udevadm: introduce new 'wait' command

Prompted by https://github.com/systemd/systemd/pull/22717#issuecomment-1067348496.

The new command 'udevadm wait' waits for device or device symlink being
created. This may be useful to wait for a device is processed by udevd
after e.g. formatting or partitioning the device.

shell-completion/bash/udevadm

@@ -32,7 +32,7 @@ __get_all_sysdevs() {
 
 __get_all_devs() {
     local i
-    for i in /dev/* /dev/*/*; do
+    for i in /dev/* /dev/*/* /dev/*/*/*; do
         echo $i
     done
 }
@@ -64,9 +64,10 @@ _udevadm() {
         [MONITOR_ARG]='-s --subsystem-match -t --tag-match'
         [TEST]='-a --action -N --resolve-names'
         [TEST_BUILTIN]='-a --action'
+        [WAIT]='-t --timeout --initialized=no --removed --settle'
     )
 
-    local verbs=(info trigger settle control monitor test-builtin test)
+    local verbs=(info trigger settle control monitor test-builtin test wait)
     local builtins=(blkid btrfs hwdb input_id keyboard kmod net_id net_setup_link path_id usb_id uaccess)
 
     for ((i=0; i < COMP_CWORD; i++)); do
@@ -245,6 +246,25 @@ _udevadm() {
             fi
             ;;
 
+        'wait')
+            if __contains_word "$prev" ${OPTS[WAIT]}; then
+                case $prev in
+                    *)
+                        comps=''
+                        ;;
+                esac
+                COMPREPLY=( $(compgen -W '$comps' -- "$cur") )
+                return 0
+            fi
+
+            if [[ $cur = -* ]]; then
+                comps="${OPTS[COMMON]} ${OPTS[WAIT]}"
+            else
+                comps=$( __get_all_devs )
+                local IFS=$'\n'
+            fi
+            ;;
+
         *)
             comps=${VERBS[*]}
             ;;

shell-completion/zsh/_udevadm

@@ -104,6 +104,17 @@ _udevadm_test-builtin(){
     fi
 }
 
+(( $+functions[_udevadm_wait] )) ||
+_udevadm_wait(){
+    _arguments \
+        '--timeout=[Maximum number of seconds to wait for the devices being created.]' \
+        '--initialized=[Wait for devices being initialized by systemd-udevd.]:boolean:(yes no)' \
+        '--removed[Wait for devices being removed.]' \
+        '--settle[Also wait for udev queue being empty.]' \
+        '--help[Print help text.]' \
+        '*::devpath:_files -P /dev/ -W /dev'
+}
+
 (( $+functions[_udevadm_mounts] )) ||
 _udevadm_mounts(){
     local dev_tmp dpath_tmp mp_tmp mline