storage: add comment about the idea to lock an oid before reporting a resolvable conflict

Currently, another argument not to lock is that we would not be able to test
incremental resolution anymore. We can think about this again when deadlock
resolution is implemented.

neo/storage/transactions.py

@@ -267,6 +267,13 @@ class TransactionManager(object):
             logging.info('Possible deadlock on %r:%r with %r',
                 dump(oid), dump(ttid), dump(locking_tid))
             raise ConflictError(ZERO_TID)
+        # XXX: Consider locking before reporting a conflict:
+        #      - That would speed up the case of cascading conflict resolution
+        #        by avoiding incremental resolution, assuming that the time to
+        #        resolve a conflict is often constant: "C+A vs. B -> C+A+B"
+        #        rarely costs more than "C+A vs. C+B -> C+A+B".
+        #      - That would slow down of cascading unresolvable conflicts but
+        #        if that happens, the application should be reviewed.
         if previous_serial is None:
             previous_serial = self._app.dm.getLastObjectTID(oid)
         if previous_serial is not None and previous_serial != serial: