[core] Introduced the CSharedResource

[maxsharabayko]

NB! The description is outdated, as now issue #2234 is being addressed in this PR.

Introducing the CSharedResource class. Not yet used in the code. Potentially the m_ConnectionLock can be converted to the CSharedResource.

[ethouris]

A similar thing has been already introduced for the use of group locking - the apiAcquire and apiRelease pair and the scoped wrapper for it GroupKeeper.

This is currently built in into the group, but could be abstracted out for more purposes, and maybe in future it could be also used for sockets in order to prevent any still hanging around risks of deleting a socket being in use in another thread.

[gou4shi1]

It looks like std::recursive_mutex.
If C++11 is enabled, we can just use std::recursive_mutex?

[ethouris]

The "shared resource" type is introduced exactly in order to not lock mutexes guarding the container access in order to run some longer procedure on an object contained in this container.

[gou4shi1]

The "shared resource" type is introduced exactly in order to not lock mutexes guarding the container access in order to run some longer procedure on an object contained in this container.

It looks like std::shared_ptr?

[ethouris]

No. This is for objects potentially being used by multiple threads at a time and at some point they have to be deleted.

[gou4shi1]

No. This is for objects potentially being used by multiple threads at a time and at some point they have to be deleted.

If all users of the shared_ptr (from different threads) end, the resource will be deleted automatically.

[gou4shi1]

It looks like std::shared_ptr?

For example, m_Sockets could be std::map<SOCKETID, std::shared_ptr<CUDTSocket>>, all other containers only store std::weak_ptr<CUDTSocket> (meaning that they don't hold the longtime ownership of that socket), if some function need a temporary ownership of that socket, it can just use std::weak_ptr<CUDTSocket>::lock(), without need to hold m_GlobControlLock in the whole function to guard the socket existence.

[maxsharabayko]

Closing as outdated and partially covered by PR #2981 (a SharedMutex class).

[codecov]

Codecov Report

Attention: Patch coverage is 89.83051% with 6 lines in your changes missing coverage. Please review.

Project coverage is 65.93%. Comparing base (1dacc2a) to head (dfe367a).
Report is 410 commits behind head on master.

Files	Patch %	Lines
srtcore/sync.cpp	85.71%	3 Missing ⚠️
srtcore/sync.h	91.30%	2 Missing ⚠️
srtcore/core.cpp	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2293      +/-   ##
==========================================
+ Coverage   65.67%   65.93%   +0.25%     
==========================================
  Files          95       95              
  Lines       19520    19569      +49     
==========================================
+ Hits        12819    12902      +83     
+ Misses       6701     6667      -34     

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