Robert G sends us a WTF from the outside, but I think we can come up with some hypotheses about what actually happened.

Robert was traveling by train. For complicated train reasons, the car he was seated on was not actually scheduled to continue with the train he was booked to ride, the replacement can missed its scheduled arrival, and the result was that Robert was left at a station needing to rebook his travel.

Now, given that this is in a region of the world with a fairly robust train network, this represented a hiccup of thirty minutes in a many hour trip. Not a particularly big deal. But Robert didn't want to get stranded any longer than he had to, so he took out his phone, fired up the train company's app, and made sure he had a reservation on the next train. He got the confirmation, and grabbed a coffee while he waited.

Not quite thirty minutes later, Robert boarded the train and went towards his reserved seat. Another passenger approached from the opposite direction, and they met at the seat. "Excuse me," Robert said, "but I reserved this one."

"No, I reserved this one."

Being reasonable people, they both pulled out their phones and pulled up their apps. Sure enough, each one of them was confirmed for that seat. Since the email confirmation had more details than the app provided, they both pulled up the email. Same seat. Both confirmed. The timestamp of the reservation? The same, down to the second.

"I don't think the conductor will let us sit in each other's laps," the other passenger said.

As it turned out, there was enough room on the train that no lap sitting was required. Everyone got a seat, even if it wasn't the one they reserved.

But the story stuck with Robert. A major transit provider had a reservation system that clearly had a race condition. It seemed like the most basic example of race conditions too, the kind you're shown in college to explain the basic concept.

"Check if the seat is reserved, and if it's not, reserve it." Because that sequence isn't atomic, two different threads could do the check, see it's not reserved, and then reserve it. I suppose it's at least something that one reservation doesn't overwrite the other, which at least indicates that a reservation is its own record.

The real question is this: this has to happen with some frequency, so how has nobody fixed it?

The real answer: they probably just don't care, either because they literally don't care, or because their software is such a trashfire that this doesn't even rise to the top ten bugs. It's likely a mix of both.

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