simulator: Wait for DDA to complete before exiting

When the simulator finishes processing the last gcode, it exits.  This
leaves causes us to exit without completing the last commanded move.

Rework how we handle end-of-file parsing and wait for the dda queue to
empty before exiting at the end of file processing.

simulator/serial_sim.c

@@ -8,6 +8,7 @@
 #include <sys/stat.h>
 #include <stdlib.h>
 
+#include "dda_queue.h"
 #include "serial.h"
 #include "simulator.h"
 
@@ -95,29 +96,36 @@ uint8_t serial_rxchars(void) {
     ioctl(serial_fd, FIONREAD, &rx_chars_nb);
     return rx_chars_nb;
   }
-  // File always has more data
-  return 1;
+
+  // An open file always has more data
+  if (gcode_fd) return 1;
+
+  // No more gcode data; wait for DDA queue to drain
+  if (queue_empty()) {
+    sim_info("Gcode processing completed.");
+    exit(0);
+  }
+
+  // Nothing to read from
+  return 0;
 }
 
 // read one character
 uint8_t serial_popchar(void) {
-  uint8_t c;
-  ssize_t count;
+  uint8_t c = 0;
+  ssize_t count = 0;
   int fd = serial_fd ? serial_fd : gcode_fd;
 
   sim_assert(serial_initialised, "serial interface not initialised");
   sim_assert(serial_rxchars() > 0, "no chars to read");
-  count = read(fd, &c, 1);
-  if (gcode_fd && !count) {
+  while (fd && !count) {
+    count = read(fd, &c, 1);
+    if (!gcode_fd || count)
+      break;
     // EOF: try to open next file
     open_file();
-    if (gcode_fd || serial_fd)
-      return serial_popchar();
-
-    sim_info("Gcode processing completed.");
-    exit(0);
+    fd = serial_fd ? serial_fd : gcode_fd;
   }
-  sim_assert(count == 1, "no character in serial RX buffer");
   return c;
 }