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Why?

Most compilers give no guarantees in which order they lay out the data. I love deterministic processes as much as everyone. But randomized approaches have their advantages too. And if a compiler has reasons to randomize output e.g. for speed than it’s a trade off to consider.

Thread finishes work

grabs lock

writes to file

writes to index

releases lock

That's not a race condition. The output order doesn't matter, but it is nondeterministic.

Why is it a bug? I write a program to download four files. I do so in parallel. Sometimes X finishes first, sometimes Y finishes first, and the files are written to disk in a different order. Why do I want to serialize this operation?

The end result is a set of four files. You don't care about the order they are laid out on the disk and the next steps shouldn't let the order of those files influence the end result.

Let's assume there is a latent bug in the compiler that gets triggered if file four is the first one. Good luck debugging that.

But parent claimed that creating a set with a different order was inherently a bug. Not that depending on the order of an unordered set was a bug.

It's the internal structure of the files.