git-fetch-branch

#!/usr/bin/env bash
set -euo pipefail

usage() {
  cat <<'EOF'
usage: git fetch-branch [--dry-run] [--remote <remote>] <user:branch>

Fetch a branch from another user's GitHub fork of this repository into a local
branch named <user>/<branch>, and configure it to track the fork branch so
future `git pull` commands pull from that fork.

Example:
  git fetch-branch takashiidobe:handle-canonicalization-of-bool-cond-cmp

This fetches:

  https://github.com/takashiidobe/<repo>.git handle-canonicalization-of-bool-cond-cmp 

Into local branch:

    handle-canonicalization-of-bool-cond-cmp

And tracks branch handle-canonicalization-of-bool-cond-cmp from remote Takashiidobe.
EOF
}

die() {
  printf 'git fetch-branch: %s\n' "$*" >&2
  exit 1
}

run() {
  printf '+'
  printf ' %q' "$@"
  printf '\n'
  if [[ "${dry_run}" != true ]]; then
    "$@"
  fi
}

github_slug_from_url() {
  local url=$1
  local slug

  url=${url%.git}
  url=${url%/}
  case "$url" in
    https://github.com/*)
      slug=${url#https://github.com/}
      ;;
    http://github.com/*)
      slug=${url#http://github.com/}
      ;;
    git://github.com/*)
      slug=${url#git://github.com/}
      ;;
    git@github.com:*)
      slug=${url#git@github.com:}
      ;;
    ssh://git@github.com/*)
      slug=${url#ssh://git@github.com/}
      ;;
    *)
      return 1
      ;;
  esac

  [[ "$slug" == */* ]] || return 1
  printf '%s\n' "$slug"
}

dry_run=false
remote=origin

while (($#)); do
  case "$1" in
    --dry-run|-n)
      dry_run=true
      shift
      ;;
    --no-push)
      shift
      ;;
    --remote)
      (($# >= 2)) || die "--remote requires a value"
      remote=$2
      shift 2
      ;;
    --help|-h)
      usage
      exit 0
      ;;
    --)
      shift
      break
      ;;
    -*)
      die "unknown option: $1"
      ;;
    *)
      break
      ;;
  esac
done

(($# == 1)) || { usage >&2; exit 2; }

spec=$1
[[ "$spec" == *:* ]] || die "expected <user:branch>, got '$spec'"

user=${spec%%:*}
source_branch=${spec#*:}
if [[ "$source_branch" == "${user}/"* ]]; then
  target_branch="$source_branch"
else
  target_branch="${user}/${source_branch}"
fi

[[ -n "$user" ]] || die "empty GitHub username in '$spec'"
[[ -n "$source_branch" ]] || die "empty branch name in '$spec'"
[[ "$user" =~ ^[A-Za-z0-9]([A-Za-z0-9-]{0,37}[A-Za-z0-9])?$ ]] ||
  die "invalid GitHub username: '$user'"

git rev-parse --is-inside-work-tree >/dev/null 2>&1 ||
  die "not inside a git worktree"

git check-ref-format --branch "$source_branch" >/dev/null ||
  die "invalid source branch name: '$source_branch'"
git check-ref-format --branch "$target_branch" >/dev/null ||
  die "invalid target branch name: '$target_branch'"

repo_root=$(git rev-parse --show-toplevel)
origin_url=$(git -C "$repo_root" remote get-url "$remote") ||
  die "remote '$remote' does not exist"

repo_name=${origin_url##*/}
repo_name=${repo_name%.git}
repo_name=${repo_name%/}
[[ -n "$repo_name" ]] || die "could not infer repository name from '$remote' URL: $origin_url"

fork_url="https://github.com/${user}/${repo_name}.git"
fork_slug="${user}/${repo_name}"
source_ref="refs/heads/${source_branch}"
target_ref="refs/heads/${target_branch}"
remote_tracking_ref="refs/remotes/${user}/${source_branch}"

if existing_fork_url=$(git -C "$repo_root" remote get-url "$user" 2>/dev/null); then
  if existing_fork_slug=$(github_slug_from_url "$existing_fork_url"); then
    [[ "$existing_fork_slug" == "$fork_slug" ]] ||
      die "remote '$user' points to '$existing_fork_url', not '$fork_url'"
  else
    die "remote '$user' already exists with non-GitHub URL: $existing_fork_url"
  fi
else
  run git -C "$repo_root" remote add "$user" "$fork_url"
fi

fetch_refspecs=("+${source_ref}:${remote_tracking_ref}")
if [[ "$(git -C "$repo_root" branch --show-current)" != "$target_branch" ]]; then
  fetch_refspecs+=("+${source_ref}:${target_ref}")
fi

run git -C "$repo_root" fetch "$user" "${fetch_refspecs[@]}"
run git -C "$repo_root" config "branch.${target_branch}.remote" "$user"
run git -C "$repo_root" config "branch.${target_branch}.merge" "$source_ref"

printf 'Branch ready: %s\n' "$target_branch"