#!/usr/bin/perl
#
# buildah-vendor-treadmill - daily vendor of latest-buildah onto latest-podman
#
package Podman::BuildahVendorTreadmill;

use v5.14;
use utf8;
use open qw( :encoding(UTF-8) :std );

use strict;
use warnings;

use File::Temp                  qw(tempfile);
use JSON;
use LWP::UserAgent;
use POSIX                       qw(strftime);

(our $ME = $0) =~ s|.*/||;
our $VERSION = '0.3';

# For debugging, show data structures using DumpTree($var)
#use Data::TreeDumper; $Data::TreeDumper::Displayaddress = 0;

###############################################################################
# BEGIN user-customizable section

# Page describing this process in much more detail
our $Docs_URL =
    'https://github.com/containers/podman/wiki/Buildah-Vendor-Treadmill';

# github path to buildah
our $Buildah = 'github.com/containers/buildah';

# FIXME FIXME FIXME: add 'main'? I hope we never need this script for branches.
our $Treadmill_PR_Title = 'DO NOT MERGE: buildah vendor treadmill';

# Github API; this is where we query to find out the active treadmill PR
our $API_URL = 'https://api.github.com/graphql';

# Temporary file used to preserve current treadmill patches. This file
# should only exist very briefly while we perform branch operations.
our $Patch_File = "0000-$ME.patch";

# Use colors if available and if stdout is a tty
our $C_Highlight = '';
our $C_Warning = '';
our $C_Reset = '';
eval '
    use Term::ANSIColor;
    if (-t 1) {
        $C_Highlight = color("green");
        $C_Warning   = color("bold red");
        $C_Reset     = color("reset");

    }
    $SIG{__WARN__} = sub { print STDERR $C_Warning, "@_", $C_Reset; };

';

# END   user-customizable section
###############################################################################

###############################################################################
# BEGIN boilerplate args checking, usage messages

sub usage {
    print  <<"END_USAGE";
Usage: $ME [OPTIONS] [--sync | --pick | --reset ]

$ME is (2022-04-20) **EXPERIMENTAL**

$ME is intended to solve the problem of vendoring
buildah into podman.

Call me with one of two options:

    --sync  The usual case. Mostly used by Ed. Called from a
            development branch, this just updates everything so
            we vendor in latest-buildah (main) on top of
            latest-podman (main). With a few sanity checks.

    --pick  Used for really-truly vendoring in a new buildah; will
            cherry-pick a commit on your buildah-vendor working branch

    --reset Used after vendoring buildah into main, when there
            really aren't any buildah patches to keep rolling.

For latest documentation and best practices, please see:

    $Docs_URL

OPTIONS:

  --help         display this message
  --version      display program name and version
END_USAGE

    exit;
}

# Command-line options.  Note that this operates directly on @ARGV !
our %action;
our $debug   = 0;
our $force_old_main = 0;        # in --pick, proceeds even if main is old
our $force_testing = 0;         # in --sync, test even no podman/buildah changes
our $verbose = 0;
our $NOT     = '';              # print "blahing the blah$NOT\n" if $debug
sub handle_opts {
    use Getopt::Long;
    GetOptions(
        'sync'       => sub { $action{sync}++  },
        'pick'       => sub { $action{pick}++  },
        'reset'      => sub { $action{reset}++ },

        'force-old-main'  => \$force_old_main,
        'force-testing'   => \$force_testing,

        'debug!'     => \$debug,
        'dry-run|n!' => sub { $NOT = ' [NOT]' },
        'verbose|v'  => \$verbose,

        help         => \&usage,
        version      => sub { print "$ME version $VERSION\n"; exit 0 },
    ) or die "Try `$ME --help' for help\n";
}

# END   boilerplate args checking, usage messages
###############################################################################

############################## CODE BEGINS HERE ###############################

# The term is "modulino".
__PACKAGE__->main()                                     unless caller();

# Main code.
sub main {
    # Note that we operate directly on @ARGV, not on function parameters.
    # This is deliberate: it's because Getopt::Long only operates on @ARGV
    # and there's no clean way to make it use @_.
    handle_opts();                      # will set package globals

    # Fetch command-line arguments.  Barf if too many.
    # FIXME: if called with arg, that's the --sync branch?
    # FIXME: if called with --pick + arg, that's the PR?
    die "$ME: Too many arguments; try $ME --help\n"                 if @ARGV;

    my @action = keys(%action);
    die "$ME: Please invoke me with one of --sync or --pick\n"
        if ! @action;
    die "$ME: Please invoke me with ONLY one of --sync or --pick\n"
        if @action > 1;

    my $handler = __PACKAGE__->can("do_@action")
        or die "$ME: No handler available for --@action\n";

    # We've validated the command-line args. Before running action, check
    # that repo is clean. None of our actions can be run on a dirty repo.
    assert_clean_repo();

    $handler->();
}

###############################################################################
# BEGIN sync and its helpers

sub do_sync {
    # Preserve current branch name, so we can come back after switching to main
    my $current_branch = git_current_branch();

    # Branch HEAD must be the treadmill commit.
    my $commit_message = git('log', '-1', '--format=%s', 'HEAD');
    print "[$commit_message]\n"         if $verbose;
    $commit_message =~ /buildah.*treadmill/
        or die "$ME: HEAD must be a 'buildah treadmill' commit.\n";

    # ...and previous commit must be a scratch buildah vendor
    $commit_message = git('log', '-1', '--format=%B', 'HEAD^');
    $commit_message =~ /DO NOT MERGE.* vendor in buildah.*JUNK COMMIT/s
        or die "$ME: HEAD^ must be a DO NOT MERGE / JUNK COMMIT commit\n";
    assert_buildah_vendor_commit('HEAD^');

    # Looks good so far.
    my $buildah_old = vendored_buildah();
    print "-> buildah old = $buildah_old\n";

    # Pull main, and pivot back to this branch
    pull_main();
    git('checkout', '-q', $current_branch);

    # Preserve local patches. --always will generate empty patches (e.g.,
    # after a buildah vendor when everything is copacetic); --no-signature
    # prevents a buildup of "-- 2.35" (git version) lines at the end.
    git('format-patch', '--always', '--no-signature', "--output=$Patch_File", 'HEAD^');
    progress("Treadmill patches saved to $Patch_File");

    #
    # Danger Will Robinson! This is where it gets scary: a failure here
    # can leave us in a state where we could lose the treadmill patches.
    # Proceed with extreme caution.
    #
    local $SIG{__DIE__} = sub {
        print STDERR $C_Warning, "@_", <<"END_FAIL_INSTRUCTIONS";

This is not something I can recover from. Your human judgment is needed.

You will need to recover from this manually. Your best option is to
look at the source code for this script.

Your treadmill patches are here: $Patch_File
END_FAIL_INSTRUCTIONS

        exit 1;
    };

    my $forkpoint = git_forkpoint();
    my $rebased;

    # Unlikely to fail
    git('reset', '--hard', 'HEAD^^');

    # Rebase branch. Also unlikely to fail
    my $main_commit = git('rev-parse', 'main');
    if ($forkpoint eq $main_commit) {
        progress("[Already rebased on podman main]");
    }
    else {
        progress("Rebasing on podman main...");
        git('rebase', '--empty=keep', 'main');
        $rebased = 1;
    }

    # This does have a high possibility of failing.
    progress("Vendoring in buildah...");
    system('go', 'mod', 'edit', '--require' => "${Buildah}\@main") == 0
        or die "$ME: go mod edit failed";
    system('make', 'vendor') == 0
        or die "$ME: make vendor failed";
    my $buildah_new = vendored_buildah();
    print "-> buildah new = $buildah_new\n";

    # Tweak .cirrus.yml so we run bud tests first in CI (to fail fast).
    tweak_cirrus_test_order();

    # 'make vendor' seems to git-add files under buildah itself, but not
    # under other changed modules. Add those now, otherwise we fail
    # the dirty-tree test in CI.
    if (my @v = git('status', '--porcelain', '--untracked=all', 'vendor')) {
        if (my @untracked = grep { /^\?\?\s/ } @v) {
            my %repos = map {
                s!^.*?vendor/[^/]+/([^/]+/[^/]+)/.*$!$1!; $_ => 1;
            } @untracked;
            my $repos = join(', ', sort keys %repos);
            progress("Adding untracked files under $repos");
            git('add', 'vendor');
        }
    }

    # Commit everything.
    git_commit_buildah($buildah_new);

    # And, finally, this has the highest possibility of failing
    progress('Reapplying preserved patches');
    git('am', '--empty=keep', $Patch_File);

    # It worked! Clean up: remove our local die() handler and the patch file
    undef $SIG{__DIE__};
    unlink $Patch_File;

    # if buildah is unchanged, and we did not pull main, exit cleanly
    my $change_message = '';
    if ($buildah_new eq $buildah_old) {
        if (! $rebased) {
            $change_message = "Nothing has changed (same buildah, same podman).";
            if ($force_testing) {
                $change_message .= " Testing anyway due to --force-testing.";
            }
            else {
                progress($change_message);
                progress("Not much point to testing this, but use --force-testing to continue.");
                exit 0;
            }
        }
        else {
            $change_message = "Podman has bumped, but Buildah is unchanged. There's probably not much point to testing this.";
        }
    }
    else {
        my $samenew = ($rebased ? 'new' : 'same');
        $change_message = "New buildah, $samenew podman. Good candidate for pushing.";
    }
    progress($change_message);

    build_and_check_podman();

    progress("All OK. It's now up to you to 'git push --force'");
    progress(" --- Reminder: $change_message");
}

###############
#  pull_main  #  Switch to main, and pull latest from github
###############
sub pull_main {
    progress("Pulling podman main...");
    git('checkout', '-q', 'main');
    git('pull', '-r', git_upstream(), 'main');
}

#############################
#  tweak_cirrus_test_order  #  Run bud tests first, to fail fast & early
#############################
sub tweak_cirrus_test_order {
    my $cirrus_yml = '.cirrus.yml';
    my $tmpfile = "$cirrus_yml.tmp.$$";
    unlink $tmpfile;

    progress("Tweaking test order in $cirrus_yml to run bud tests early");
    open my $in, '<', $cirrus_yml
        or do {
            warn "$ME: Cannot read $cirrus_yml: $!\n";
            warn "$ME: Will continue anyway\n";
            return;
        };
    open my $out, '>'. $tmpfile
        or die "$ME: Cannot create $tmpfile: $!\n";
    my $current_task = '';
    my $in_depend;
    while (my $line = <$in>) {
        chomp $line;
        if ($line =~ /^(\S+)_task:$/) {
            $current_task = $1;
            undef $in_depend;
        }
        elsif ($line =~ /^(\s+)depends_on:$/) {
            $in_depend = $1;
        }
        elsif ($in_depend && $line =~ /^($in_depend\s+-\s+)(\S+)/) {
            if ($current_task eq 'buildah_bud_test') {
                # Buildah bud test now depends on validate, so it runs early
                $line = "${1}validate";
            }
            elsif ($2 eq 'validate' && $current_task ne 'success') {
                # Other tests that relied on validate, now rely on bud instead
                $line = "${1}buildah_bud_test";
            }
        }
        else {
            undef $in_depend;
        }

        print { $out } $line, "\n";
    }
    close $in;
    close $out
        or die "$ME: Error writing $tmpfile: $!\n";
    chmod 0644 => $tmpfile;
    rename $tmpfile => $cirrus_yml
        or die "$ME: Could not rename $tmpfile: $!\n";
}

############################
#  build_and_check_podman  #  Run quick (local) sanity checks before pushing
############################
sub build_and_check_podman {
    my $errs = 0;

    # Confirm that we can still build podman
    progress("Running 'make' to confirm that podman builds cleanly...");
    system('make') == 0
        or die "$ME: 'make' failed with new buildah. Cannot continue.\n";

    # See if any new options need man pages. (C_Warning will highlight errs)
    progress('Cross-checking man pages...');
    print $C_Warning;
    $errs += system('hack/xref-helpmsgs-manpages');
    print $C_Reset;

    # Confirm that buildah-bud patches still apply. This requires knowing
    # the name of the directory created by the bud-tests script.
    progress("Confirming that buildah-bud-tests patches still apply...");
    system('rm -rf test-buildah-*');
    if (system('test/buildah-bud/run-buildah-bud-tests', '--no-test')) {
        # Error
        ++$errs;
        warn "$ME: Leaving test-buildah- directory for you to investigate\n";
    }
    else {
        # Patches apply cleanly. Clean up
        system('rm -rf test-buildah-*');
    }

    return if !$errs;
    warn <<"END_WARN";
$ME: Errors found. I have to stop now for you to fix them.
    Your best bet now is:
      1) Find and fix whatever needs to be fixed; then
      2) git commit -am'fixme-fixme'; then
      3) git rebase -i main:
         a) you are now in an editor window
         b) move the new fixme-fixme commit up a line, to between the
            'buildah vendor treadmill' and 'vendor in buildah @ ...' lines
         c) change 'pick' to 'squash' (or just 's')
         d) save & quit to continue the rebase
         e) back to a new editor window
         f) change the commit message: remove fixme-fixme, add a description
            of what you actually fixed. If possible, reference the PR (buildah
            or podman) that introduced the failure
         g) save & quit to continue the rebase

    Now, for good measure, rerun this script.

    For full documentation, refer to

        $Docs_URL
END_WARN
    exit 1;
}

# END   sync and its helpers
###############################################################################
# BEGIN pick and its helpers
#
# This is what gets used on a real vendor-new-buildah PR

sub do_pick {
    my $current_branch = git_current_branch();

    # Confirm that current branch is a buildah-vendor one
    assert_buildah_vendor_commit('HEAD');
    progress("HEAD is a buildah vendor commit. Good.");

    # Identify and pull the treadmill PR
    my $treadmill_pr = treadmill_pr();
    my $treadmill_branch = "$ME/pr$treadmill_pr/tmp$$";
    progress("Fetching treadmill PR $treadmill_pr into $treadmill_branch");
    git('fetch', '-q', git_upstream(), "pull/$treadmill_pr/head:$treadmill_branch");

    # Compare merge bases of our branch and the treadmill one
    progress("Checking merge bases");
    check_merge_bases($treadmill_pr, $treadmill_branch);

    # read buildah go.mod from it, and from current tree, and compare
    my $buildah_on_treadmill = vendored_buildah($treadmill_branch);
    my $buildah_here         = vendored_buildah();
    if ($buildah_on_treadmill ne $buildah_here) {
        warn "$ME: Warning: buildah version mismatch:\n";
        warn "$ME: on treadmill:   $buildah_on_treadmill\n";
        warn "$ME: on this branch: $buildah_here\n";
        # FIXME: should this require --force? A yes/no prompt?
        # FIXME: I think not, because usual case will be a true tagged version
        warn "$ME: Continuing anyway\n";
    }

    cherry_pick($treadmill_pr, $treadmill_branch);

    # Clean up
    git('branch', '-D', $treadmill_branch);

    build_and_check_podman();

    progress("Looks good! Please 'git commit --amend' and edit commit message before pushing.");
}

##################
#  treadmill_pr  #  Returns ID of open podman PR with the desired subject
##################
sub treadmill_pr {
    my $query = <<'END_QUERY';
{
  search(
    query: "buildah vendor treadmill repo:containers/podman",
    type: ISSUE,
    first: 10
  ) {
    edges { node { ... on PullRequest { number state title } } }
  }
}
END_QUERY

    my $ua = LWP::UserAgent->new;
    $ua->agent("$ME " . $ua->agent);              # Identify ourself

    my %headers = (
        'Accept'        => "application/vnd.github.antiope-preview+json",
        'Content-Type'  => "application/json",
    );

    # Use github token if available, but don't require it. (All it does is
    # bump up our throttling limit, which shouldn't be an issue) (unless
    # someone invokes this script hundreds of times per minute).
    if (my $token = $ENV{GITHUB_TOKEN}) {
        $headers{Authorization} = "bearer $token";
    }
    $ua->default_header($_ => $headers{$_}) for keys %headers;

    # Massage the query: escape quotes, put it all in one line, collapse spaces
    $query =~ s/\"/\\"/g;
    $query =~ s/\n/\\n/g;
    $query =~ s/\s+/ /g;
    # ...and now one more massage
    my $postquery = qq/{ "query": \"$query\" }/;

    print $postquery, "\n"            if $debug;
    my $res = $ua->post($API_URL, Content => $postquery);
    if ((my $code = $res->code) != 200) {
        print $code, " ", $res->message, "\n";
        exit 1;
    }

    # Got something. Confirm that it has all our required fields
    my $content = decode_json($res->content);
    use Data::Dump; dd $content         if $debug;
    exists $content->{data}
        or die "$ME: No '{data}' section in response\n";
    exists $content->{data}{search}
        or die "$ME: No '{data}{search}' section in response\n";
    exists $content->{data}{search}{edges}
        or die "$ME: No '{data}{search}{edges}' section in response\n";

    # Confirm that there is exactly one such PR
    my @prs = @{ $content->{data}{search}{edges} };
    @prs > 0
        or die "$ME: WEIRD! No 'buildah vendor treadmill' PRs found!\n";
    @prs = grep { $_->{node}{title} eq $Treadmill_PR_Title } @prs
        or die "$ME: No PRs found with title '$Treadmill_PR_Title'\n";
    @prs = grep { $_->{node}{state} eq 'OPEN' } @prs
        or die "$ME: Found '$Treadmill_PR_Title' PRs, but none are OPEN\n";
    @prs == 1
        or die "$ME: Multiple OPEN '$Treadmill_PR_Title' PRs found!\n";

    # Yay. Found exactly one.
    return $prs[0]{node}{number};
}

#######################
#  check_merge_bases  #  It's OK if our branch is newer than treadmill
#######################
sub check_merge_bases {
    my $treadmill_pr     = shift;       # e.g., 12345
    my $treadmill_branch = shift;       # e.g., b-v-p/pr12345/tmpNNN

    # Fetch latest main, for accurate comparison
    git('fetch', '-q', git_upstream(), 'main');

    my $forkpoint_cur       = git_forkpoint();
    my $forkpoint_treadmill = git_forkpoint($treadmill_branch);

    print "fork cur: $forkpoint_cur\nfork tm:  $forkpoint_treadmill\n"
        if $debug;
    if ($forkpoint_cur eq $forkpoint_treadmill) {
        progress("Nice. This branch is up-to-date wrt treadmill PR $treadmill_pr");
        return;
    }

    # They differ.
    if (git_is_ancestor($forkpoint_cur, $forkpoint_treadmill)) {
        warn <<"END_WARN";
$ME: treadmill PR $treadmill_pr is based on
    a newer main than this branch. This means it might have
    more up-to-date patches.

END_WARN

        if ($force_old_main) {
            warn "$ME: Proceeding due to --force-old-main\n";
            return;
        }

        # Cannot continue. Clean up side branch, and bail.
        git('branch', '-D', $treadmill_branch);
        warn "$ME: You might want to consider rebasing on latest main.\n";
        warn "$ME: Aborting. Use --force-old-main to continue without rebasing.\n";
        exit 1;
    }
    else {
        progress("Your branch is based on a newer main than treadmill PR $treadmill_pr. This is usually OK.");
    }
}

#################
#  cherry_pick  #  cherry-pick a commit, updating its commit message
#################
sub cherry_pick {
    my $treadmill_pr     = shift;       # e.g., 12345
    my $treadmill_branch = shift;       # e.g., b-v-p/pr12345/tmpNNN

    progress("Cherry-picking from $treadmill_pr");

    # Create a temp script. Do so in /var/tmp because sometimes $TMPDIR
    # (e.g. /tmp) has noexec.
    my ($fh, $editor) = tempfile( "$ME.edit-commit-message.XXXXXXXX", DIR => "/var/tmp" );
    printf { $fh } <<'END_EDIT_SCRIPT', $ME, $VERSION, $treadmill_pr;
#!/bin/bash

if [[ -z "$1" ]]; then
    echo "FATAL: Did not get called with an arg" >&2
    exit 1
fi

msgfile=$1
if [[ ! -e $msgfile ]]; then
    echo "FATAL: git-commit file does not exist: $msgfile" >&2
    exit 1
fi

tmpfile=$msgfile.tmp
rm -f $tmpfile

cat >$tmpfile <<EOF
WIP: Fixes for vendoring Buildah

This commit was automatically cherry-picked
by %s v%s
from the buildah vendor treadmill PR, #%s

/vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
> The git commit message from that PR is below. Please review it,
> edit as necessary, then remove this comment block.
\^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

EOF

# Strip the "DO NOT MERGE" header from the treadmill PR, print only
# the "Changes as of YYYY-MM-DD" and subsequent lines
sed -ne '/^Changes as of/,$ p' <$msgfile >>$tmpfile
mv $tmpfile $msgfile

END_EDIT_SCRIPT
    close $fh
        or die "$ME: Error writing $editor: $!\n";
    chmod 0755 => $editor;
    local $ENV{EDITOR} = $editor;
    git('cherry-pick', '--allow-empty', '--edit', $treadmill_branch);
    unlink $editor;
}

# END   pick and its helpers
###############################################################################
# BEGIN reset and its helpers

sub do_reset {
    my $current_branch = git_current_branch();

    # Make sure side branch == main (i.e., there are no commits on the branch)
    if (git('rev-parse', $current_branch) ne git('rev-parse', 'main')) {
        die "$ME: for --reset, $current_branch must == main\n";
    }

    # Pull main, and pivot back to this branch
    pull_main();
    git('checkout', '-q', $current_branch);

    git('rebase', '--empty=keep', 'main');
    git_commit_buildah('[none]');

    my $ymd = strftime("%Y-%m-%d", localtime);
    git('commit', '--allow-empty', '-s', '-m' => <<"END_COMMIT_MESSAGE");
$Treadmill_PR_Title

As you run --sync, please update this commit message with your
actual changes.

Changes since $ymd:
END_COMMIT_MESSAGE

    progress("Done. You may now run --sync.\n");
}

# END   reset and its helpers
###############################################################################
# BEGIN general-purpose helpers

##############
#  progress  #  Progris riport Dr Strauss says I shud rite down what I think
##############
sub progress {
    print $C_Highlight, "|\n+---> @_\n", $C_Reset;
}

#######################
#  assert_clean_repo  #  Don't even think of running with local changes
#######################
sub assert_clean_repo {
    # Our patch file should only exist for brief moments during a sync run.
    # If it exists at any other time, something has gone very wrong.
    if (-e $Patch_File) {
        warn <<"END_WARN";
$ME: File exists: $Patch_File

   This means that something went very wrong during an earlier sync run.
   Your git branch may be in an inconsistent state. Your work to date
   may be lost. This file may be your only hope of recovering it.

   This is not something a script can resolve. You need to look at this
   file, compare to your git HEAD, and manually reconcile any differences.
END_WARN
        exit 1;
    }

    # OK so far. Now check for modified files.
    if (my @changed = git('status', '--porcelain', '--untracked=no')) {
        warn "$ME: Modified files in repo:\n";
        warn "    $_\n" for @changed;
        exit 1;
    }

    # ...and for untracked files under vendor/
    if (my @v = git('status', '--porcelain', '--untracked=all', 'vendor')) {
        warn "$ME: Untracked vendor files:\n";
        warn "    $_\n" for @v;
        exit 1;
    }
}

########################
#  git_current_branch  #  e.g., 'vendor_buildah'
########################
sub git_current_branch() {
    my $b = git('rev-parse', '--abbrev-ref=strict', 'HEAD');

    # There is no circumstance in which we can ever be called from main
    die "$ME: must run from side branch, not main\n" if $b eq 'main';
    return $b;
}

###################
#  git_forkpoint  #  Hash at which branch (default: cur) branched from main
###################
sub git_forkpoint {
    return git('merge-base', '--fork-point', 'main', @_);
}

#####################
#  git_is_ancestor  #  Is hash1 an ancestor of hash2?
#####################
sub git_is_ancestor {
    # Use system(), not git(), because we don't want to abort on exit status
    my $rc = system('git', 'merge-base', '--is-ancestor', @_);
    die "$ME: Cannot continue\n"        if $? > 256; # e.g., Not a valid object

    # Translate shell 0/256 status to logical 1/0
    return !$rc;
}

##################
#  git_upstream  #  Name of true github upstream
##################
sub git_upstream {
    for my $line (git('remote', '-v')) {
        my ($remote, $url, $type) = split(' ', $line);
        if ($url =~ m!github\.com.*containers/(podman|libpod)!) {
            if ($type =~ /fetch/) {
                return $remote;
            }
        }
    }

    die "$ME: did not find a remote with 'github.com/containers/podman'\n";
}

########################
#  git_commit_buildah  #  Do the buildah commit
########################
sub git_commit_buildah {
    my $buildah_version = shift;

    # When called by --reset, this can be empty
    git('commit', '-as', '--allow-empty', '-m', <<"END_COMMIT_MESSAGE");
DO NOT MERGE: vendor in buildah \@ $buildah_version

This is a JUNK COMMIT from $ME v$VERSION.

DO NOT MERGE! This is just a way to keep the buildah-podman
vendoring in sync. Refer to:

   $Docs_URL
END_COMMIT_MESSAGE
}

#########
#  git  #  Run a git command
#########
sub git {
    my @cmd = ('git', @_);
    print "\$ @cmd\n"                   if $verbose || $debug;
    open my $fh, '-|', @cmd
        or die "$ME: Cannot fork: $!\n";
    my @results;
    while (my $line = <$fh>) {
        chomp $line;
        push @results, $line;
    }
    close $fh
        or die "$ME: command failed: @cmd\n";

    return wantarray ? @results : join("\n", @results);
}

##################################
#  assert_buildah_vendor_commit  #  Fails if input arg is not a buildah vendor
##################################
sub assert_buildah_vendor_commit {
    my $ref = shift;                    # in: probably HEAD or HEAD^

    my @deltas = git('diff', '--name-only', "$ref^", $ref);

    # It's OK if there are no deltas, e.g. immediately after a buildah vendor PR
    return if !@deltas;

    # It's OK if there are more modified files than just these.
    # It's not OK if any of these are missing.
    my @expect = qw(go.mod go.sum vendor/modules.txt);
    my @missing;
    for my $expect (@expect) {
        if (! grep { $_ eq $expect } @deltas) {
            push @missing, "$expect is unchanged";
        }
    }

    if (! grep { m!^vendor/\Q$Buildah\E/! } @deltas) {
        push @missing, "no changes under $Buildah";
    }

    return if !@missing;

    warn "$ME: $ref does not look like a buildah vendor commit:\n";
    warn "$ME:  - $_\n" for @missing;
    die "$ME: Cannot continue\n";
}

######################
#  vendored_buildah  #  Returns currently-vendored buildah
######################
sub vendored_buildah {
    my $gomod_file = 'go.mod';
    my @gomod;
    if (@_) {
        # Called with a branch argument; fetch that version of go.mod
        $gomod_file = "@_:$gomod_file";
        @gomod = git('show', $gomod_file);
    }
    else {
        # No branch argument, read file
        open my $fh, '<', $gomod_file
          or die "$ME: Cannot read $gomod_file: $!\n";
        while (my $line = <$fh>) {
            chomp $line;
            push @gomod, $line;
        }
        close $fh;
    }

    for my $line (@gomod) {
        if ($line =~ m!^\s+\Q$Buildah\E\s+(\S+)!) {
            return $1;
        }
    }

    die "$ME: Could not find buildah in $gomod_file!\n";
}

# END   general-purpose helpers
###############################################################################