Heathrow tourist trap buses

So I slept in the Sheraton Heathrow hotel last night. It's a half-an-hour walk away from the airport, or a five minute bus drive.

If you go to any service desk and ask for the best way to get to the hotel, they'll offer you a ticket for the 'HotelHoppa' service, which will stop right in front of your hotel entrance, and costs £4 for a one-way trip (even though the bus sides declare a £4.5 charge)

However, if you take the regular "London United" bus service (which drives all over London, not just near the airport like the HotelHoppa service) then the same trip is completely free of charge. You may have to walk about a hundred yards more to reach your hotel, since the bus will stop at a regular bus stop rather than at the hotel entrance, but that's hardly a problem.

What a ripoff.

So, to clarify, don't take this bus:

Instead, take this bus:

This message brought by your friendly neighbourhood anti-ripoff service.

Compilers should not segfault.

Ever.

(this service message brought to you by the society for reimplementing what used to be a bad idea in another language)

DebConf 12 is over

As I write this, DebConf12 has actually been over for more than a week.

Usually during DebConf, I'm a more proficient blogger than during other times of the year, since I'm not doing as much work that I have to remain quiet about at that time, which simply means I have more to blog about. Unfortunately, however, this year the server on which this blog runs managed to become unavailable on the second day of DebCamp, an issue that I could only fix once I was back in Belgium, which meant I haven't been able to blog as much as I would've wanted to.

Anyway, all I can say is that both DebConf and DebCamp have been a huge success for me again, this year. I'd started the second rewrite of ipcfg a few weeks before DebCamp, and spent most of DebCamp whipping the basic framework into shape. It's nowhere near ready yet, but I'm confident it'll get further this time than it did before.

During DebConf, I was also responsible for doing the group photo. I'd done that (together with Andrew McMillan) during DebConf8 before (if you look closely, you'll see me pressing the remote which actually triggers that picture), and I still like that picture. This time around, we didn't have a beach nearby (at least not as nearby as during DebConf8), so we had to do things slightly differently. Rather than doing one large picture, I took several pictures and used Hugin to stitch them together.

Unfortunately however, some things did go wrong with the group photo. For starters, the weather wasn't very helpful, in that I noticed it was about to rain. As such, the main bunch of pictures were taken slightly before the announced time of 14:30, which meant that some people had missed it. To make matters worse, I had rushed to the group and handed the camera to someone else, so that I could gimp myself onto the picture too; but that meant that some people who were arriving as these pictures were being taken thought they were on the picture, when they really weren't. I've fixed most of these by just gimping them into the picture as well, but I might've missed some of them. If you're one of them, talk to me! Preferably now.

If you're not one of them (or even if you are), hold off on doing things like printing it in large format or some such. I'll re-upload improved versions onto the same location once they're ready.

Runner scripts

Bert Desmet blogs about having to run a script more than once per minute, and using a runner script (started from cron) to do that. Unfortunately, his implementation contains a race condition:

if ! ps -p $(cat /path/to/runner.pid);
  then
    echo $$ > /path/to/runner.pid

If the script is accidentally started twice on approximately the same moment, chances are pretty high you'll see them both executing the ps call at approximately the same time, concluding that no other script is running, and then both executing the echo call at the same time. Result: your script will be running twice, yet you'll have only one pid file.

Of course, if you do indeed run the script only from cron, and just once every 30 minutes, as in Bert's case, chances that you'll hit this race condition are pretty low. However, race conditions are bad, especially if they're easy to avoid.

set -o noclobber
if ! echo $$ > /var/run/runner.pid
then
  echo pidfile exists; exiting
  exit 0
fi

This code uses the noclobber shell option, which causes the shell to issue an error state if the file we try to overwrite with > already exists, and to do so in an atomic operation (so, no race condition there: either the file exists and we fail, or the file does not exist yet and we create it with our PID). Of course, that leaves out the check for is the other process running? Adding a second check for that is fairly complex to do right, as this cannot be done in an atomic operation; so it's usually better use exit traps to clean up the file:

set -o noclobber
if ! echo $$ > /var/run/runner.pid
then
  echo pidfile exists; exiting
  exit 0
fi
trap EXIT rm -f /var/run/runner.pid

This will fail if the system dies when the script is running, but then since we use /var/run (which is cleared on bootup, either because it's a tmpfs or because the initscripts clean it) this means the file should only ever be stale if some genius kills our process with kill -9 or kill -11.