test for eas going backward

beacon-chain/db/kv/custody.go

@@ -46,6 +46,14 @@ func (s *Store) UpdateCustodyInfo(ctx context.Context, earliestAvailableSlot pri
 			return nil
 		}
 
+		// When increasing custody group count, prevent earliestAvailableSlot from going backward.
+		if storedEarliestAvailableSlot != 0 && earliestAvailableSlot < storedEarliestAvailableSlot {
+			return errors.Errorf(
+				"cannot decrease earliest available slot from %d to %d when increasing custody group count from %d to %d",
+				storedEarliestAvailableSlot, earliestAvailableSlot, storedGroupCount, custodyGroupCount,
+			)
+		}
+
 		storedGroupCount, storedEarliestAvailableSlot = custodyGroupCount, earliestAvailableSlot
 
 		// Store the earliest available slot.

beacon-chain/db/kv/custody_test.go

@@ -134,6 +134,31 @@ func TestUpdateCustodyInfo(t *testing.T) {
 		require.Equal(t, initialSlot, storedSlot)
 		require.Equal(t, initialCount, storedCount)
 	})
+
+	t.Run("prevent earliest available slot from going backward when increasing group count", func(t *testing.T) {
+		const (
+			initialSlot  = primitives.Slot(200)
+			initialCount = uint64(5)
+			newSlot      = primitives.Slot(100) // Lower than initial - should be rejected
+			newCount     = uint64(10)           // Higher than initial - would trigger update
+		)
+
+		db := setupDB(t)
+
+		// Initialize custody info with slot 200 and group count 5
+		_, _, err := db.UpdateCustodyInfo(ctx, initialSlot, initialCount)
+		require.NoError(t, err)
+
+		// Try to increase group count (5 -> 10) but with a lower earliest slot (200 -> 100)
+		// This should fail because we can't move earliest available slot backward when increasing custody
+		_, _, err = db.UpdateCustodyInfo(ctx, newSlot, newCount)
+		require.ErrorContains(t, "cannot decrease earliest available slot", err)
+
+		// Verify the database wasn't updated
+		storedSlot, storedCount := getCustodyInfoFromDB(t, db)
+		require.Equal(t, initialSlot, storedSlot)
+		require.Equal(t, initialCount, storedCount)
+	})
 }
 
 func TestUpdateEarliestAvailableSlot(t *testing.T) {

changelog/satushh-eas-bug.md

@@ -0,0 +1,2 @@
+### Fixed
+- prevent eas from going backward when increasing cgc