Merge pull request #1446 from AntelopeIO/drain_ship_session_strand_11x

[1.1.5] fix state history race on disconnect: drain session strand before destroying session

Author: spoonincode

plugins/state_history_plugin/include/eosio/state_history_plugin/session.hpp

@@ -25,6 +25,8 @@ class session_base {
 
    virtual void block_applied(const chain::block_num_type applied_block_num) = 0;
 
+   virtual void drain_strand() = 0;
+
    virtual ~session_base() = default;
 };
 
@@ -53,6 +55,11 @@ class session final : public session_base {
       awake_if_idle();
    }
 
+   // allow main thread to drain the strand before destruction -- some awake_if_idle() post()s may be inflight
+   void drain_strand() {
+      boost::asio::post(strand, boost::asio::use_future([](){})).get();
+   }
+
 private:
    std::string get_remote_endpoint_string() const {
       try {

plugins/state_history_plugin/state_history_plugin.cpp

@@ -117,6 +117,12 @@ struct state_history_plugin_impl {
                                                   },
                                                   [this](session_base* conn) {
                                                      app().executor().post(priority::high, exec_queue::read_write, [conn, this]() {
+                                                        //Main thread may have post()s inflight to session strand (via block_applied() -> awake_if_idle()) that
+                                                        // could execute during destruction. Drain any possible post() before destruction. This is in main
+                                                        // thread now so guaranteed no new block_applied() will be called during these lines below, and the
+                                                        // session has already indicated it is "done" so it will not be running any operations of its own
+                                                        // on the strand
+                                                        conn->drain_strand();
                                                         connections.erase(connections.find(conn));
                                                      });
                                                   }, _log));