Freezing on non-systemd GNU/Linux distributions when activated by udev

[BalkanMadman]

Gentoo GNU/Linux. I suppose it is because of some kind of data race when it gets executed twice in a very short amount of time. Relevant code snippet from udev rule:

# Initialize iOS devices into "deactivated" USB configuration state and activate usbmuxd
SUBSYSTEM=="usb", ENV{DEVTYPE}=="usb_device", ENV{PRODUCT}=="5ac/12[9a][0-9a-f]/*|5ac/8600/*", ACTION=="add", ENV{USBMUX_SUPPORTED}="1", ATTR{bConfigurationValue}="0", OWNER="usbmux", RUN+="/usr/sbin/usbmuxd --user usbmux --udev"

# Make sure properties don't get lost when bind action is called
SUBSYSTEM=="usb", ENV{DEVTYPE}=="usb_device", ENV{PRODUCT}=="5ac/12[9a][0-9a-f]/*|5ac/8600/*", ACTION=="bind", ENV{USBMUX_SUPPORTED}="1", OWNER="usbmux", RUN+="/usr/sbin/usbmuxd --user usbmux --udev"

The latter follows the former almost immediately, causing some kind of a data race (I guess).

Got it working by applying the next fix (simply removed the run part when device is added):

--- /lib/udev/rules.d/39-usbmuxd.rules	2023-01-22 00:01:59.127987197 +0200
+++ /etc/udev/rules.d/39-usbmuxd.rules	2023-01-27 11:51:07.222984812 +0200
@@ -4,7 +4,7 @@
 SUBSYSTEM=="usb", ENV{DEVTYPE}=="usb_device", ENV{PRODUCT}=="5ac/12[9a][0-9a-f]/*|5ac/8600/*", TAG+="systemd"

 # Initialize iOS devices into "deactivated" USB configuration state and activate usbmuxd
-SUBSYSTEM=="usb", ENV{DEVTYPE}=="usb_device", ENV{PRODUCT}=="5ac/12[9a][0-9a-f]/*|5ac/8600/*", ACTION=="add", ENV{USBMUX_SUPPORTED}="1", ATTR{bConfigurationValue}="0", OWNER="usbmux", RUN+="/usr/sbin/usbmuxd --user usbmux --udev"
+SUBSYSTEM=="usb", ENV{DEVTYPE}=="usb_device", ENV{PRODUCT}=="5ac/12[9a][0-9a-f]/*|5ac/8600/*", ACTION=="add", ENV{USBMUX_SUPPORTED}="1", ATTR{bConfigurationValue}="0", OWNER="usbmux"

 # Make sure properties don't get lost when bind action is called
 SUBSYSTEM=="usb", ENV{DEVTYPE}=="usb_device", ENV{PRODUCT}=="5ac/12[9a][0-9a-f]/*|5ac/8600/*", ACTION=="bind", ENV{USBMUX_SUPPORTED}="1", OWNER="usbmux", RUN+="/usr/sbin/usbmuxd --user usbmux --udev"

[BalkanMadman]

Fixed in e55e6e7

[BalkanMadman]

It turns out the commit e55e6e7 didn't actually fix the problem.

I investigated this a bit further and found that the problem may be in one of the functions in https://github.com/libimobiledevice/usbmuxd/blob/master/src/usb.c. I don't remember which one exactly; the corresponding logs will be attached later.
So, there are two issues with the current handling of udev activation:

1. usbmuxd is a daemon — a long living process — and udev manpage especially discourages starting daemons with RUN attribute (this is what is done on non-systemd builds). Being unconditionally killed may be the reason why it freezes.
2. The actual reason behind freezing lies somewhere in usbmuxd code, i.e. there is a bug.

To deal with 1. we need to provide the actual OpenRC service file and implement udev activation via it. I've already done, so wait for the draft PR shortly after the publication of this comment.
The other part of the issue needs to be traced down though.

[BalkanMadman]

The issue on the Gentoo bugzilla: https://bugs.gentoo.org/show_bug.cgi?id=910706