Jul 01, 2024

zsh fallback completions on empty results

Usually zsh does fine wrt tab-completion, but sometimes you just get nothing when pressing tab, either due to somewhat-broken completer or it working as intended but there's seemingly being "nothing" to complete.

Recently latter started happening after redirection characters, e.g. on cat myfile > <TAB>, and that finally prompted me to re-examine why I even put up with this crap.

Because in vast majority of cases, completion should use files, except for commands as the first thing on the line, and maybe some other stuff way more rarely, almost as an exception. But completing nothing at all seems like an obvious bug to me, as if I wanted nothing, wouldn't have pressed the damn tab key in the first place.

One common way to work around the lack of file-completions when needed, is to define special key for just those, like shift-tab:

zstyle ':completion:complete-files:*' completer _files
bindkey "\e[Z" complete-files

If using that becomes a habit everytime one needs files, that'd be a good solution, but I still use generic "tab" by default, and expect file-completion from it in most cases, so why not have it fallback to file-completion if whatever special thing zsh has otherwise fails - i.e. suggest files/paths instead of nothing.

Looking at _complete_debug output (can be bound/used instead of tab-completion), it's easy to find where _main_complete dispatcher picks completer script, and that there is apparently no way to define fallback of any kind there, but easy enough to patch one in, at least.

Here's the hack I ended up with for /etc/zsh/zshrc:

## Make completion always fallback to next completer if current returns 0 results
# This allows to fallback to _file completion properly when fancy _complete fails
# Patch requires running zsh as root at least once, to apply it (or warn/err about it)

_patch_completion_fallbacks() {
  local patch= p=/usr/share/zsh/functions/Completion/Base/_main_complete
  if [[ "$p".orig -nt "$p" ]]; then return; fi
  read -r -d '' patch <<'EOF'
--- _main_complete      2024-06-09 01:10:28.352215256 +0500
+++ _main_complete.new  2024-06-09 01:10:51.087404762 +0500
@@ -210,18 +210,19 @@
     fi

     _comp_mesg=
     if [[ -n "$call" ]]; then
       if "${(@)argv[3,-1]}"; then
         ret=0
         break 2
       fi
     elif "$tmp"; then
+      [[ $compstate[nmatches] -gt 0 ]] || continue
       ret=0
       break 2
     fi
     (( _matcher_num++ ))
   done
   [[ -n "$_comp_mesg" ]] && break

   (( _completer_num++ ))
 done
EOF
  patch --dry-run -stN "$p" <<< "$patch" &>/dev/null \
    || { echo >&2 "zshrc :: WARNING: zsh fallback-completions patch fails to apply"; return; }
  cp -a "$p" "$p".orig && patch -stN "$p" <<< "$patch" && touch "$p".orig \
    || { echo >&2 "zshrc :: ERROR: failed to apply zsh fallback-completions patch"; return; }
  echo >&2 'zshrc :: NOTE: patched zsh _main_complete routine to allow fallback-completions'
}
[[ "$UID" -ne 0 ]] || _patch_completion_fallbacks
unset _patch_completion_fallbacks

This would work with multiple completers defined like this:

zstyle ':completion:*' completer _complete _ignored _files

Where _complete _ignored is the default completer-chain, and will try whatever zsh has for the command first, and then if those return nothing, instead of being satisfied with that, patched-in continue will keep going and run next completer, which is _files in this case.

A patch with generous context is to find the right place and bail if upstream code changes, but otherwise, whenever first running the shell as root, fix the issue until next zsh package update (and then patch will run/fix it again).

Doubt it'd make sense upstream in this form, as presumably current behavior is locked-in over years, but an option for something like this would've been nice. I'm content with a hack for now though, it works too.