PopSavePoint discards lock information

[lth]

Expected behavior

If you have:

txn->SetSavePoint();
txn->Put("A")
txn->SetSavePoint();
txn->Put("B")
txn->PopSavePoint()
txn->RollbackToSavePoint()

Then the write batch is rolled back to the first savepoint, meaning that both keys A and B are rolled back. I also expect that keys A and B will both be unlocked as well.

Actual behavior

Only A is unlocked, and B remains locked. The problem is that every savepoint tracks keys locked since its last savepoint, and that information is used for unlocking if RollbackToSavePoint is called. When PopSavePoint is called, instead of simply popping off the stack and discarding that information, the key set should be merged with the key set of the previous savepoint instead, so that all keys get unlocked.

Steps to reproduce the behavior

Add this to utilities/transactions/transaction_test.cc

TEST_P(TransactionTest, SavepointTest4) {
  WriteOptions write_options;
  ReadOptions read_options;
  TransactionOptions txn_options;
  Status s;

  txn_options.lock_timeout = 1;  // 1 ms
  Transaction* txn1 = db->BeginTransaction(write_options, txn_options);
  ASSERT_TRUE(txn1);

  txn1->SetSavePoint(); // 1
  s = txn1->Put("A", "");
  ASSERT_OK(s);

  txn1->SetSavePoint(); // 2
  s = txn1->Put("B", "");
  ASSERT_OK(s);

  s = txn1->PopSavePoint(); // Remove 2
  ASSERT_OK(s);

  ASSERT_OK(txn1->RollbackToSavePoint());  // Rollback to 1

  // Verify that everything was rolled back.
  std::string value;
  s = txn1->Get(read_options, "A", &value);
  ASSERT_TRUE(s.IsNotFound());

  s = txn1->Get(read_options, "B", &value);
  ASSERT_TRUE(s.IsNotFound());

  // Nothing should be locked
  Transaction* txn2 = db->BeginTransaction(write_options, txn_options);
  ASSERT_TRUE(txn2);

  s = txn2->Put("A", "");
  ASSERT_OK(s);

  s = txn2->Put("B", "");
  ASSERT_OK(s); // This fails

  delete txn2;
  delete txn1;
}