loop_sdcard: Add loopable SD card file sections

To support continuous belt printing, add nestable repeat
loop support via an `[sdcard_loop]` module.

Supported G-Code:
  - SDCARD_LOOP_BEGIN COUNT=n  ; Loop for N times, or infinitely if N is 0
  - SDCARD_LOOP_END            ; End of loop
  - SDCARD_LOOP_DESIST         ; Complete all loops without iterating

Marlin M808 compatibility example in `config/sample-macros.cfg`:
  - M808 Ln        ; Loop for N times, or infinitely if N is 0
  - M808           ; End of loop
  - M808 K         ; Complete all loops without iterating

Added unit tests in test/klippy/sdcard_loop.test

See https://reprap.org/wiki/G-code#M808:_Set_or_Goto_Repeat_Marker

Signed-off-by: Jason S. McMullan <jason.mcmullan@gmail.com>

klippy/extras/sdcard_loop.py

@@ -0,0 +1,73 @@
+# Sdcard file looping support
+#
+# Copyright (C) 2021  Jason S. McMullan <jason.mcmullan@gmail.com>
+#
+# This file may be distributed under the terms of the GNU GPLv3 license.
+
+import logging
+
+class SDCardLoop:
+    def __init__(self, config):
+        printer = config.get_printer()
+        self.sdcard = printer.load_object(config, "virtual_sdcard")
+        self.gcode = printer.lookup_object('gcode')
+        self.gcode.register_command(
+            "SDCARD_LOOP_BEGIN", self.cmd_SDCARD_LOOP_BEGIN,
+            desc=self.cmd_SDCARD_LOOP_BEGIN_help)
+        self.gcode.register_command(
+            "SDCARD_LOOP_END", self.cmd_SDCARD_LOOP_END,
+            desc=self.cmd_SDCARD_LOOP_END_help)
+        self.gcode.register_command(
+            "SDCARD_LOOP_DESIST", self.cmd_SDCARD_LOOP_DESIST,
+            desc=self.cmd_SDCARD_LOOP_DESIST_help)
+        self.loop_stack = []
+    cmd_SDCARD_LOOP_BEGIN_help = "Begins a looped section in the SD file."
+    def cmd_SDCARD_LOOP_BEGIN(self, gcmd):
+        count = gcmd.get_int("COUNT", minval=0)
+        if not self.loop_begin(count):
+            raise gcmd.error("Only permitted in SD file.")
+    cmd_SDCARD_LOOP_END_help = "Ends a looped section in the SD file."
+    def cmd_SDCARD_LOOP_END(self, gcmd):
+        if not self.loop_end():
+            raise gcmd.error("Only permitted in SD file.")
+    cmd_SDCARD_LOOP_DESIST_help = "Stops iterating the current loop stack."
+    def cmd_SDCARD_LOOP_DESIST(self, gcmd):
+        if not self.loop_desist():
+            raise gcmd.error("Only permitted outside of a SD file.")
+    def loop_begin(self, count):
+        if not self.sdcard.is_cmd_from_sd():
+            # Can only run inside of an SD file
+            return False
+        self.loop_stack.append((count, self.sdcard.get_file_position()))
+        return True
+    def loop_end(self):
+        if not self.sdcard.is_cmd_from_sd():
+            # Can only run inside of an SD file
+            return False
+        # If the stack is empty, no need to skip back
+        if len(self.loop_stack) == 0:
+            return True
+        # Get iteration count and return position
+        count, position = self.loop_stack.pop()
+        if count == 0: # Infinite loop
+            self.sdcard.set_file_position(position)
+            self.loop_stack.append((0, position))
+        elif count == 1: # Last repeat
+            # Nothing to do
+            pass
+        else:
+            # At the next opportunity, seek back to the start of the loop
+            self.sdcard.set_file_position(position)
+            # Decrement the count by 1, and add the position back to the stack
+            self.loop_stack.append((count - 1, position))
+        return True
+    def loop_desist(self):
+        if self.sdcard.is_cmd_from_sd():
+            # Can only run outside of an SD file
+            return False
+        logging.info("Desisting existing SD loops")
+        self.loop_stack = []
+        return True
+
+def load_config(config):
+    return SDCardLoop(config)