WriteUnPrepared: Enable WAL during crash recovery (facebook#6418)

Summary:
Unfortunately, it seems like mysqld reuses xids across machine restarts. When that happens, we could have something like the following happening:

```
BEGIN_PREPARE(unprepared) Put(a) END_PREPARE(xid = 1)
-- crash and recover with Put(a) rolled back as it was not prepared
BEGIN_PREPARE(prepared) Put(b) END_PREPARE(xid = 1)
COMMIT(xid = 1)
-- crash and recover with both a, b
```

To solve this, we will have to log the rollback batch into the WAL during recovery.

WritePrepared already logs the rollback batch into the WAL, if a rollback happens after prepare, so there is no problem there.
Pull Request resolved: facebook#6418

Differential Revision: D19896151

Pulled By: lth

fbshipit-source-id: 2ff65ddc5fe75efd57736fed4b7cd7a109d26609

utilities/transactions/write_unprepared_txn_db.cc

@@ -21,10 +21,23 @@ Status WriteUnpreparedTxnDB::RollbackRecoveredTransaction(
   assert(rtxn->unprepared_);
   auto cf_map_shared_ptr = WritePreparedTxnDB::GetCFHandleMap();
   auto cf_comp_map_shared_ptr = WritePreparedTxnDB::GetCFComparatorMap();
+  // In theory we could write with disableWAL = true during recovery, and
+  // assume that if we crash again during recovery, we can just replay from
+  // the very beginning. Unfortunately, the XIDs from the application may not
+  // necessarily be unique across restarts, potentially leading to situations
+  // like this:
+  //
+  // BEGIN_PREPARE(unprepared) Put(a) END_PREPARE(xid = 1)
+  // -- crash and recover with Put(a) rolled back as it was not prepared
+  // BEGIN_PREPARE(prepared) Put(b) END_PREPARE(xid = 1)
+  // COMMIT(xid = 1)
+  // -- crash and recover with both a, b
+  //
+  // We could just write the rollback marker, but then we would have to extend
+  // MemTableInserter during recovery to actually do writes into the DB
+  // instead of just dropping the in-memory write batch.
+  //
   WriteOptions w_options;
-  // If we crash during recovery, we can just recalculate and rewrite the
-  // rollback batch.
-  w_options.disableWAL = true;
 
   class InvalidSnapshotReadCallback : public ReadCallback {
    public: