Adding Line Error Resend Control

Initial commit with Line Error Resend control to tackle resend loops
with host >> printer high latency allowing GCode in flight.
eg, Octoprint>[wifi]>ESP3D>>Printer

Existing Resend method, clears RX_Buffer, sends request
    Unable to clear consecutive N lines already release from Octoprint
    host(In-flight). Printer receives expecting requested line,
    triggering additional resend request. Printer eventually receives
    correct line, multiple times, triggering further resend requests
    for next line.

Marlin/Configuration_adv.h

@@ -2405,6 +2405,21 @@
   //#define SERIAL_XON_XOFF
 #endif
 
+/**
+ * Line Number Error Handler
+ * 
+ * Add function to delay resend requests for line number 
+ * errors, for Hosts that send N# line number with GCode.
+ * Prevents resend cycles causing stuttering and failure
+ * on systems with latency between Host and Printer.
+ * eg. Octoprint through ESP8266(WIFI) pass-through
+*/
+#define RESEND_HANDLER
+#if ENABLED(RESEND_HANDLER)
+  #define RESEND_HANDLER_DROP_GCODE 1   //Number of GCode lines to drop before resend request sent to Host. Octprint>>ESP3D; Min 5
+  //#define RESEND_HANDLER_NOTICE       //Send additional details to host terminal
+#endif
+
 #if ENABLED(SDSUPPORT)
   // Enable this option to collect and display the maximum
   // RX queue usage after transferring a file to SD.

Marlin/src/gcode/queue.cpp

@@ -62,6 +62,9 @@ PGMSTR(G28_STR, "G28");
 
 GCodeQueue::SerialState GCodeQueue::serial_state[NUM_SERIAL] = { 0 };
 GCodeQueue::RingBuffer GCodeQueue::ring_buffer = { 0 };
+#if ENABLED(RESEND_HANDLER)
+  GCodeQueue::ResendCtrl GCodeQueue::resend_ctrl = { 0 };
+#endif
 
 #if NO_TIMEOUTS > 0
   static millis_t last_command_time = 0;
@@ -280,6 +283,25 @@ void GCodeQueue::flush_and_request_resend(const serial_index_t serial_ind) {
   SERIAL_ECHOLNPGM(STR_OK);
 }
 
+/**
+ * Notify host a resend has been skipped, with the offending and expected Line number
+ * Or
+ * send "ok" to keep comms alive
+*/
+  void GCodeQueue::ln_num_error_notice(const serial_index_t serial_ind, const long host_gcode_N) { //~8ms to send @500000 through ESP8266(ESP3D WIFI)>>Octoprint
+    #if HAS_MULTI_SERIAL
+      if (!serial_ind.valid()) return;              // Optimization here, skip if the command came from SD or Flash Drive
+      PORT_REDIRECT(SERIAL_PORTMASK(serial_ind));   // Reply to the serial port that sent the command
+    #endif
+    #if ENABLED(RESEND_HANDLER_NOTICE)
+      serial_echo_start(); // { serial_print(F("Echo:")); }
+      SERIAL_ECHOLNPGM("Host sent incorrect line : ", host_gcode_N);
+      serial_echo_start(); // { serial_print(F("Echo:")); }
+      SERIAL_ECHOLNPGM("Line expected : ", serial_state[serial_ind.index].last_N + 1);
+    #endif
+    SERIAL_ECHOLNPGM(STR_OK); //send Ok to continue action from Host
+  }
+
 static bool serial_data_available(serial_index_t index) {
   const int a = SERIAL_IMPL.available(index);
   #if ENABLED(RX_BUFFER_MONITOR) && RX_BUFFER_SIZE
@@ -467,11 +489,41 @@ void GCodeQueue::get_serial_commands() {
             if (n2pos) npos = n2pos;
           }
 
-          const long gcode_N = strtol(npos + 1, nullptr, 10);
-
+          const long gcode_N = strtol(npos + 1, nullptr, 10); //Host sent GCode Line# from RX_buffer
+
+          /**
+           * Resend Handler - Received line # != expected line number
+           * 
+           * Standard behaviour - Clear RX_Buffer; resend request to host
+           * Extended behaviour - Allow ignore_resend_max # of errors to be ignored. 
+           * Required when latency present between Marlin>>Host allowing GCode in-flight to cause comms issue 
+           * on resend requests, such as stuttering and print failure.
+           * Ignore_resend_max should not exceed expected in-flight GCode + RX_Buffer. RX_Buffer is cleared in 
+           * gcode_line_error, not ln_num_error_notice as next serial in buffer may be the required line.
+          */
           if (gcode_N != serial.last_N + 1 && !M110) {
             // In case of error on a serial port, don't prevent other serial port from making progress
-            gcode_line_error(F(STR_ERR_LINE_NO), p);
+            #if ENABLED(RESEND_HANDLER)
+              const serial_index_t serial_ind = p;
+
+              if ((resend_ctrl.ignore_resend_count < resend_ctrl.ignore_resend_max - 1)) {            //Threshold eliminated resends
+                if(serial_state[serial_ind.index].last_N != resend_ctrl.last_error_N){                //Is first error instance
+                  ln_num_error_notice(p, gcode_N);
+                  resend_ctrl.last_error_N = serial_state[serial_ind.index].last_N;         //Set last ignored error line
+                  resend_ctrl.ignore_resend_count = 0;                                      //Reset count as first instance                                         
+                }
+                else {
+                  ln_num_error_notice(p, gcode_N);
+                }
+                resend_ctrl.ignore_resend_count += 1; //Capture anything that doesn't fall in prev if_stmt. Nothing should miss
+              }
+              else{                                         //Exceeded maximum deleted requests or is a new resend request
+                resend_ctrl.ignore_resend_count = 0;          //Reset counter
+                gcode_line_error(F(STR_ERR_LINE_NO), p);      //Send resend request
+              }
+            #else
+              gcode_line_error(F(STR_ERR_LINE_NO), p);      //Send resend request
+            #endif
             break;
           }
 

Marlin/src/gcode/queue.h

@@ -100,6 +100,21 @@ class GCodeQueue {
     inline char* peek_next_command_string() { return peek_next_command().buffer; }
   };
 
+  /**
+   * Resend handler to deal with Host>>Printer latency
+  */
+  #if ENABLED(RESEND_HANDLER)
+    struct ResendCtrl {
+      long last_error_N = 0; //Record the last requested resend line number
+      uint8_t ignore_resend_count = 0;
+      const uint8_t ignore_resend_max = RESEND_HANDLER_DROP_GCODE; //Number of resends requests deleted //LH this can be constant
+    };
+
+    static ResendCtrl resend_ctrl; //resend ctrl variables
+
+    static void ln_num_error_notice(const serial_index_t serial_ind, const long host_gcode_N);
+  #endif
+
   /**
    * The ring buffer of commands
    */