Before this change the code used to count references to CNode objects manually via CNode::nRefCount. Unneeded CNodes were scheduled for deletion by putting them in CConnman::m_nodes_disconnected and were deleted after their reference count reached zero. Deleting consists of calling PeerManager::FinalizeNode() and destroying the CNode object.
Replace this scheme with std::shared_ptr. This simplifies the code and removes:
CNode::nRefCount
CNode::GetRefCount()
CNode::AddRef()
CNode::Release()
CConnman::m_nodes_disconnected
CConnman::NodesSnapshot
Now creating a snapshot of CConnman::m_nodes is done by simply copying it (under the mutex).
Call PeerManager::FinalizeNode() from the destructor of CNode, which is called when the reference count reaches 0.
This removes brittle code and replaces it by a code from the standard library which is more robust. All the below are possible with the manual reference counting and not possible with shared_ptr:
- Use an object without adding a reference to it
- Add too many references to an object, mismatch the add/remove by having more “add”
- Forget to reduce the reference count, mismatch the add/remove by having less “remove”
- Go with negative reference count by mistakenly having too many “remove”
- Destroy an object while there are references to it
There is zero learning curve for the average C++ programmer who knows how shared_ptr works. Not so much with the manual reference counting because one has to learn about AddRef(), Release(), check and maintain where are they called and their relationship with m_nodes_disconnected.
This has some history in #28222 and #10738. See below #32015 (comment) and #32015 (comment).
