Allow command hooks to provide additional status output

[garethgeorge]

Tl;dr logs are captured for command hooks today but these are verbose and intended to capture "everything" for debugging. Command hooks should be able to provide concise summaries (viewable at a glance in the UI) of their result.

Proposal

command hooks should be exec'd with an extra open file descriptor (fd 3) where a command hook can write status information. A command hook can set it's exit status by exiting with 0 (success) or non-zero (failure).

fd3 will be a unix pipe consumed by backrest and buffered in memory. The message written here will be stored in the operation representing the hook and will be presented in the UI.

A hypothetical usage example:

#!/bin/bash
if ping -q -c 1 -W 1 google.com >/dev/null; then
  echo "Internet connection is up" &>3 
  exit 0
else
  echo "Internet connection is down" &>3
  exit 1
fi

will skip (or not skip) a backup based on the result of a ping to google.com and writes a descriptive status message to fd3 which will be viewable in the UI for a user to easily understand the result.

or more simply

echo "FOO" &>3

writes text to file descriptor 3 in a shell script.

The ultimate goal is for Backrest to facilitate a basic scripting model (built on unix principles) for users to customize backup scheduling, I'll try to collect a cookbook of worked-out scripts at https://garethgeorge.github.io/backrest/cookbooks/command-hook-examples .

Note: ExtraFiles is a Unix only feature, this will not be supported on Windows