#!/bin/bash
set -e

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

Force-push our HEAD to the given GitHub pull request branch.

Useful for a maintainer to run just before pushing to master,
after tweaking the branch and/or rebasing to latest.  This causes
GitHub to see the subsequent push to master as representing a
merge of the PR, rather than requiring the PR to be manually
(and to the casual observer misleadingly) closed instead.

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

pr_id="$1"
remote="${2:-upstream}"

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

remote_url="$(git config remote."$remote".url)"
repo_fq="$(echo "$remote_url" | perl -lne 'print $1 if (
    m, ^ git\@github\.com:
         ([^/]+ / [^/]+?)
         (?:\.git)?
       $ ,x )')"

if [ -z "$repo_fq" ]; then
    # We're pretty specific about what we expect the URL to look like;
    # there are probably more cases we could legitimately cover, which
    # we can add if/when they come up for someone.
    echo "error: couldn't parse remote URL as GitHub repo" >&2
    exit 1
fi

# See https://developer.github.com/v3/pulls/#get-a-single-pull-request .
# This is the old REST API; the new GraphQL API does look neat, but it
# seems to require authentication even for simple lookups of public data,
# and that'd be a pain for a simple script like this.
pr_url=https://api.github.com/repos/"${repo_fq}"/pulls/"${pr_id}"
pr_details="$(curl -s "$pr_url")"

pr_jq () {
    echo "$pr_details" | jq "$@"
}

if [ "$(pr_jq -r .message)" = "Not Found" ]; then
    echo "Invalid PR URL: $pr_url"
    exit 1
fi

if [ "$(pr_jq .maintainer_can_modify)" != "true" ]; then
    # This happens when the PR has already been merged or closed, or
    # if the contributor has turned off the (default) setting to allow
    # maintainers of the target repo to push to their PR branch.
    #
    # The latter seems to be rare (in Greg's experience doing the
    # manual equivalent of this script for many different
    # contributors, none have ever chosen this setting), but give a
    # decent error message if it does happen.
    echo "error: PR already closed, or contributor has disallowed pushing to branch" >&2
    exit 1
fi

pr_head_repo_fq="$(pr_jq -r .head.repo.full_name)"
pr_head_refname="$(pr_jq -r .head.ref)"

set -x
exec git push git@github.com:"$pr_head_repo_fq" +@:"$pr_head_refname"