#! /bin/sh
#
# usage: fetchgit git_rev git_url out_link
#
# Clone the specified Git repository and make it available as out_link.
#
set -euf

git_rev=$1
git_url=$2
out_link=$3

# Put all bases in the same place as out_link.
# Notice how out_link must not clash with cache_dir and work_dir.
cache_base=$(dirname "$out_link")
work_base=$(dirname "$out_link")

# cache_dir points to a (maybe non-existent) directory, where a shared cache of
# the repository should be maintained.  The shared cache is used to create
# multiple working trees of the repository.
cache_dir=$cache_base/$(echo "$git_url" | urlencode)

# work_dir points to a (maybe non-existent) directory, where a specific
# revision of the repository is checked out.
work_dir=$work_base/$(echo "$git_rev" | urlencode)

cache_git() {
  git --git-dir="$cache_dir" "$@"
}

work_git() {
  git -C "$work_dir" "$@"
}

is_up_to_date() {
  test -d "$cache_dir" &&
  test -d "$work_dir" &&
  test "$(cache_git rev-parse "$git_rev")" = "$git_rev" &&
  test "$(work_git rev-parse HEAD)" = "$git_rev"
}

# Notice how the remote name "origin" has been chosen arbitrarily.
if ! is_up_to_date; then
  if ! test -d "$cache_dir"; then
    mkdir -p "$cache_dir"
    cache_git init --bare
  fi
  if ! cache_git_url=$(cache_git config remote.origin.url); then
    cache_git remote add origin "$git_url"
  elif test "$cache_git_url" != "$git_url"; then
    cache_git remote set-url origin "$git_url"
  fi
  cache_git fetch origin
  if ! test -d "$work_dir"; then
    git clone -n --shared "$cache_dir" "$work_dir"
  fi
  commit_name=$(cache_git rev-parse "$git_rev")
  work_git checkout "$commit_name" -- "$(readlink -f "$work_dir")"
  work_git checkout -q "$commit_name"
  work_git submodule init
  work_git submodule update
fi
work_git clean -dxf

# Relative links are nicer, and actually we know that work_dir and out_link are
# the same.  But, for robustness, check anyway.. :)
if test "$(dirname "$work_dir")" = "$(dirname "$out_link")"; then
  ln -snf "$(basename "$work_dir")" "$out_link"
else
  ln -snf "$work_dir" "$out_link"
fi