* Enclose most operations that update the database in transactions.

* Open all database tables (Db objects) at initialisation time, not
  every time they are used.  This is necessary because tables have to
  outlive all transactions that refer to them.

src/normalise.cc

@@ -9,7 +9,9 @@
 
 void registerSuccessor(const FSId & id1, const FSId & id2)
 {
-    nixDB.setString(noTxn, dbSuccessors, id1, id2);
+    Transaction txn(nixDB);
+    nixDB.setString(txn, dbSuccessors, id1, id2);
+    txn.commit();
 }