Support locking multiple variables at once?

[tewalds]

Do we need/want to support something equivalent to https://en.cppreference.com/w/cpp/thread/scoped_lock ?

[tewalds]

folly solves this with a free function that returns a tuple of mutex_locked:

    void fun(Synchronized<vector<int>>& a, Synchronized<vector<int>>& b) {
      auto [lockedA, lockedB] = folly::acquireLocked(a, b);
      ... use lockedA and lockedB ...
    }

The two ways to lock multiple locks without deadlocks is std::lock or std::scoped_lock. I guess the obvious implementation then is std::lock, followed by a tuple of mutex_protected where each wraps a lock_guard constructed with std::adopt_lock.

I guess the goal of standardization would be an overload of std::lock that accepts not just Mutex but also mutex_protected. The return value would need careful consideration given that std::lock currently returns void.

[jbcoe]

As a first pass, maybe we support only multiple mutex-protected objects; not a mix of mutexes and mutex-protected objects.

[tewalds]

This is added in #26, though adding xyz::try_lock would also be good. std::lock returns void, so changing that to return a tuple of mutex_locked makes sense. std::try_lock returns -1 on success or an index to the lock failure, which is less clear how you'd translate into returning the tuple or error. Maybe a std::expected?

[jbcoe]

We should call this function something other than lock to avoid overloads having different return type-shapes.