Use less syscalls in FileDesc::set_{nonblocking,cloexec}

[tbu-]

Only set the flags if they differ from what the OS reported, use
FIONBIO to atomically set the non-blocking IO flag on Linux.

[rust-highfive]

r? @aturon

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[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit efeb42b has been approved by alexcrichton

[nagisa]

FIONBIO is supported by pretty much every recent BSD as well.

[tbu-]

@nagisa It's hard to find docs on ioctls, do you know any sources?

[nagisa]

@tbu- not really. This is a pretty obscure API and hence rarely documented. I know it to work because I had opportunity to test it out in an actual system.

mio may be a good indicator it “works” as it uses this API unconditionally.

[joshtriplett]

You may also want to document, in the documentation for set_cloexec, that you often want to open with cloexec already set rather than setting it afterward, and show how to do that. (Doing it within open avoids a race that can leak file descriptors to child processes.)

[tbu-]

@joshtriplett This is private API as far as I can tell, and it's only used in places where we already pass O_CLOEXEC. It's apparantly just for old kernels which are either unaware of O_CLOEXEC or have a bug relating to it.

[joshtriplett]

@tbu- Ah, OK; thanks for the clarification.