Async git information in a ZSH prompt

This isn't really packaged up in the most consumable way, but this is the program I use to generate my shell prompt. It runs in about 12ms on my machine even in large git repos, but renders an indication of whether or not there are uncommitted changes in the repo. It does this by running git status in the background and redrawing the prompt when it's done.

The key here is that running zle reset-prompt will cause the prompt to be regenerated, so we configure ZSH to run that when it handles SIGALRM. The prompt program forks and disowns a process that continues to run after the prompt is printed, and sends SIGALRM to the shell when the background process is done.

This is the chunk I put in my ~/.zshrc:

PROMPT='$(
  PS1_EXEC_NO=$__ps1_exec_no \
  EXIT_STATUS=$? \
  SHADOWENV_DATA=${__shadowenv_data%%:*} \
  SHELL_PID=$$ \
  async-shell-prompt
)'

function __ps1_exec_incr() {
  __ps1_exec_no=$((__ps1_exec_no+1))
}
precmd_functions+=(__ps1_exec_incr)
TRAPALRM() {
  # Without this conditional, we sometimes get the following error. It seems to
  # happen when SIGALRM lands while executing a command; that is, between ZLE
  # instances.
  #   TRAPALRM:zle:1: widgets can only be called when ZLE is active
  if [[ -n "$WIDGET" ]]; then
    zle reset-prompt
  fi
}

The __ps1_exec_no is incremented on precmd, so that when we call the program multiple times via zle reset-prompt, it is able to understand that it still corresponds to the same prompt line.

The program here is pretty rough: the code sucks and it's very bad for leaking files in /tmp.