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diff --git a/src/shrpx_http2_session.h b/src/shrpx_http2_session.h
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+/*
+ * nghttp2 - HTTP/2 C Library
+ *
+ * Copyright (c) 2012 Tatsuhiro Tsujikawa
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+#ifndef SHRPX_HTTP2_SESSION_H
+#define SHRPX_HTTP2_SESSION_H
+
+#include "shrpx.h"
+
+#include <unordered_set>
+#include <memory>
+
+#include <openssl/ssl.h>
+
+#include <ev.h>
+
+#include <nghttp2/nghttp2.h>
+
+#include "llhttp.h"
+
+#include "shrpx_connection.h"
+#include "buffer.h"
+#include "template.h"
+
+using namespace nghttp2;
+
+namespace shrpx {
+
+class Http2DownstreamConnection;
+class Worker;
+class Downstream;
+struct DownstreamAddrGroup;
+struct DownstreamAddr;
+struct DNSQuery;
+
+struct StreamData {
+  StreamData *dlnext, *dlprev;
+  Http2DownstreamConnection *dconn;
+};
+
+enum class FreelistZone {
+  // Http2Session object is not linked in any freelist.
+  NONE,
+  // Http2Session object is linked in address scope
+  // http2_extra_freelist.
+  EXTRA,
+  // Http2Session object is about to be deleted, and it does not
+  // belong to any linked list.
+  GONE
+};
+
+enum class Http2SessionState {
+  // Disconnected
+  DISCONNECTED,
+  // Connecting proxy and making CONNECT request
+  PROXY_CONNECTING,
+  // Tunnel is established with proxy
+  PROXY_CONNECTED,
+  // Establishing tunnel is failed
+  PROXY_FAILED,
+  // Connecting to downstream and/or performing SSL/TLS handshake
+  CONNECTING,
+  // Connected to downstream
+  CONNECTED,
+  // Connection is started to fail
+  CONNECT_FAILING,
+  // Resolving host name
+  RESOLVING_NAME,
+};
+
+enum class ConnectionCheck {
+  // Connection checking is not required
+  NONE,
+  // Connection checking is required
+  REQUIRED,
+  // Connection checking has been started
+  STARTED,
+};
+
+class Http2Session {
+public:
+  Http2Session(struct ev_loop *loop, SSL_CTX *ssl_ctx, Worker *worker,
+               const std::shared_ptr<DownstreamAddrGroup> &group,
+               DownstreamAddr *addr);
+  ~Http2Session();
+
+  // If hard is true, all pending requests are abandoned and
+  // associated ClientHandlers will be deleted.
+  int disconnect(bool hard = false);
+  int initiate_connection();
+  int resolve_name();
+
+  void add_downstream_connection(Http2DownstreamConnection *dconn);
+  void remove_downstream_connection(Http2DownstreamConnection *dconn);
+
+  void remove_stream_data(StreamData *sd);
+
+  int submit_request(Http2DownstreamConnection *dconn, const nghttp2_nv *nva,
+                     size_t nvlen, const nghttp2_data_provider *data_prd);
+
+  int submit_rst_stream(int32_t stream_id, uint32_t error_code);
+
+  int terminate_session(uint32_t error_code);
+
+  nghttp2_session *get_session() const;
+
+  int resume_data(Http2DownstreamConnection *dconn);
+
+  int connection_made();
+
+  int do_read();
+  int do_write();
+
+  int on_read(const uint8_t *data, size_t datalen);
+  int on_write();
+
+  int connected();
+  int read_clear();
+  int write_clear();
+  int tls_handshake();
+  int read_tls();
+  int write_tls();
+  // This is a special write function which just stop write event
+  // watcher.
+  int write_void();
+
+  int downstream_read_proxy(const uint8_t *data, size_t datalen);
+  int downstream_connect_proxy();
+
+  int downstream_read(const uint8_t *data, size_t datalen);
+  int downstream_write();
+
+  int noop();
+  int read_noop(const uint8_t *data, size_t datalen);
+  int write_noop();
+
+  void signal_write();
+
+  struct ev_loop *get_loop() const;
+
+  ev_io *get_wev();
+
+  Http2SessionState get_state() const;
+  void set_state(Http2SessionState state);
+
+  void start_settings_timer();
+  void stop_settings_timer();
+
+  SSL *get_ssl() const;
+
+  int consume(int32_t stream_id, size_t len);
+
+  // Returns true if request can be issued on downstream connection.
+  bool can_push_request(const Downstream *downstream) const;
+  // Initiates the connection checking if downstream connection has
+  // been established and connection checking is required.
+  void start_checking_connection();
+  // Resets connection check timer to timeout |t|.  After timeout, we
+  // require connection checking.  If connection checking is already
+  // enabled, this timeout is for PING ACK timeout.
+  void reset_connection_check_timer(ev_tstamp t);
+  void reset_connection_check_timer_if_not_checking();
+  // Signals that connection is alive.  Internally
+  // reset_connection_check_timer() is called.
+  void connection_alive();
+  // Change connection check state.
+  void set_connection_check_state(ConnectionCheck state);
+  ConnectionCheck get_connection_check_state() const;
+
+  bool should_hard_fail() const;
+
+  void submit_pending_requests();
+
+  DownstreamAddr *get_addr() const;
+
+  const std::shared_ptr<DownstreamAddrGroup> &get_downstream_addr_group() const;
+
+  int handle_downstream_push_promise(Downstream *downstream,
+                                     int32_t promised_stream_id);
+  int handle_downstream_push_promise_complete(Downstream *downstream,
+                                              Downstream *promised_downstream);
+
+  // Returns number of downstream connections, including pushed
+  // streams.
+  size_t get_num_dconns() const;
+
+  // Adds to group scope http2_avail_freelist.
+  void add_to_avail_freelist();
+  // Adds to address scope http2_extra_freelist.
+  void add_to_extra_freelist();
+
+  // Removes this object from any freelist.  If this object is not
+  // linked from any freelist, this function does nothing.
+  void remove_from_freelist();
+
+  // Removes this object form any freelist, and marks this object as
+  // not schedulable.
+  void exclude_from_scheduling();
+
+  // Returns true if the maximum concurrency is reached.  In other
+  // words, the number of currently participated streams in this
+  // session is equal or greater than the max concurrent streams limit
+  // advertised by server.  If |extra| is nonzero, it is added to the
+  // number of current concurrent streams when comparing against
+  // server initiated concurrency limit.
+  bool max_concurrency_reached(size_t extra = 0) const;
+
+  DefaultMemchunks *get_request_buf();
+
+  void on_timeout();
+
+  // This is called periodically using ev_prepare watcher, and if
+  // group_ is retired (backend has been replaced), send GOAWAY to
+  // shutdown the connection.
+  void check_retire();
+
+  // Returns address used to connect to backend.  Could be nullptr.
+  const Address *get_raddr() const;
+
+  // This is called when SETTINGS frame without ACK flag set is
+  // received.
+  void on_settings_received(const nghttp2_frame *frame);
+
+  bool get_allow_connect_proto() const;
+
+  using ReadBuf = Buffer<8_k>;
+
+  Http2Session *dlnext, *dlprev;
+
+private:
+  Connection conn_;
+  DefaultMemchunks wb_;
+  ev_timer settings_timer_;
+  // This timer has 2 purpose: when it first timeout, set
+  // connection_check_state_ = ConnectionCheck::REQUIRED.  After
+  // connection check has started, this timer is started again and
+  // traps PING ACK timeout.
+  ev_timer connchk_timer_;
+  // timer to initiate connection.  usually, this fires immediately.
+  ev_timer initiate_connection_timer_;
+  ev_prepare prep_;
+  DList<Http2DownstreamConnection> dconns_;
+  DList<StreamData> streams_;
+  std::function<int(Http2Session &)> read_, write_;
+  std::function<int(Http2Session &, const uint8_t *, size_t)> on_read_;
+  std::function<int(Http2Session &)> on_write_;
+  // Used to parse the response from HTTP proxy
+  std::unique_ptr<llhttp_t> proxy_htp_;
+  Worker *worker_;
+  // NULL if no TLS is configured
+  SSL_CTX *ssl_ctx_;
+  std::shared_ptr<DownstreamAddrGroup> group_;
+  // Address of remote endpoint
+  DownstreamAddr *addr_;
+  nghttp2_session *session_;
+  // Actual remote address used to contact backend.  This is initially
+  // nullptr, and may point to either &addr_->addr,
+  // resolved_addr_.get(), or HTTP proxy's address structure.
+  const Address *raddr_;
+  // Resolved IP address if dns parameter is used
+  std::unique_ptr<Address> resolved_addr_;
+  std::unique_ptr<DNSQuery> dns_query_;
+  Http2SessionState state_;
+  ConnectionCheck connection_check_state_;
+  FreelistZone freelist_zone_;
+  // true if SETTINGS without ACK is received from peer.
+  bool settings_recved_;
+  // true if peer enables RFC 8441 CONNECT protocol.
+  bool allow_connect_proto_;
+};
+
+nghttp2_session_callbacks *create_http2_downstream_callbacks();
+
+} // namespace shrpx
+
+#endif // SHRPX_HTTP2_SESSION_H