`amtool` ZSH completion is broken

What did you do?
amtool --completion-script-zsh >> ~/.zshrc

What did you expect to see?
Proper tab-completion of subcommands similar to that of Bash

What did you see instead? Under which circumstances?
amtool <TAB> results in:

amtool: error: expected command but got ""

Environment

• System information:

Linux 4.14.336-256.559.amzn2.x86_64 x86_64, but applies to all OSes afaik.

• Alertmanager version:

amtool, version 0.27.0 (branch: HEAD, revision: 0aa3c2aad14cff039931923ab16b26b7481783b5)
  build user:       root@22cd11f671e9
  build date:       20240228-11:51:20
  go version:       go1.21.7
  platform:         linux/amd64
  tags:             netgo

• Prometheus version: N/A

• Alertmanager configuration file: N/A

• Prometheus configuration file: N/A

• Logs: N/A

Solution:

The issue seems to be the way ZSH slices arrays which includes the end index. This is contrary to bash where the end index is not included. Therefore, when $CURRENT-1 is the total number of words, the last word (which is an empty string for a command line ending with space) is included. This causes amtool --completion-bash to receive an extra empty argument.

This is fixed by subtracting 2 instead of 1, like so:

_amtool() {
    local matches
    matches=($( ${words[1]} --completion-bash "${words[@]:1:$CURRENT-2}" ))
    compadd -a matches

    if [[ $compstate[nmatches] -eq 0 && $words[$CURRENT] != -* ]]; then
        _files
    fi
}

compdef _amtool amtool