#! /usr/bin/perl

use POSIX;
use strict;
use warnings;

my $default = '/etc/default/openvswitch-switch';

my (%config) = load_config($default);
if (@ARGV) {
    foreach my $arg (@ARGV) {
        my ($key, $value) = $arg =~ /^([^=]+)=(.*)/
          or die "bad argument '$arg'\n";
        if ($value ne '') {
            $config{$key} = $value;
        } else {
            delete $config{$key};
        }
    }
    save_config($default, %config);
}
print "$_=$config{$_}\n" foreach sort(keys(%config));

sub load_config {
    my ($file) = @_;

    # Get the list of the variables that the shell sets automatically.
    my (%auto_vars) = read_vars("set -a && env");

    # Get the variables from $default.
    my (%config) = read_vars("set -a && . '$default' && env");

    # Subtract.
    delete @config{keys %auto_vars};

    return %config;
}

sub read_vars {
    my ($cmd) = @_;
    local @ENV;
    if (!open(VARS, '-|', $cmd)) {
        print STDERR "$cmd: failed to execute: $!\n";
        return ();
    }
    my (%config);
    while (<VARS>) {
        my ($var, $value) = /^([^=]+)=(.*)$/ or next;
        $config{$var} = $value;
    }
    close(VARS);
    return %config;
}

sub shell_escape {
    local $_ = $_[0];
    if ($_ eq '') {
        return '""';
    } elsif (m&^[-a-zA-Z0-9:./%^_+,]*$&) {
        return $_;
    } else {
        s/'/'\\''/;
        return "'$_'";
    }
}

sub shell_assign {
    my ($var, $value) = @_;
    return $var . '=' . shell_escape($value);
}

sub save_config {
    my ($file, %config) = @_;
    my (@lines);
    if (open(FILE, '<', $file)) {
        @lines = <FILE>;
        chomp @lines;
        close(FILE);
    }

    # Replace all existing variable assignments.
    for (my ($i) = 0; $i <= $#lines; $i++) {
        local $_ = $lines[$i];
        my ($var, $value) = /^\s*([^=#]+)=(.*)$/ or next;
        if (exists($config{$var})) {
            $lines[$i] = shell_assign($var, $config{$var});
            delete $config{$var};
        } else {
            $lines[$i] = "#$lines[$i]";
        }
    }

    # Find a place to put any remaining variable assignments.
  VAR:
    for my $var (keys(%config)) {
        my $assign = shell_assign($var, $config{$var});

        # Replace the last commented-out variable assignment to $var, if any.
        for (my ($i) = $#lines; $i >= 0; $i--) {
            local $_ = $lines[$i];
            if (/^\s*#\s*$var=/) {
                $lines[$i] = $assign;
                next VAR;
            }
        }

        # Find a place to add the var: after the final commented line
        # just after a line that contains "$var:".
        for (my ($i) = 0; $i <= $#lines; $i++) {
            if ($lines[$i] =~ /^\s*#\s*$var:/) {
                for (my ($j) = $i + 1; $j <= $#lines; $j++) {
                    if ($lines[$j] !~ /^\s*#/) {
                        splice(@lines, $j, 0, $assign);
                        next VAR;
                    }
                }
            }
        }

        # Just append it.
        push(@lines, $assign);
    }

    open(NEWFILE, '>', "$file.tmp") or die "$file.tmp: create: $!\n";
    print NEWFILE join('', map("$_\n", @lines));
    close(NEWFILE);
    rename("$file.tmp", $file) or die "$file.tmp: rename to $file: $!\n";
}