Fix MSETNX not allow overriding the same key

[enjoy-binbin]

The changes in #337 causing this issue:

127.0.0.1:6379> flushall
OK
127.0.0.1:6379> msetnx k v1 k v2
(integer) 1
127.0.0.1:6379> get k
"v2

127.0.0.1:6666> flushall
OK
127.0.0.1:6666> msetnx k v1 k v2
(integer) 0
127.0.0.1:6666> get k
"v1"

Redis allow we overriding the same key but kvrocks will
fail in this case. The way to handle it is to revert the
changes in #337.

So this PR is based on that, after reverting the changes
of #337, we can reuse the logic of MSet. And hulk mentions
that MSETNX is an exclusive command and we better to lock
those keys and then remove the exclusive flag from the command.

So we use MultiLockGuard before Exists call to lock multi
keys. And change MSet to support non-lock calls. And also
remove the exclusive flag from MSETNX command.

Co-authored-by: hulk hulk.website@gmail.com

[mapleFU]

Didn't review it carefully, just a tiny round.

[mapleFU]

After a round of review I get to understand this patch. There is a problem:

class MultiLockGuard {
 public:
  explicit MultiLockGuard(LockManager *lock_mgr, const std::vector<std::string> &keys) : lock_mgr_(lock_mgr) {
    locks_ = lock_mgr_->MultiGet(keys);
    for (const auto &iter : locks_) {
      iter->lock();
    }
  }

  ~MultiLockGuard() {
    // Lock with order `A B C` and unlock should be `C B A`
    for (auto iter = locks_.rbegin(); iter != locks_.rend(); ++iter) {
      (*iter)->unlock();
    }
  }

  MultiLockGuard(const MultiLockGuard &) = delete;
  MultiLockGuard &operator=(const MultiLockGuard &) = delete;

 private:
  LockManager *lock_mgr_ = nullptr;
  std::vector<std::mutex *> locks_;
};

Previously, the lock happens per-key. It was a bit wierd but would not introduce problem. Now, exclusive is removed, so it might introduce the deadlock problem.

A proposal is that using std::lock( https://en.cppreference.com/w/cpp/thread/lock ) instead of loop in MultiLockGuard.

@PragmaTwice would that be possible?

[git-hulk]

@mapleFU I think it won't have the deadlock for the multi-keys lock since it will sort keys first: https://github.com/apache/kvrocks/blob/unstable/src/storage/lock_manager.cc#L43

[mapleFU]

Nice I didn't find out that.