[Bug 1778833] Re: tty symlink incorrect after disconnect/reconnect, fixed by manual udevadm trigger

[zach]

For the record, this behavior disappears if I change the rule from:
SUBSYSTEMS=="usb", ATTRS{whatever}=="whatever", SYMLINK+="ttyWhatever"
to:
SUBSYSTEM=="tty", ATTRS{whatever}=="whatever", SYMLINK+="ttyWhatever"

The first rule didn't actually match the TTY device at the leaf of the
device tree. I suspect it must have been matching the cdc_acm USB device
that is one level above the TTY device in the hierarchy.

So in all likelihood, the "correct" rule is the second one, And this
rule does work the same way regardless of whether the device is
disconnected and reconnected, or `udevadm trigger` is run.

I do think the current udev behavior is still a bug, though: either the
first rule should always fail, or should always work. The fact that the
symlink works correctly with the first rule only when udevadm is
manually triggered, but not when the device is removed and re-added,
cannot be the expected behavior. (Right?)

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https://bugs.launchpad.net/bugs/1778833

Title:
  tty symlink incorrect after disconnect/reconnect, fixed by manual
  udevadm trigger

Status in udev package in Ubuntu:
  New

Bug description:
  I recently upgraded from Ubuntu 17.10 to 18.04 and encountered what appears to be a regression in udev. On 17.10 (and previous), the following udev rule to rename a USB serial device worked perfectly:
  SUBSYSTEMS=="usb", ATTRS{product}=="IOTool", ATTRS{manufacturer}=="zplab.wustl.edu", ATTRS{serial}=="0xFFFF", SYMLINK+="ttyIOTool"

  This would yield symlinks as expected:
  > ls -l /dev/ttyIOTool
  lrwxrwxrwx 1 root root 7 Jun 26 21:34 /dev/ttyIOTool -> ttyACM1

  On 18.04, the same symlinks are made correctly at boot time. However,
  if the device is unplugged and then re-plugged, the symlink instead
  goes to /dev/bus/usb:

  > ls -l /dev/ttyIOTool     
  lrwxrwxrwx 1 root root 15 Jun 26 21:33 /dev/ttyIOTool -> bus/usb/003/013

  This is a problem, because /dev/bus/usb devices can't be opened as
  serial ports in the way that /dev/tty devices can.

  However, manually triggering udev via `udevadm trigger` returns the
  symlinks to the expected state. I am fairly certain that this behavior
  represents a bug, because there shouldn't be any difference in
  behavior between an auto-triggered event with a disconnect /
  reconnect, and one manually triggered via udevadm trigger, right?

  Version information: udevadm --version gives 237 on the 18.04 system
  (with this broken behavior) and 234 on an identical 17.10 system with
  the correct behavior. On a similar 16.10 system with udevadm version
  231, such symlinks also work fine.

  Note also that there are several reports of similar udev behavior around the web, which have left the users fairly perplexed; e.g.:
  https://github.com/psi46/elComandante/issues/14
  https://github.com/oskarpearson/mmeowlink/issues/52

  Please let me know if there's anything further I can do to help debug
  this problem. I can probably work around this by using /dev/serial/by-
  id, but if anyone has other workarounds to try (different udev rules,
  perhaps?), I would be grateful as well.

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