to_copy = True
         # we need to copy this group
 
         # we can't "commit" the group to the ledger (or the number of rows)