Rewriting the GNU Coreutils in Rust [LWN.net]

Rewriting the GNU Coreutils in Rust

Posted Jun 9, 2021 23:17 UTC (Wed) by Sesse (subscriber, #53779)
In reply to: Rewriting the GNU Coreutils in Rust by epa
Parent article: Rewriting the GNU Coreutils in Rust

Most syscalls can very well return EINTR also on modern Linux. You may think it is cruft, but you cannot just remove it.

Rewriting the GNU Coreutils in Rust

Posted Jun 10, 2021 5:34 UTC (Thu) by epa (subscriber, #39769) [Link] (7 responses)

I agree, my point is that hopefully Rust and its standard library handle this kind of thing for you, so you don't need boilerplate in every call site.

Rewriting the GNU Coreutils in Rust

Posted Jun 10, 2021 7:50 UTC (Thu) by Sesse (subscriber, #53779) [Link] (4 responses)

That's assuming all you want to do is restart. Frequently, that's not what you want; the reason this behavior persists is so that you _can_ abort syscalls. (You can turn it off in the signal handler by setting SA_RESTART, that's got nothing to do with the language.)

Say that you have some long-running computation that you want to be able to abort cleanly. Now further assume that your thread is stuck in a syscall; say either waiting for a write to file on NFS, or waiting for data from socket. Let's say a recvfrom(). You trap SIGINT, and in your handler, you set a “please shut down” flag. (You can't do much else in a signal handler!) Now, if this didn't abort your recvfrom() with an EINTR, how would you ever get to actually check that flag and shut down?

Rewriting the GNU Coreutils in Rust

Posted Jun 15, 2021 12:44 UTC (Tue) by epa (subscriber, #39769) [Link] (3 responses)

What you say is quite right in general but I am talking about coreutils specifically. Those are short-lived command line programs with no event loop. (And, I would suggest, any program where the programmer hasn't explicitly decided to allow for signal handling and interruptions. The default should be to retry automatically, simulating a world in which EINTR doesn't exist, but of course the standard library should include an interruptible variant of each operation you can call explicitly and handle the error explicitly.)

Rewriting the GNU Coreutils in Rust

Posted Jun 15, 2021 19:02 UTC (Tue) by nix (subscriber, #2304) [Link] (2 responses)

> Those are short-lived command line programs with no event loop

Many, perhaps most of the commands in coreutils are or can operate as filters. These can be arbitrarily long-lived and spend nearly all their time blocked on I/O.

Rewriting the GNU Coreutils in Rust

Posted Jun 16, 2021 5:03 UTC (Wed) by epa (subscriber, #39769) [Link] (1 responses)

Right, but they are not event driven. At least not in the classical implementation. Maybe with the Rust implementation it’s much more parallelized, there is cleanup work to do on being interrupted, and so you do need to explicitly check for system calls giving EINTR and do something other than just retrying. I still think Rust’s error handling mechanisms will make that cleaner than in C.

Rewriting the GNU Coreutils in Rust

Posted Jun 16, 2021 14:46 UTC (Wed) by nix (subscriber, #2304) [Link]

Oh, definitely. The need to wrap everything in either if statements or EINTR-checking while loops makes robust C unbelievably ugly compared to Rust of corresponding robustness, where you usually just need a trailing ?.

Rewriting the GNU Coreutils in Rust

Posted Jun 10, 2021 15:45 UTC (Thu) by mathstuf (subscriber, #69389) [Link] (1 responses)

> Rust and its standard library handle this kind of thing for you

That's not good in a system's language. You want to get the errors as they occur. Now, the stdlib can provide combinators to say "restart on EINTR" around these calls to abstract at least that away, but Sesse's point is that you can't just hide it away in all cases.

Rewriting the GNU Coreutils in Rust

Posted Jun 11, 2021 20:09 UTC (Fri) by notriddle (subscriber, #130608) [Link]

That's what it does.

In the documentation for Write, it says that the regular write call can return Interrupted. The wrapper that automatically retries is called write_all.