If any system user entry exceeds `GETPW_R_SIZE_MAX`, retreiving any user is impossible about crystal HOT 2 OPEN

Thanks for digging into this 🫶

I'm wishing we'd implement more of these system functions in Crystal. It's nice and easy to use libc functions, but they have a complexity as well. And there are many pitfalls for using them correctly, demonstrated by the multiple different bugs we have identified in these relatively small pieces of code (#14619, #14615 and this one).
Of course, rolling our own implementation also adds complexity and we need to ensure correctness. We have some prior art to help with that though. And overall the implementations could end up simpler and more efficient. Looking up some string in /etc/passwd should be easy enough. And we only need to allocate memory for the field we're actually looking for.
Adding nscd might be a bit more complicated, although I'm wondering if that's even much useful anymore... We might just get away without it.

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Adding nscd might be a bit more complicated, although I'm wondering if that's even much useful anymore... We might just get away without it.

I think /etc/passwd and /etc/groups will basically always fit into a single 4K memory page, so there's probably no difference to sending a lookup request to nscd over a unix socket (musl also only uses nscd as a fallback, so it's probably alright).

Manually implementing the file parsing also allows us to trivially implement methods like System::User#each.

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