Add JSON-RPC capability

Inline nlohmann/json for the JSON parsing itself, and handle requests
with the same SCGI interface as XML-RPC.

Based off the work in https://github.com/jesec/rtorrent

src/rpc/rpc_manager.h

@@ -0,0 +1,88 @@
+#ifndef RTORRENT_RPC_MANAGER_H
+#define RTORRENT_RPC_MANAGER_H
+
+#include <cstdint>
+#include <functional>
+
+#include "rpc/command.h"
+#include "rpc/jsonrpc.h"
+#include "rpc/xmlrpc.h"
+
+namespace core {
+class Download;
+}
+
+namespace torrent {
+class File;
+class Object;
+class Tracker;
+} // namespace torrent
+
+namespace rpc {
+
+class rpc_error : public torrent::base_error {
+public:
+  rpc_error(int type, std::string msg) :
+      m_type(type), m_msg(msg) {}
+  virtual ~rpc_error() throw() {}
+
+  virtual int         type() const throw() { return m_type; }
+  virtual const char* what() const throw() { return m_msg.c_str(); }
+
+private:
+  int         m_type;
+  std::string m_msg;
+};
+
+class RpcManager {
+public:
+  using slot_download          = std::function<core::Download*(const char*)>;
+  using slot_file              = std::function<torrent::File*(core::Download*, uint32_t)>;
+  using slot_tracker           = std::function<torrent::Tracker*(core::Download*, uint32_t)>;
+  using slot_peer              = std::function<torrent::Peer*(core::Download*, const torrent::HashString&)>;
+  using slot_response_callback = std::function<bool(const char*, uint32_t)>;
+
+  enum RPCType { XML,
+                 JSON };
+
+  RpcManager()  = default;
+  ~RpcManager() = default;
+
+  void           initialize();
+  void           cleanup();
+  bool           is_initialized() const;
+
+  int64_t        size_limit() { return m_xmlrpc.size_limit(); };
+  void           set_size_limit(uint64_t size) { m_xmlrpc.set_size_limit(size); };
+  int            dialect() { return m_xmlrpc.dialect(); }
+  void           set_dialect(int dialect) { m_xmlrpc.set_dialect(dialect); }
+
+  bool           is_type_enabled(RPCType type) const;
+  void           set_type_enabled(RPCType type, bool enabled);
+
+  bool           process(RPCType type, const char* in_buffer, uint32_t length, slot_response_callback callback);
+
+  void           insert_command(const char* name, const char* parm, const char* doc);
+  static void    object_to_target(const torrent::Object& obj, int callFlags, rpc::target_type* target);
+
+  slot_download& slot_find_download() { return m_slot_find_download; }
+  slot_file&     slot_find_file() { return m_slot_find_file; }
+  slot_tracker&  slot_find_tracker() { return m_slot_find_tracker; }
+  slot_peer&     slot_find_peer() { return m_slot_find_peer; }
+
+private:
+  XmlRpc        m_xmlrpc;
+  JsonRpc       m_jsonrpc;
+
+  bool          m_initialized        = false;
+  bool          m_is_jsonrpc_enabled = true;
+  bool          m_is_xmlrpc_enabled  = true;
+
+  slot_download m_slot_find_download;
+  slot_file     m_slot_find_file;
+  slot_tracker  m_slot_find_tracker;
+  slot_peer     m_slot_find_peer;
+};
+} // namespace rpc
+
+#endif