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Comment by pjc50 | Hacker News Reader

"Check before" is a race condition nightmare - for anything where you care about concurrency, you must do the thing and then see whether it succeeded or not, assuming that the underlying layer is basically sound in this regard.

The world is full of bugs of the form "does this file exist? no? OK, open it for writing", which is exploitable by dropping a symlink in there between the check and the open.

The classic primitive CMPXCHG works like this too; you use it as you would a "store", but one that might fail if another thread has changed a value in the meantime.

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That case is even much more exceptional: where the record existed when you checked but didn't in the millisecond afterwards when you actually opened it. But if that is common, then I would guess a concurrent check for existence and open if it is might be necessary.

That’s a very, very common source of subtle bugs (often exploitable as security bugs) that are hard to spot and hard to test for.

It’s good practice to design APIs to make race conditions less likely, by explicitly not splitting operations across multiple calls.

Separating “exists” and “get” into separate calls is a disaster.

I would compare this to a map API: an access could throw if a key wasn't present, but you can check to see if the key was in there first anyways. As long as the map isn't being used concurrently, it isn't that much of a problem, and if it is being used concurrently, then you need a different design. In the former case, separating "exists" and "get" would be considered over-engineering.

I guess databases and file systems are different because concurrent access is the norm, not the exception, and APIs should be designed accordingly. But for most of your in-memory structures, you probably don't want them dealing with concurrency because they aren't going to be able do that very well anyways (instead, deal with concurrency outside of the data structure and have them throw when inconsistencies from bad concurrency policies arise).

For most implementations of maps (hashtables and various tree-ish things), even in the single-threaded case, doing .exists and then .get is about twice as expensive as doing a .get that returns an Option (or nullable reference, default value, whatever)- you have to do the map lookup twice.

(Okay, probably not twice as slow, since the cache will be hot, but still.)

Not twice as slow. But you are forgetting the case where you know the element is already in the map, and you want to call map.get() without wrapping it in an option.

C# does this fairly nicely with an "out" parameter: https://docs.microsoft.com/en-us/dotnet/api/system.collectio...

Or if you "know" it's there call the version that throws an exception if it isn't.

I haven't used that aspect of the Dictionary API at all in the 9 years programming C#, it is just too unwieldy to have to define a separate local variable to hold the result of the get. It also doesn't scope the out assignment (if the get fails, you can still read the out variable bound). I'm rarely in the case where I don't expect something to be in a map anyways, and when I do, I have an extension method with a default value returned if the key doesn't exist (or in some cases populate). Keeping expressions clean is important.

Exceptional doesn’t mean uncommon, it means your program is in an invalid state. Your programming style is inherently unsafe, and you are derelict of your most basic responsibility as a software professional if you apply it on any software of consequence. This is why we can’t have nice things.

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