#!/usr/bin/perl

use strict;
use warnings;

use 5.010;
use POSIX qw(_exit);
use DBI;
use File::Temp;
use Time::HiRes qw(usleep);

$| = 1;
my $dbfile = "/tmp/test.db";
unlink $dbfile;

my $dbh = DBI->connect("dbi:SQLite:$dbfile", "", "", {RaiseError => 1});

sub lazy { usleep rand 10000 }

sub DBI::db::prepare_retry {
    my $dbh = shift;
    while (1) {
        my $sth = eval { $dbh->prepare(@_) };
        return $sth if defined $sth;
        if ($dbh->err == 5) {
            print "!";
            sleep 1;
            redo;
        }
        die ($@ || $dbh->errstr);
    }
}

$dbh->do("create table queue (id integer primary key, child, n, take)");
for my $w ('00'..'20') {
    fork or do {
        print "<$w enter>";
        eval {
            my $dbh = DBI->connect("dbi:SQLite:$dbfile", "", "", {RaiseError => 1});
            # $dbh->do("PRAGMA journal_mode = TRUNCATE");
            $dbh->do("PRAGMA synchronous = OFF");
            my $sth = $dbh->prepare_retry("insert into queue values (NULL, ?, ?, NULL)");
            print "<$w with sth>";
            for (1..1000) {
                lazy;
                local $@;
                eval {
                    $sth->execute($w, $_);
                    print $w;
                };
                if ($@) {
                    if ($dbh->err == 5) {
                        print "!";
                        sleep 1;
                        redo;
                    }
                    print "<$w error: $@>"
                }
            };
        };
        $@ and print "<$w error: $@>";
        print "<$w exit>";
        _exit(0);
    }
}

# sleep 1;

for my $w ('a'..'h') {
    fork or do {
        print "<$w enter>";
        eval {
            my $dbh = DBI->connect("dbi:SQLite:$dbfile", "", "", {RaiseError => 1});
            $dbh->do("PRAGMA synchronous = OFF");
            my $sth0 = $dbh->prepare_retry("select id, child, n, take from queue where take is NULL limit 1");
            my $sth1 = $dbh->prepare_retry("update queue set take=? where id=? and take is NULL");
            my $sth2 = $dbh->prepare_retry("delete from queue where id=?");
            print "<$w with sth1 and sth2>";
            my ($id, $child);
            while (1) {
                lazy;
                eval {
                    $sth0->execute;
                    ($id, $child) = $sth0->fetchrow_array;
                    if (defined $id) {
                        if ($sth1->execute($w, $id)) {
                            # do something here!
                            $sth2->execute($id);
                            print $w
                        }
                        else {
                            print "!";
                        }
                    }
                };
                if ($@) {
                    if ($dbh->err == 5) {
                        print "!";
                        sleep 1;
                        redo;
                    }
                    else {
                        print "<$w error: " . $dbh->errstr . "(".$dbh->err.")>";
                        die;
                    }
                }
                last unless defined $id;
            }
        };
        # $@ and print "<$w error: " . $dbh->errstr . "(".$dbh->err.")>";
        print "<$w exit>";
        exit(0);
    }
}

1 while (wait > 0);

my $sth = $dbh->prepare("select count(*) from queue");
$sth->execute;
my $row = $sth->fetchrow_arrayref;
print "\n@$row\n";