[net processing] Add Peer

Peer is a struct for holding per-peer data. This structure is not
protected by cs_main since it does not contain validation-critical data.

Author: jnewbery

src/net_processing.cpp

@@ -471,6 +471,43 @@ static CNodeState *State(NodeId pnode) EXCLUSIVE_LOCKS_REQUIRED(cs_main) {
     return &it->second;
 }
 
+/**
+ * Data structure for an individual peer. This struct is not protected by
+ * cs_main since it does not contain validation-critical data.
+ *
+ * Memory is owned by shared pointers and this object is destructed when
+ * the refcount drops to zero.
+ *
+ * TODO: move most members from CNodeState to this structure.
+ * TODO: move remaining application-layer data members from CNode to this structure.
+ */
+struct Peer {
+    /** Same id as the CNode object for this peer */
+    const NodeId m_id{0};
+
+    Peer(NodeId id) : m_id(id) {}
+};
+
+using PeerRef = std::shared_ptr<Peer>;
+
+/**
+ * Map of all Peer objects, keyed by peer id. This map is protected
+ * by the global g_peer_mutex. Once a shared pointer reference is
+ * taken, the lock may be released. Individual fields are protected by
+ * their own locks.
+ */
+Mutex g_peer_mutex;
+static std::map<NodeId, PeerRef> g_peer_map GUARDED_BY(g_peer_mutex);
+
+/** Get a shared pointer to the Peer object.
+ *  May return nullptr if the Peer object can't be found. */
+static PeerRef GetPeerRef(NodeId id)
+{
+    LOCK(g_peer_mutex);
+    auto it = g_peer_map.find(id);
+    return it != g_peer_map.end() ? it->second : nullptr;
+}
+
 static void UpdatePreferredDownload(const CNode& node, CNodeState* state) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
 {
     nPreferredDownload -= state->fPreferredDownload;
@@ -838,6 +875,11 @@ void PeerLogicValidation::InitializeNode(CNode *pnode) {
         LOCK(cs_main);
         mapNodeState.emplace_hint(mapNodeState.end(), std::piecewise_construct, std::forward_as_tuple(nodeid), std::forward_as_tuple(addr, pnode->IsInboundConn(), pnode->IsManualConn()));
     }
+    {
+        PeerRef peer = std::make_shared<Peer>(nodeid);
+        LOCK(g_peer_mutex);
+        g_peer_map.emplace_hint(g_peer_map.end(), nodeid, std::move(peer));
+    }
     if(!pnode->IsInboundConn())
         PushNodeVersion(*pnode, *connman, GetTime());
 }
@@ -865,6 +907,10 @@ void PeerLogicValidation::ReattemptInitialBroadcast(CScheduler& scheduler) const
 void PeerLogicValidation::FinalizeNode(NodeId nodeid, bool& fUpdateConnectionTime) {
     fUpdateConnectionTime = false;
     LOCK(cs_main);
+    {
+        LOCK(g_peer_mutex);
+        g_peer_map.erase(nodeid);
+    }
     CNodeState *state = State(nodeid);
     assert(state != nullptr);