Implement inheriting more than 3 file descriptors for UNIX Commands

[ipetkov]

• This allows creating sub-processes through std::io::process::Command with file descriptors beyond stdio
• Support is still missing under Windows

[rust-highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[thestinger]

We really need to switch to posix_spawn rather than digging deeper into this hole. Along with performance simplicity, there are issues like #14232. It should likely all be thrown out and rewritten as part of the runtime rewrite.

[alexcrichton]

I have investigated posix_spawn in the past because I agree I would love to use it! I could not, however, figure out how to change the working directory of the child process, which is a fairly critical aspect, so I was unable to make progress on that front. Do you know if it's possible to change the current directory of the child with posix_spawn?

[alexcrichton]

@ipetkov thanks for the patch! I'll try to take a closer look tomorrow. I'm pretty wary of implementing support for a feature such as this while ignoring windows, especially without an expected frame of time to have it implemented. Would you be willing to implement the windows side of the API as well?

[ipetkov]

@alexcrichton I'd be willing to take a stab at implementing the Windows side of the API, though I don't have much experience with the WINAPI itself, and I'm not sure what is the correct way to go about implementing this.

After a few Google searches it looks like UpdateProcThreadAttribute function might be able to accomplish setting the child's file descriptors in the right order (though I'm not sure how it would work for leaving holes without opening handles to NUL), however, WinXP does not support this call...

Alternatively the files to inherit could be shuffled around via _dup/_dup2 etc., but without a direct call to fork() the file descriptors will be modified in the parent. Resetting them back to their original state before returning to the caller seems like a difficult problem to handle.

[alexcrichton]

Most of my inspiration for these apis came from libuv's implementation, and you can also take a look at their windows implementation of more than 3 stdio file descriptors.

That being said, now that I look at it, it looks significantly complicated... We may be able to implement a somewhat stripped down version to get the tests passing and then we can add more features later on.

[alexcrichton]

Closing out of inactivity, and I think that with the runtime removal this functionality is likely to just disappear as it's unimplemented entirely right now. It would of course be nice to add support for this, but we'll need to figure out a story for windows at the same time as well.