[net] Move asmap into NetGroupManager

src/netgroup.h

@@ -5,9 +5,41 @@
 #ifndef BITCOIN_NETGROUP_H
 #define BITCOIN_NETGROUP_H
 
+#include <vector>
+
 /**
  * Netgroup manager
  */
-class NetGroupManager {};
+class NetGroupManager {
+public:
+    explicit NetGroupManager(std::vector<bool> asmap)
+        : m_asmap{std::move(asmap)}
+    {}
+
+    /* Get a reference to (const) asmap. May be held as long as NetGroupManager
+     * exists, since the data is const. */
+    const std::vector<bool>& GetAsmap() const { return m_asmap; }
+
+private:
+    /** Compressed IP->ASN mapping, loaded from a file when a node starts.
+     *
+     * This mapping is then used for bucketing nodes in Addrman and for
+     * ensuring we connect to a diverse set of peers in Connman. The map is
+     * empty if no file was provided.
+     *
+     * If asmap is provided, nodes will be bucketed by AS they belong to, in
+     * order to make impossible for a node to connect to several nodes hosted
+     * in a single AS. This is done in response to Erebus attack, but also to
+     * generally diversify the connections every node creates, especially
+     * useful when a large fraction of nodes operate under a couple of cloud
+     * providers.
+     *
+     * If a new asmap is provided, the existing addrman records are
+     * re-bucketed.
+     *
+     * This is initialized in the constructor, const, and therefore is
+     * thread-safe. */
+    const std::vector<bool> m_asmap;
+};
 
 #endif // BITCOIN_NETGROUP_H