#!/usr/bin/env bash
set -e

usage() {
    cat >&2 <<EOF
usage: $0 PULL_REQUEST_ID [REMOTE]

Fetch the given GitHub pull request branch, and reset our
current branch to it.

Useful for anyone reading or reviewing a PR, in order to
run the code and to study it with a full local set of tools.

REMOTE defaults to the value of the Git config variable
\`zulip.zulipRemote\` if set, else to \`upstream\`.

If the Git config variable \`zulip.prPseudoRemote\` is set,
e.g. with:
  git config zulip.prPseudoRemote pr
then the PR branch is also recorded as a local ref, like a
remote-tracking branch as if the PRs make up a "remote".  In
the example, PR #1234 is recorded as \`pr/1234\`, or in full
\`refs/remotes/pr/1234\`.  This is useful for keeping track of
the PR branch while comparing with other code, and for using
the reflog to compare with previous versions of the same PR.

See also \`push-to-pull-request\`.
EOF
    exit 1
}

remote_default="$(git config zulip.zulipRemote || echo upstream)"
pseudo_remote="$(git config zulip.prPseudoRemote || echo)"

request_id="$1"
remote=${2:-"$remote_default"}

if [ -z "$request_id" ]; then
    usage
fi

this_dir=${BASH_SOURCE[0]%/*}
# shellcheck source=lib/git-tools.bash
. "${this_dir}"/lib/git-tools.bash

require_clean_work_tree 'reset to PR'

if [ -z "$pseudo_remote" ]; then
    set -x
    git fetch "$remote" "pull/$request_id/head"
    git reset --hard FETCH_HEAD
else
    target_ref=refs/remotes/"$pseudo_remote"/"$request_id"
    set -x
    git fetch "$remote" +"pull/$request_id/head:$target_ref"
    git reset --hard "$target_ref"
fi