fix(mine): fallback to test pool when assign_splits hashes all tasks to test

skillopt_sleep/mine.py

@@ -337,13 +337,36 @@ def _promote_one(*, to: str, from_splits: set[str]) -> None:
         else:
             t.split = "train"
 
+    import logging
+
     # Guarantee val (the gate) is non-empty when we have >=2 real tasks.
-    # Only promote from train so hash-assigned test tasks stay untouched.
+    # Only promote from train so hash-assigned test tasks stay untouched,
+    # UNLESS train is also empty, in which case we must borrow from test.
     if len(real) >= 2 and not any(t.split == "val" for t in real):
-        _promote_one(to="val", from_splits={"train"})
-    # Guarantee a train pool exists when possible; never borrow from test.
+        if not any(t.split == "train" for t in real):
+            logging.getLogger("skillopt_sleep").warning(
+                f"assign_splits: all {len(real)} real tasks hashed into test "
+                f"(val_fraction={val_fraction}, test_fraction={test_fraction}, seed={seed}). "
+                "Borrowing from test to satisfy non-empty val."
+            )
+            _promote_one(to="val", from_splits={"test"})
+        else:
+            _promote_one(to="val", from_splits={"train"})
+
+    # Guarantee a train pool exists when possible; never borrow from test
+    # UNLESS val has only 1 task (so taking from val would re-empty the gate)
+    # and test tasks are available.
     if not any(t.split == "train" for t in tasks) and len(real) >= 2:
-        _promote_one(to="train", from_splits={"val"})
+        val_count = sum(1 for t in real if t.split == "val")
+        if val_count <= 1 and any(t.split == "test" for t in real):
+            logging.getLogger("skillopt_sleep").warning(
+                f"assign_splits: pulling from test to satisfy non-empty train "
+                f"because val has only {val_count} real task "
+                f"(val_fraction={val_fraction}, test_fraction={test_fraction}, seed={seed})."
+            )
+            _promote_one(to="train", from_splits={"test"})
+        else:
+            _promote_one(to="train", from_splits={"val"})
     return tasks
 
 

tests/test_split_hardening_2x3.py

@@ -117,6 +117,20 @@ def test_hash_assigned_test_not_demoted_for_val_top_up(self):
             if t.id in test_ids:
                 self.assertEqual(t.split, "test")
 
+    def test_val_and_train_pull_from_test_when_empty(self):
+        """When all real tasks hash to test, val and train borrow from test to satisfy non-empty guarantees."""
+        # Seed 42 with this configuration was observed to put all tasks into test
+        tasks = assign_splits(
+            [_task(f"t{i}", f"task {i}") for i in range(5)],
+            val_fraction=0.10,
+            test_fraction=0.80,
+            seed=42,
+        )
+        splits = {t.split for t in tasks}
+        self.assertIn("val", splits, "val must not be empty even if it needs to borrow from test")
+        self.assertIn("train", splits, "train must not be empty even if it needs to borrow from test")
+        self.assertIn("test", splits, "test must still contain remaining tasks")
+
 
 class Pass1ApproachCFractionBoundaries(unittest.TestCase):
     """Pass 1 / approach C: reject invalid fraction knobs early."""