fix: never auto-detect another device's ttyACM port; udev rule as 70-

README.md

@@ -155,7 +155,7 @@ telling ModemManager to leave the port alone is cleaner — `./install.sh --udev
 or by hand:
 
 ```bash
-sudo cp packaging/99-skydimo.rules /etc/udev/rules.d/
+sudo cp packaging/70-skydimo.rules /etc/udev/rules.d/
 sudo udevadm control --reload-rules && sudo udevadm trigger
 ```
 

install.sh

@@ -88,7 +88,7 @@ do_uninstall() {
     ok "removed program files, service and launcher"
     say ""
     say "Your settings were left at $CONFDIR/config.json"
-    say "The udev rule, if you installed it, is at /etc/udev/rules.d/99-skydimo.rules"
+    say "The udev rule, if you installed it, is at /etc/udev/rules.d/70-skydimo.rules"
     exit 0
 }
 
@@ -334,13 +334,17 @@ command -v update-desktop-database >/dev/null 2>&1 && \
     update-desktop-database "$DESKTOPDIR" 2>/dev/null || true
 ok "desktop entry: $DESKTOPDIR/$APP_NAME.desktop"
 
-cp "$SRC_DIR/packaging/99-skydimo.rules" "$PREFIX/"
+cp "$SRC_DIR/packaging/70-skydimo.rules" "$PREFIX/"
+rm -f "$PREFIX/99-skydimo.rules"
 cp "$SRC_DIR/README.md" "$PREFIX/" 2>/dev/null || true
 
 if [ "$DO_UDEV" = 1 ]; then
     step "Installing the udev rule"
-    if sudo install -m 0644 "$SRC_DIR/packaging/99-skydimo.rules" \
-            /etc/udev/rules.d/99-skydimo.rules; then
+    # Older versions installed it as 99-, which sorts after 73-seat-late.rules
+    # and so never got the uaccess ACL.
+    if sudo install -m 0644 "$SRC_DIR/packaging/70-skydimo.rules" \
+            /etc/udev/rules.d/70-skydimo.rules; then
+        sudo rm -f /etc/udev/rules.d/99-skydimo.rules
         sudo udevadm control --reload-rules && sudo udevadm trigger || true
         ok "ModemManager will leave the adapter alone"
     else

packaging/70-skydimo.rules

@@ -0,0 +1,14 @@
+# Keep ModemManager away from the Skydimo adapter's CH340 bridge, hand it to the
+# logged-in user, and name it /dev/skydimo so the driver never has to guess
+# which serial port is the strip. Without the ModemManager part, it probes the
+# port with AT commands for a few seconds after plug-in, which can collide with
+# the driver's handshake.
+#
+# The 70- prefix matters: TAG+="uaccess" is acted on by 73-seat-late.rules, so a
+# rule sorted after it (99-...) sets the tag too late and the user gets no ACL.
+# Install with:
+#   sudo cp ~/.local/share/skydimo/70-skydimo.rules /etc/udev/rules.d/
+#   sudo udevadm control --reload-rules && sudo udevadm trigger
+SUBSYSTEM=="tty", ATTRS{idVendor}=="1a86", ATTRS{idProduct}=="7523", \
+  ENV{ID_MM_DEVICE_IGNORE}="1", ENV{ID_MM_PORT_IGNORE}="1", TAG+="uaccess", \
+  SYMLINK+="skydimo"

src/skydimo/device.py

@@ -37,6 +37,7 @@
 from __future__ import annotations
 
 import glob
+import os
 import time
 
 import serial
@@ -45,6 +46,7 @@
 BAUD = 1_000_000
 VID_CH340 = 0x1A86
 PID_CH340 = 0x7523
+UDEV_SYMLINK = "/dev/skydimo"  # created by packaging/70-skydimo.rules
 
 HDR_MAGIC0 = 0x54  # 'T'
 HDR_MAGIC1 = 0x6E  # 'n'
@@ -68,14 +70,18 @@ class HandshakeError(RuntimeError):
 
 def find_port() -> str:
     """Locate the adapter's serial port (CH340 bridge)."""
+    # The udev rule names the adapter, so this never has to guess.
+    if os.path.exists(UDEV_SYMLINK):
+        return UDEV_SYMLINK
     for p in list_ports.comports():
         if p.vid == VID_CH340 and p.pid == PID_CH340:
             return p.device
-    # Fall back to any USB serial node; the handshake will confirm identity.
-    for pattern in ("/dev/ttyUSB*", "/dev/ttyACM*"):
-        found = sorted(glob.glob(pattern))
-        if found:
-            return found[0]
+    # Fall back to USB serial bridges only. The CH340 never enumerates as
+    # ttyACM, and ttyACM nodes are other people's CDC devices (LCD panels,
+    # microcontrollers): the DTR reset and 1 Mbaud handshake would disrupt them.
+    found = sorted(glob.glob("/dev/ttyUSB*"))
+    if found:
+        return found[0]
     raise FileNotFoundError(
         "No USB serial adapter found. Is the Skydimo adapter plugged in?"
     )

tests/test_core.py

@@ -314,5 +314,27 @@ def test_sampling_picks_up_edge_position(self):
         self.assertGreater(top.mean(), bottom.mean() + 10)
 
 
+class TestFindPort(unittest.TestCase):
+    def _find(self, exists=False, comports=(), globbed=()):
+        from unittest import mock
+        from skydimo import device
+        with mock.patch.object(device.os.path, "exists", return_value=exists), \
+             mock.patch.object(device.list_ports, "comports", return_value=list(comports)), \
+             mock.patch.object(device.glob, "glob",
+                               side_effect=lambda pat: [g for g in globbed if g.startswith(pat[:-1])]):
+            return device.find_port()
+
+    def test_prefers_udev_symlink(self):
+        self.assertEqual(self._find(exists=True, globbed=["/dev/ttyUSB0"]), "/dev/skydimo")
+
+    def test_never_falls_back_to_ttyacm(self):
+        # e.g. an LCD panel on ttyACM0 with no Skydimo adapter plugged in
+        with self.assertRaises(FileNotFoundError):
+            self._find(globbed=["/dev/ttyACM0"])
+
+    def test_falls_back_to_ttyusb(self):
+        self.assertEqual(self._find(globbed=["/dev/ttyACM0", "/dev/ttyUSB1"]), "/dev/ttyUSB1")
+
+
 if __name__ == "__main__":
     unittest.main()