path: root/src/script/backport-resolve-issue

#!/usr/bin/env python3
#
# backport-resolve-issue
#
# Based on "backport-create-issue", which was itself based on work by
# by Loic Dachary.
#
#
# Introduction
# ============
#
# This script processes GitHub backport PRs, checking for proper cross-linking
# with a Redmine Backport tracker issue and, if a PR is merged and properly
# cross-linked, it can optionally resolve the tracker issue and correctly
# populate the "Target version" field.
#
# The script takes a single positional argument, which is optional. If the
# argument is an integer, it is assumed to be a GitHub backport PR ID (e.g. "28549").
# In this mode ("single PR mode") the script processes a single GitHub backport
# PR and terminates.
#
# If the argument is not an integer, or is missing, it is assumed to be a
# commit (SHA1 or tag) to start from. If no positional argument is given, it
# defaults to the tag "BRI-{release}", which might have been added by the last run of the
# script. This mode is called "scan merge commits mode".
#
# In both modes, the script scans a local git repo, which is assumed to be
# in the current working directory. In single PR mode, the script will work
# only if the PR's merge commit is present in the current branch of the local
# git repo. In scan merge commits mode, the script starts from the given SHA1
# or tag, taking each merge commit in turn and attempting to obtain the GitHub
# PR number for each.
#
# For each GitHub PR, the script interactively displays all relevant information
# (NOTE: this includes displaying the GitHub PR and Redmine backport issue in
# web browser tabs!) and prompts the user for her preferred disposition.
#
#
# Assumptions
# ===========
#
# Among other things, the script assumes:
#
# 1. it is being run in the top-level directory of a Ceph git repo
# 2. the preferred web browser is Firefox and the command to open a browser
#    tab is "firefox"
# 3. if Firefox is running and '--no-browser' was not given, the Firefox window
#    is visible to the user and the user desires to view GitHub PRs and Tracker
#    Issues in the browser
# 4. if Firefox is not running, the user does not want to view PRs and issues
#    in a web browser
#
#
# Dependencies
# ============
#
# To run this script, first install the dependencies
#
# virtualenv v
# source v/bin/activate
# pip install gitpython python-redmine
#
# Then, copy the script from src/script/backport-resolve-issue (in the branch
# "master" - the script is not maintained anywhere else) to somewhere in your
# PATH.
#
# Finally, run the script with appropriate parameters. For example:
#
#     backport-resolve-issue --key $MY_REDMINE_KEY
#     backport-resolve-issue --user $MY_REDMINE_USER --password $MY_REDMINE_PASSWORD
#
#
# Copyright Notice
# ================
#
# Copyright (C) 2019, SUSE LLC
#
# Author: Nathan Cutler <ncutler@suse.com>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as
# published by the Free Software Foundation, either version 3 of the
# License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU Affero General Public License for more details.
#
# You should have received a copy of the GNU Affero General Public License
# along with this program.  If not, see http://www.gnu.org/licenses/>
#
import argparse
import logging
import json
import os
import re
import sys
import time
from redminelib import Redmine  # https://pypi.org/project/python-redmine/
from redminelib.exceptions import ResourceAttrError
from git import Repo
from git.exc import GitCommandError

github_endpoint = "https://github.com/ceph/ceph"
redmine_endpoint = "https://tracker.ceph.com"
project_name = "Ceph"
status2status_id = {}
project_id2project = {}
tracker2tracker_id = {}
version2version_id = {}
delay_seconds = 5
browser_cmd = "firefox"
no_browser = False
ceph_release = None
dry_run = False
redmine = None
bri_tag = None
github_token_file = "~/.github_token"
github_token = None
github_user = None
redmine_key_file = "~/.redmine_key"
redmine_key = None

def browser_running():
    global browser_cmd
    retval = os.system("pgrep {} >/dev/null".format(browser_cmd))
    if retval == 0:
        return True
    return False

def ceph_version(repo, sha1=None):
    if sha1:
        return repo.git.describe('--match', 'v*', sha1).split('-')[0]
    return repo.git.describe('--match', 'v*').split('-')[0]

def commit_range(args):
    global bri_tag
    if len(args.pr_or_commit) == 0:
        return '{}..HEAD'.format(bri_tag)
    elif len(args.pr_or_commit) == 1:
        pass
    else:
        logging.warn("Ignoring positional parameters {}".format(args.pr_or_commit[1:]))
    commit = args.pr_or_commit[0]
    return '{}..HEAD'.format(commit)

def connect_to_redmine(a):
    global redmine_key
    global redmine_key_file
    redmine_key = read_from_file(redmine_key_file)
    if a.user and a.password:
        logging.info("Redmine username and password were provided; using them")
        return Redmine(redmine_endpoint, username=a.user, password=a.password)
    elif redmine_key:
        logging.info("Redmine key was read from '%s'; using it" % redmine_key_file)
        return Redmine(redmine_endpoint, key=redmine_key)
    else:
        usage()

def derive_github_user_from_token(gh_token):
    retval = None
    if gh_token:
        curl_opt = "-u :{} --silent".format(gh_token)
        cmd = "curl {} https://api.github.com/user".format(curl_opt)
        logging.debug("Running curl command ->{}<-".format(cmd))
        json_str = os.popen(cmd).read()
        github_api_result = json.loads(json_str)
        if "login" in github_api_result:
            retval = github_api_result['login']
            if "message" in github_api_result:
                assert False, \
                    "GitHub API unexpectedly returned ->{}<-".format(github_api_result['message'])
    return retval

def ensure_bri_tag_exists(repo, release):
    global bri_tag
    bri_tag = "BRI-{}".format(release)
    bri_tag_exists = ''
    try:
        bri_tag_exists = repo.git.show_ref(bri_tag)
    except GitCommandError as err:
        logging.error(err)
    logging.debug("git show-ref {} returned ->{}<-".format(bri_tag, bri_tag_exists))
    if not bri_tag_exists:
        c_v = ceph_version(repo)
        logging.info("No {} tag found: setting it to {}".format(bri_tag, c_v))
        repo.git.tag(bri_tag, c_v)

def get_issue_release(redmine_issue):
    for field in redmine_issue.custom_fields:
        if field['name'] == 'Release':
            return field['value']
    return None

def get_project(r, p_id):
    if p_id not in project_id2project:
        p_obj = r.project.get(p_id, include='trackers')
        project_id2project[p_id] = p_obj
    return project_id2project[p_id]

def has_tracker(r, p_id, tracker_name):
    for tracker in get_project(r, p_id).trackers:
        if tracker['name'] == tracker_name:
            return True
    return False

def parse_arguments():
    parser = argparse.ArgumentParser()
    parser.add_argument("--user", help="Redmine user")
    parser.add_argument("--password", help="Redmine password")
    parser.add_argument("--debug", help="Show debug-level messages",
                        action="store_true")
    parser.add_argument("--dry-run", help="Do not write anything to Redmine",
                        action="store_true")
    parser.add_argument("--no-browser", help="Do not use web browser even if it is running",
                        action="store_true")
    parser.add_argument("pr_or_commit", nargs='*',
                        help="GitHub PR ID, or last merge commit successfully processed")
    return parser.parse_args()

def populate_ceph_release(repo):
    global ceph_release
    current_branch = repo.git.rev_parse('--abbrev-ref', 'HEAD')
    release_ver_full = ceph_version(repo)
    logging.info("Current git branch is {}, {}".format(current_branch, release_ver_full))
    release_ver = release_ver_full.split('.')[0] + '.' + release_ver_full.split('.')[1]
    try:
        ceph_release = ver_to_release()[release_ver]
    except KeyError:
        assert False, \
            "Release version {} does not correspond to any known stable release".format(release_ver)
    logging.info("Ceph release is {}".format(ceph_release))

def populate_status_dict(r):
    for status in r.issue_status.all():
        status2status_id[status.name] = status.id
    logging.debug("Statuses {}".format(status2status_id))
    return None

def populate_tracker_dict(r):
    for tracker in r.tracker.all():
        tracker2tracker_id[tracker.name] = tracker.id
    logging.debug("Trackers {}".format(tracker2tracker_id))
    return None

# not used currently, but might be useful
def populate_version_dict(r, p_id):
    versions = r.version.filter(project_id=p_id)
    for version in versions:
        version2version_id[version.name] = version.id
    return None

def print_inner_divider():
    print("-----------------------------------------------------------------")

def print_outer_divider():
    print("=================================================================")

def process_merge(repo, merge, merges_remaining):
    backport = None
    sha1 = merge.split(' ')[0]
    possible_to_resolve = True
    try:
        backport = Backport(repo, merge_commit_string=merge)
    except AssertionError as err:
        logging.error("Malformed backport due to ->{}<-".format(err))
        possible_to_resolve = False
    if tag_merge_commits:
        if possible_to_resolve:
            prompt = ("[a] Abort, "
                      "[i] Ignore and advance {bri} tag, "
                      "[u] Update tracker and advance {bri} tag (default 'u') --> "
                      .format(bri=bri_tag)
                     )
            default_input_val = "u"
        else:
            prompt = ("[a] Abort, "
                      "[i] Ignore and advance {bri} tag (default 'i') --> "
                      .format(bri=bri_tag)
                     )
            default_input_val = "i"
    else:
        if possible_to_resolve:
            prompt = "[a] Abort, [i] Ignore, [u] Update tracker (default 'u') --> "
            default_input_val = "u"
        else:
            if merges_remaining > 1:
                prompt = "[a] Abort, [i] Ignore --> "
                default_input_val = "i"
            else:
                return False
    input_val = input(prompt)
    if input_val == '':
        input_val = default_input_val
    if input_val.lower() == "a":
        exit(-1)
    elif input_val.lower() == "i":
        pass
    else:
        input_val = "u"
    if input_val.lower() == "u":
        if backport:
            backport.resolve()
        else:
            logging.warn("Cannot determine which issue to resolve. Ignoring.")
    if tag_merge_commits:
        if backport:
            tag_sha1(repo, backport.merge_commit_sha1)
        else:
            tag_sha1(repo, sha1)
    return True

def read_from_file(fs):
    retval = None
    full_path = os.path.expanduser(fs)
    try:
        with open(full_path, "r") as f:
            retval = f.read().strip()
    except FileNotFoundError:
        pass
    return retval

def releases():
    return ('argonaut', 'bobtail', 'cuttlefish', 'dumpling', 'emperor',
            'firefly', 'giant', 'hammer', 'infernalis', 'jewel', 'kraken',
            'luminous', 'mimic', 'nautilus', 'octopus')

def report_params(a):
    global dry_run
    global no_browser
    if a.dry_run:
        dry_run = True
        logging.warning("Dry run: nothing will be written to Redmine")
    if a.no_browser:
        no_browser = True
        logging.warning("Web browser will not be used even if it is running")

def set_logging_level(a):
    if a.debug:
        logging.basicConfig(level=logging.DEBUG)
    else:
        logging.basicConfig(level=logging.INFO)
    return None

def tag_sha1(repo, sha1):
    global bri_tag
    repo.git.tag('--delete', bri_tag)
    repo.git.tag(bri_tag, sha1)

def ver_to_release():
    return {'v9.2': 'infernalis', 'v10.2': 'jewel', 'v11.2': 'kraken',
            'v12.2': 'luminous', 'v13.2': 'mimic', 'v14.2': 'nautilus',
            'v15.2': 'octopus'}

def usage():
    logging.error("Redmine credentials are required to perform this operation. "
                  "Please provide either a Redmine key (via {}) "
                  "or a Redmine username and password (via --user and --password). "
                  "Optionally, one or more issue numbers can be given via positional "
                  "argument(s). In the absence of positional arguments, the script "
                  "will loop through all merge commits after the tag \"BRI-{release}\". "
                  "If there is no such tag in the local branch, one will be created "
                  "for you.".format(redmine_key_file)
                 )
    exit(-1)


class Backport:

    def __init__(self, repo, merge_commit_string):
        '''
        The merge commit string should look something like this:
            27ff851953 Merge pull request #29678 from pdvian/wip-40948-nautilus
        '''
        global browser_cmd
        global ceph_release
        global github_token
        global github_user
        self.repo = repo
        self.merge_commit_string = merge_commit_string
        #
        # split merge commit string on first space character
        merge_commit_sha1_short, self.merge_commit_description = merge_commit_string.split(' ', 1)
        #
        # merge commit SHA1 from merge commit string
        p = re.compile('\\S+')
        self.merge_commit_sha1_short = p.match(merge_commit_sha1_short).group()
        assert self.merge_commit_sha1_short == merge_commit_sha1_short, \
            ("Failed to extract merge commit short SHA1 from merge commit string ->{}<-"
             .format(merge_commit_string)
            )
        logging.debug("Short merge commit SHA1 is {}".format(self.merge_commit_sha1_short))
        self.merge_commit_sha1 = self.repo.git.rev_list(
            '--max-count=1',
            self.merge_commit_sha1_short,
            )
        logging.debug("Full merge commit SHA1 is {}".format(self.merge_commit_sha1))
        self.merge_commit_gd = repo.git.describe('--match', 'v*', self.merge_commit_sha1)
        self.populate_base_version()
        self.populate_target_version()
        self.populate_github_url()
        #
        # GitHub PR description and merged status from GitHub
        curl_opt = "--silent"
        # if GitHub token was provided, use it to avoid throttling -
        if github_token and github_user:
            curl_opt = "-u {}:{} {}".format(github_user, github_token, curl_opt)
        cmd = (
            "curl {} https://api.github.com/repos/ceph/ceph/pulls/{}"
            .format(curl_opt, self.github_pr_id)
            )
        logging.debug("Running curl command ->{}<-".format(cmd))
        json_str = os.popen(cmd).read()
        github_api_result = json.loads(json_str)
        if "title" in github_api_result and "body" in github_api_result:
            self.github_pr_title = github_api_result["title"]
            self.github_pr_desc = github_api_result["body"]
        else:
            logging.error("GitHub API unexpectedly returned: {}".format(github_api_result))
            logging.info("Curl command was: {}".format(cmd))
            sys.exit(-1)
        self.mogrify_github_pr_desc()
        self.github_pr_merged = github_api_result["merged"]
        if not no_browser:
            if browser_running():
                os.system("{} {}".format(browser_cmd, self.github_url))
        pr_title_trunc = self.github_pr_title
        if len(pr_title_trunc) > 60:
            pr_title_trunc = pr_title_trunc[0:50] + "|TRUNCATED"
        print('''\n\n=================================================================
GitHub PR URL:    {}
GitHub PR title:  {}
Merge commit:     {} ({})
Merged:           {}
Ceph version:     base {}, target {}'''
              .format(self.github_url, pr_title_trunc, self.merge_commit_sha1,
                      self.merge_commit_gd, self.github_pr_merged, self.base_version,
                      self.target_version
                     )
             )
        if no_browser or not browser_running():
            print('''----------------------- PR DESCRIPTION --------------------------
{}
-----------------------------------------------------------------'''.format(self.github_pr_desc))
        assert self.github_pr_merged, "GitHub PR {} has not been merged!".format(self.github_pr_id)
        #
        # obtain backport tracker from GitHub PR description
        self.extract_backport_trackers_from_github_pr_desc()
        #
        for bt in self.backport_trackers:
            # does the Backport Tracker description link back to the GitHub PR?
            p = re.compile('http.?://github.com/ceph/ceph/pull/\\d+')
            bt.get_tracker_description()
            try:
                bt.github_url_from_tracker = p.search(bt.tracker_description).group()
            except AttributeError:
                pass
            if bt.github_url_from_tracker:
                p = re.compile('\\d+')
                bt.github_id_from_tracker = p.search(bt.github_url_from_tracker).group()
                logging.debug("GitHub PR from Tracker: URL is ->{}<- and ID is {}"
                              .format(bt.github_url_from_tracker, bt.github_id_from_tracker))
                assert bt.github_id_from_tracker == self.github_pr_id, \
                    "GitHub PR ID {} does not match GitHub ID from tracker {}".format(
                        self.github_pr_id,
                        bt.github_id_from_tracker,
                        )
            print_inner_divider()
            if bt.github_url_from_tracker:
                logging.info("Tracker {} links to PR {}".format(bt.issue_url(), self.github_url))
            else:
                logging.warning("Backport Tracker {} does not link to PR - will update"
                                .format(bt.issue_id))
            #
            # does the Backport Tracker's release field match the Ceph release?
            tracker_release = get_issue_release(bt.redmine_issue)
            assert ceph_release == tracker_release, \
                (
                    "Backport Tracker {} is a {} backport - expected {}"
                    .format(bt.issue_id, tracker_release, ceph_release)
                )
            #
            # is the Backport Tracker's "Target version" custom field populated?
            try:
                ttv = bt.get_tracker_target_version()
            except:
                logging.info("Backport Tracker {} target version not populated yet!"
                             .format(bt.issue_id))
                bt.set_target_version = True
            else:
                bt.tracker_target_version = ttv
                logging.info("Backport Tracker {} target version already populated "
                             "with correct value {}"
                             .format(bt.issue_id, bt.tracker_target_version))
                bt.set_target_version = False
                assert bt.tracker_target_version == self.target_version, \
                    (
                        "Tracker target version {} is wrong; should be {}"
                        .format(bt.tracker_target_version, self.target_version)
                    )
            #
            # is the Backport Tracker's status already set to Resolved?
            resolved_id = status2status_id['Resolved']
            if bt.redmine_issue.status.id == resolved_id:
                logging.info("Backport Tracker {} status is already set to Resolved"
                             .format(bt.issue_id))
                bt.set_tracker_status = False
            else:
                logging.info("Backport Tracker {} status is currently set to {}"
                             .format(bt.issue_id, bt.redmine_issue.status))
                bt.set_tracker_status = True
        print_outer_divider()

    def populate_base_version(self):
        self.base_version = ceph_version(self.repo, self.merge_commit_sha1)

    def populate_target_version(self):
        x, y, z = self.base_version.split('v')[1].split('.')
        maybe_stable = "v{}.{}".format(x, y)
        assert ver_to_release()[maybe_stable], \
            "SHA1 {} is not based on any known stable release ({})".format(sha1, maybe_stable)
        tv = "v{}.{}.{}".format(x, y, int(z) + 1)
        if tv in version2version_id:
            self.target_version = tv
        else:
            raise Exception("Version {} not found in Redmine".format(tv))

    def mogrify_github_pr_desc(self):
        if not self.github_pr_desc:
            self.github_pr_desc = ''
        p = re.compile('<!--.+-->', re.DOTALL)
        new_str = p.sub('', self.github_pr_desc)
        if new_str == self.github_pr_desc:
            logging.debug("GitHub PR description not mogrified")
        else:
            self.github_pr_desc = new_str

    def populate_github_url(self):
        global github_endpoint
        # GitHub PR ID from merge commit string
        p = re.compile('(pull request|PR) #(\\d+)')
        try:
            self.github_pr_id = p.search(self.merge_commit_description).group(2)
        except AttributeError:
            assert False, \
                (
                    "Failed to extract GitHub PR ID from merge commit string ->{}<-"
                    .format(self.merge_commit_string)
                )
        logging.debug("Merge commit string: {}".format(self.merge_commit_string))
        logging.debug("GitHub PR ID from merge commit string: {}".format(self.github_pr_id))
        self.github_url = "{}/pull/{}".format(github_endpoint, self.github_pr_id)

    def extract_backport_trackers_from_github_pr_desc(self):
        global redmine_endpoint
        p = re.compile('http.?://tracker.ceph.com/issues/\\d+')
        matching_strings = p.findall(self.github_pr_desc)
        if not matching_strings:
            print_outer_divider()
            assert False, \
                "GitHub PR description does not contain a Tracker URL"
        self.backport_trackers = []
        for issue_url in list(dict.fromkeys(matching_strings)):
            p = re.compile('\\d+')
            issue_id = p.search(issue_url).group()
            if not issue_id:
                print_outer_divider()
                assert issue_id, \
                    "Failed to extract tracker ID from tracker URL {}".format(issue_url)
            issue_url = "{}/issues/{}".format(redmine_endpoint, issue_id)
            #
            # we have a Tracker URL, but is it really a backport tracker?
            backport_tracker_id = tracker2tracker_id['Backport']
            redmine_issue = redmine.issue.get(issue_id)
            if redmine_issue.tracker.id == backport_tracker_id:
                self.backport_trackers.append(
                    BackportTracker(redmine_issue, issue_id, self)
                    )
                print('''Found backport tracker: {}'''.format(issue_url))
        if not self.backport_trackers:
            print_outer_divider()
            assert False, \
                "No backport tracker found in PR description at {}".format(self.github_url)

    def resolve(self):
        for bt in self.backport_trackers:
            bt.resolve()


class BackportTracker(Backport):

    def __init__(self, redmine_issue, issue_id, backport_obj):
        self.redmine_issue = redmine_issue
        self.issue_id = issue_id
        self.parent = backport_obj
        self.tracker_description = None
        self.github_url_from_tracker = None

    def get_tracker_description(self):
        try:
            self.tracker_description = self.redmine_issue.description
        except ResourceAttrError:
            self.tracker_description = ""

    def get_tracker_target_version(self):
        if self.redmine_issue.fixed_version:
            logging.debug("Target version: ID {}, name {}"
                          .format(
                              self.redmine_issue.fixed_version.id,
                              self.redmine_issue.fixed_version.name
                          )
                         )
            return self.redmine_issue.fixed_version.name
        return None

    def issue_url(self):
        return "{}/issues/{}".format(redmine_endpoint, self.issue_id)

    def resolve(self):
        global delay_seconds
        global dry_run
        global redmine
        kwargs = {}
        if self.set_tracker_status:
            kwargs['status_id'] = status2status_id['Resolved']
        if self.set_target_version:
            kwargs['fixed_version_id'] = version2version_id[self.parent.target_version]
        if not self.github_url_from_tracker:
            if self.tracker_description:
                kwargs['description'] = "{}\n\n---\n\n{}".format(
                    self.parent.github_url,
                    self.tracker_description,
                    )
            else:
                kwargs['description'] = self.parent.github_url
        kwargs['notes'] = (
            "This update was made using the script \"backport-resolve-issue\".\n"
            "backport PR {}\n"
            "merge commit {} ({})\n".format(
                self.parent.github_url,
                self.parent.merge_commit_sha1,
                self.parent.merge_commit_gd,
            )
        )
        my_delay_seconds = delay_seconds
        if dry_run:
            logging.info("--dry-run was given: NOT updating Redmine")
            my_delay_seconds = 0
        else:
            logging.debug("Updating tracker ID {}".format(self.issue_id))
            redmine.issue.update(self.issue_id, **kwargs)
        if not no_browser:
            if browser_running():
                os.system("{} {}".format(browser_cmd, self.issue_url()))
                my_delay_seconds = 3
        logging.debug(
            "Delaying {} seconds to avoid seeming like a spammer"
            .format(my_delay_seconds)
        )
        time.sleep(my_delay_seconds)


if __name__ == '__main__':
    args = parse_arguments()
    set_logging_level(args)
    logging.debug(args)
    github_token = read_from_file(github_token_file)
    if github_token:
        logging.info("GitHub token was read from ->{}<-; using it".format(github_token_file))
        github_user = derive_github_user_from_token(github_token)
        if github_user:
            logging.info(
                "GitHub user ->{}<- was derived from the GitHub token".format(github_user)
                )
    report_params(args)
    #
    # set up Redmine variables
    redmine = connect_to_redmine(args)
    project = redmine.project.get(project_name)
    ceph_project_id = project.id
    logging.debug("Project {} has ID {}".format(project_name, ceph_project_id))
    populate_status_dict(redmine)
    pending_backport_status_id = status2status_id["Pending Backport"]
    logging.debug(
        "Pending Backport status has ID {}"
        .format(pending_backport_status_id)
    )
    populate_tracker_dict(redmine)
    populate_version_dict(redmine, ceph_project_id)
    #
    # construct github Repo object for the current directory
    repo = Repo('.')
    assert not repo.bare
    populate_ceph_release(repo)
    #
    # if positional argument is an integer, assume it is a GitHub PR
    if args.pr_or_commit:
        pr_id = args.pr_or_commit[0]
        try:
            pr_id = int(pr_id)
            logging.info("Examining PR#{}".format(pr_id))
            tag_merge_commits = False
        except ValueError:
            logging.info("Starting from merge commit {}".format(args.pr_or_commit))
            tag_merge_commits = True
    else:
        logging.info("Starting from BRI tag")
        tag_merge_commits = True
    #
    # get list of merges
    if tag_merge_commits:
        ensure_bri_tag_exists(repo, ceph_release)
        c_r = commit_range(args)
        logging.info("Commit range is {}".format(c_r))
        #
        # get the list of merge commits, i.e. strings that looks like:
        # "27ff851953 Merge pull request #29678 from pdvian/wip-40948-nautilus"
        merges_raw_str = repo.git.log(c_r, '--merges', '--oneline', '--no-decorate', '--reverse')
    else:
        pr_id = args.pr_or_commit[0]
        merges_raw_str = repo.git.log(
            '--merges',
            '--grep=#{}'.format(pr_id),
            '--oneline',
            '--no-decorate',
            '--reverse',
        )
    if merges_raw_str:
        merges_raw_list = merges_raw_str.split('\n')
    else:
        merges_raw_list = []  # prevent ['']
    merges_remaining = len(merges_raw_list)
    logging.info("I see {} merge(s) to process".format(merges_remaining))
    if not merges_remaining:
        logging.info("Did you do \"git pull\" before running the script?")
        if not tag_merge_commits:
            logging.info("Or maybe GitHub PR {} has not been merged yet?".format(pr_id))
    #
    # loop over the merge commits
    for merge in merges_raw_list:
        can_go_on = process_merge(repo, merge, merges_remaining)
        if can_go_on:
            merges_remaining -= 1
            print("Merges remaining to process: {}".format(merges_remaining))
        else:
            break