From b246dd8525d603277667b1f2ba499271911eae93 Mon Sep 17 00:00:00 2001
From: Luca Toniolo <10792599+grandixximo@users.noreply.github.com>
Date: Wed, 24 Jun 2026 09:30:06 +0800
Subject: [PATCH] task: reset canon length units on program open

G20/G21 from a previous run persists in canon.lengthUnits, causing
FROM_PROG_LEN() to convert differently on re-run for G-code commands
that appear before the G20/G21 line. This shows up as wrong unit
scaling (off by 25.4x) on the lead-in moves of the second run,
producing jagged velocity in the scope.

Reset canon.lengthUnits to the machine default in emcTaskPlanOpen
before reopening the interpreter.

Fixes #4194
---
 src/emc/task/emctask.cc | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/emc/task/emctask.cc b/src/emc/task/emctask.cc
index b598bd12b52..0ac5c22f3d1 100644
--- a/src/emc/task/emctask.cc
+++ b/src/emc/task/emctask.cc
@@ -14,6 +14,7 @@
 ********************************************************************/
 
 #include <stdlib.h>
+#include <math.h>		// fabs()
 #include <rtapi_string.h>	// rtapi_strlcpy()
 #include <sys/stat.h>		// struct stat
 #include <unistd.h>		// stat()
@@ -548,6 +549,17 @@ int emcTaskPlanOpen(const char *file)
 	emcStatus->task.readLine = 0;
     }
 
+    // Reset canon length units to machine default before re-opening.
+    // Without this, G20/G21 from the previous run persists in
+    // canon.lengthUnits, causing FROM_PROG_LEN() to convert differently
+    // on re-run for G-code commands that appear before G20/G21.
+    double units = GET_EXTERNAL_LENGTH_UNITS();
+    if (fabs(units - 1.0 / 25.4) < 1.0e-3) {
+        USE_LENGTH_UNITS(CANON_UNITS_INCHES);
+    } else if (fabs(units - 1.0) < 1.0e-3) {
+        USE_LENGTH_UNITS(CANON_UNITS_MM);
+    }
+
     int retval = interp.open(file);
     if (retval > INTERP_MIN_ERROR) {
 	print_interp_error(retval);