Merge bitcoin/bitcoin#21750: net: remove unnecessary check of CNode::cs_vSend

9096b13 net: remove unnecessary check of CNode::cs_vSend (Vasil Dimov)

Pull request description:

  It is not possible to have a node in `CConnman::vNodesDisconnected` and
  its reference count to be incremented - all `CNode::AddRef()` are done
  either before the node is added to `CConnman::vNodes` or while holding
  `CConnman::cs_vNodes` and the object being in `CConnman::vNodes`.

  So, the object being in `CConnman::vNodesDisconnected` and its reference
  count being zero means that it is not and will not start to be used by
  other threads.

  So, the lock of `CNode::cs_vSend` in `CConnman::DisconnectNodes()` will
  always succeed and is not necessary.

  Indeed all locks of `CNode::cs_vSend` are done either when the reference
  count is >0 or under the protection of `CConnman::cs_vNodes` and the
  node being in `CConnman::vNodes`.

ACKs for top commit:
  MarcoFalke:
    review ACK 9096b13 🏧
  jnewbery:
    utACK 9096b13

Tree-SHA512: 910899cdcdc8934642eb0c40fcece8c3b01b7e20a0b023966b9d6972db6a885cb3a9a04e9562bae14d5833967e45e2ecb3687b94d495060c3da4b1f2afb0ac8f

Author: MarcoFalke

src/net.cpp

@@ -1224,19 +1224,10 @@ void CConnman::DisconnectNodes()
         std::list<CNode*> vNodesDisconnectedCopy = vNodesDisconnected;
         for (CNode* pnode : vNodesDisconnectedCopy)
         {
-            // wait until threads are done using it
+            // Destroy the object only after other threads have stopped using it.
             if (pnode->GetRefCount() <= 0) {
-                bool fDelete = false;
-                {
-                    TRY_LOCK(pnode->cs_vSend, lockSend);
-                    if (lockSend) {
-                        fDelete = true;
-                    }
-                }
-                if (fDelete) {
-                    vNodesDisconnected.remove(pnode);
-                    DeleteNode(pnode);
-                }
+                vNodesDisconnected.remove(pnode);
+                DeleteNode(pnode);
             }
         }
     }