Restarting a bluetooth USB device on NixOS.

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    These instructions will help you create a systemd service that will restart the a bluetooth USB dongle, working around
the need to unplug/replug the device. As always, YMMV.
First, find out the Vendor ID and Product ID of your bluetooth dongle:
 $ nix-shell -p usbutils --run lsusb | grep -i bluetooth
Bus 003 Device 010: ID 1131:1004 Integrated System Solution Corp. Bluetooth Device

The Vendor ID and Product ID pair is found in the line, after ID. It is 1131:1004 for this particular device.
Then, in your configuration.nix, create a systemd service that will run
{
  # Forces a reset for specified bluetooth usb dongle.
  systemd.services.fix-generic-usb-bluetooth-dongle = {
    description = "Fixes for generic USB bluetooth dongle.";
    wantedBy = [ "post-resume.target" ];
    after = [ "post-resume.target" ];
    script = builtins.readFile ./scripts/hack.usb.reset;
    scriptArgs = "1131:1004"; # Vendor ID and Product ID here
    serviceConfig.Type = "oneshot";
  };
}
The script referenced in the script attribute is attached to this gist.

  

  

## hack.usb.reset
#!/usr/bin/env bash

#
# Resets the specified bluetooth USB adapter.
# Restarts bluetooth service.
#
# This is to work around an issue where after suspend, the bluetooth adapter is
# not working and needed to be unplugged and re-plugged.
#
# This inelegant script basically automates this.
#

set -e
set -u
PS4=" $ "
#set -x

#
# Values can be found using `lsusb`:
#
#      Bus xxx Device yyy: ID VVVV:PPPP Integrated System Solution Corp. Bluetooth Device
#                              ^    ^
#                   ID_VENDOR-/    /
#                      ID_PRODUCT-/
#
ID_VENDOR="${1/:*/}"
ID_PRODUCT="${1/*:/}"

_reset_paths() {
	for p in "$@"; do
		echo 0 > "$p"/authorized
		sleep 1
		echo 1 > "$p"/authorized
	done
}

#
# Main function for the script.
#
main() {
	echo "Resetting USB bluetooth devices."
	# Not strictly needed, but stops bluetooth.
	# It will, in any way, be started at the end.
	systemctl stop bluetooth
	sleep 4

	# Using a function allows use of local and declare.
	local p
	local found_vnd
	local found_prd
	declare -a paths

	# For all usb devices,
	for p in /sys/bus/usb/devices/*; do
		# Try to check vendor/product IDs
		found_vnd="$(cat "$p/idVendor" 2>/dev/null)" || :
		found_prd="$(cat "$p/idProduct" 2>/dev/null)" || :

		# When both match
		if [[ "$found_vnd" == "$ID_VENDOR" && "$found_prd" == "$ID_PRODUCT" ]]; then
			# Add to valid paths
			paths+=("$p")
		fi
	done

	# Reset all paths
	_reset_paths "${paths[@]}"
	sleep 1
	# Twice for good luck
	_reset_paths "${paths[@]}"

	# Waits for everything to settle down
	sleep 2

	# Restarts bluetooth.
	systemctl restart bluetooth
	echo "Done resetting USB bluetooth devices."
}

# Calls up main.
main "$@"