Introduce macro ASCS_USE_DISPATCH_IN_IO to force to use dispatch_strand in IO operations.

For recv_msg, go back to dispatch_strand if macro ASCS_PASSIVE_RECV is not defined because it
 cannot be invoked concurrently without the macro.

asio @ 9902fe7d

-Subproject commit 5302adea05c8a1c5d3923fa93afd7cf437df5937
+Subproject commit 9902fe7d1d6b5321f4ab2f4ddbfd19cf39160891

include/ascs/config.h

@@ -811,6 +811,8 @@
  * Graceful shutdown does not support sync mode anymore.
  * Use post_strand instead of dispatch_strand in send_msg and recv_msg, because we don't synchronize socket's member variable sending and reading,
  *  there's still a race condition even in the same strand because of memory synchronization.
+ *  But we can difine macro ASCS_USE_DISPATCH_IN_IO to use dispatch_strand back, because dispatch_strand can be more efficient in specific cases.
+ *  The macro will be removed after I confirmed that dispatch_strand indeed introduces race condition.
  *
  * HIGHLIGHT:
  * Make shutdown thread safe.

include/ascs/socket.h

@@ -168,20 +168,32 @@ public:
 
 #ifdef ASCS_PASSIVE_RECV
 	bool is_reading() const {return reading;}
+#ifdef ASCS_USE_DISPATCH_IN_IO
+	void recv_msg() {if (!reading && is_ready()) dispatch_in_io_strand([this]() {this->do_recv_msg();});}
+#else
 	void recv_msg() {if (!reading && is_ready()) post_in_io_strand([this]() {this->do_recv_msg();});}
+#endif
 #else
 private:
-	void recv_msg() {post_in_io_strand([this]() {this->do_recv_msg();});}
+	void recv_msg() {dispatch_in_io_strand([this]() {this->do_recv_msg();});}
 public:
 #endif
 #ifndef ASCS_EXPOSE_SEND_INTERFACE
 protected:
 #endif
+#ifdef ASCS_USE_DISPATCH_IN_IO
+#ifdef ASCS_ARBITRARY_SEND
+	void send_msg() {dispatch_in_io_strand([this]() {this->do_send_msg();});}
+#else
+	void send_msg() {if (!sending && is_ready()) dispatch_in_io_strand([this]() {this->do_send_msg();});}
+#endif
+#else
 #ifdef ASCS_ARBITRARY_SEND
 	void send_msg() {post_in_io_strand([this]() {this->do_send_msg();});}
 #else
 	void send_msg() {if (!sending && is_ready()) post_in_io_strand([this]() {this->do_send_msg();});}
 #endif
+#endif
 
 public:
 	void start_heartbeat(int interval, int max_absence = ASCS_HEARTBEAT_MAX_ABSENCE)
@@ -664,7 +676,7 @@ private:
 		return false;
 	}
 
-	//do not use dispatch_strand at here, because the handler (do_dispatch_msg) may call this function, which can lead stack overflow.
+	//do not use dispatch_strand/dispatch_in_dis_strand at here, because the handler (do_dispatch_msg) may call this function, which can lead stack overflow.
 	void dispatch_msg() {if (!dispatching) post_in_dis_strand([this]() {this->do_dispatch_msg();});}
 	void do_dispatch_msg()
 	{