journalctl: Add support for showing messages from a previous boot

Hi,

I redid the boot ID look up to use enumerate_unique.

This is quite fast if the cache is warm but painfully slow if
it isn't. It has a slight chance of returning the wrong order if
realtime clock jumps around.

This one has to do n searches for every boot ID there is plus
a sort, so it depends heavily on cache hotness. This is in contrast
to the other way of look-up through filtering by a MESSAGE_ID,
which only needs about 1 seek + whatever amount of relative IDs
you want to walk.

I also have a linked-list + (in-place) mergesort version of this
patch, which has pretty much the same runtime. But since this one
is using libc sorting and armortized allocation, I prefer this
one.

To summarize: The MESSAGE_ID way is a *lot* faster but can be
incomplete due to rotation, while the enumerate+sort will find
every boot ID out there but will be painfully slow for large
journals and cold caches.

You choose :P

Jan

shell-completion/bash/journalctl

@@ -37,19 +37,22 @@ __journal_fields=(MESSAGE{,_ID} PRIORITY CODE_{FILE,LINE,FUNC}
 _journalctl() {
         local field_vals= cur=${COMP_WORDS[COMP_CWORD]} prev=${COMP_WORDS[COMP_CWORD-1]}
         local -A OPTS=(
-                [STANDALONE]='-a --all --full
-                              --system --user
-                              -b --this-boot --disk-usage -f --follow --header
+                [STANDALONE]='-a --all --full --system --user
+                              --disk-usage -f --follow --header
                               -h --help -l --local --new-id128 -m --merge --no-pager
                               --no-tail -q --quiet --setup-keys --this-boot --verify
                               --version --list-catalog --update-catalog'
-                       [ARG]='-D --directory -F --field -o --output -u --unit --user-unit'
+                       [ARG]='-b --boot --this-boot -D --directory -F --field
+                              -o --output -u --unit --user-unit'
                 [ARGUNKNOWN]='-c --cursor --interval -n --lines -p --priority --since --until
                               --verify-key'
         )
 
         if __contains_word "$prev" ${OPTS[ARG]} ${OPTS[ARGUNKNOWN]}; then
                 case $prev in
+                        --boot|--this-boot|-b)
+                                comps=$(journalctl -F '_BOOT_ID' 2>/dev/null)
+                        ;;
                         --directory|-D)
                                 comps=$(compgen -d -- "$cur")
                                 compopt -o filenames