bash-completion: use sqlite cache when available

Use /var/cache/dnf/packages.db to make SQL requests instead of parsing
the repodata files at each tab completion. This makes completing package
names much snappier.

This is an inspiration from zsh dnf completion:
https://github.com/zsh-users/zsh/blob/zsh-5.8.1/Completion/Redhat/Command/_dnf#L7-L33

= changelog =
msg: Use sqlite cache to make bash completion snappier
type: enhancement
resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1815895

etc/bash_completion.d/dnf

@@ -106,6 +106,21 @@ __dnf_repopkgs_subcmds="
  upgrade upgrade-to
 "
 
+__dnf_cache_file="/var/cache/dnf/packages.db"
+
+_dnf_query_db()
+{
+ local table=$1
+ local prefix=$2
+ local query="select pkg from $table where pkg like \"$prefix%\""
+ if [ "$table" = "available" ]; then
+ # The available table contains both installed and non-installed
+ # packages. Exclude the installed packages.
+ query="$query and pkg not in (select pkg from installed)"
+ fi
+ sqlite3 -batch -init /dev/null "$__dnf_cache_file" "$query"
+}
+
 __dnf_python_exec=
 _dnf_set_python_exec()
 {
@@ -148,7 +163,23 @@ _dnf_show_packages()
  shift
 
  if ! _dnf_is_path "$cur"; then
- COMPREPLY+=( $(compgen -W "$( _dnf_commands_helper $cmd "$@" "$cur" )") )
+ if [ -r "$__dnf_cache_file" ] && [ -x /usr/bin/sqlite3 ]; then
+ case "$cmd$*" in
+ listinstalled|listupdates|\
+ dg|downgrade|\
+ ri|rei|reinstall|\
+ rm|remove*|erase*|\
+ um|u-m|upgrade|upgrade-n*|\
+ up|up-min|update|update-n*)
+ COMPREPLY+=( $(_dnf_query_db installed "$cur") )
+ ;;
+ *)
+ COMPREPLY+=( $(_dnf_query_db available "$cur") )
+ ;;
+ esac
+ else
+ COMPREPLY+=( $(compgen -W "$( _dnf_commands_helper $cmd "$@" "$cur" )") )
+ fi
  fi
 
  [[ $COMPREPLY ]] && return