Added direct support for fchown and dup3

[koutheir]

Added direct support for fchown and dup3.

Fixes #939.

[koutheir]

In order to be able to use dup3() on FreeBSD, I also added the pull request #1101 to the libc crate.

[asomers]

Please add "Fixes #939" to your commit message.

[asomers]

Don't forget the CHANGELOG. Make it clear that dup3 has been removed from OSX.

[koutheir]

Don't forget the CHANGELOG. Make it clear that dup3 has been removed from OSX.

The previous implementation of dup3 was buggy because it was vulnerable to a race condition. I would argue that it should have never been implemented for OSX (or at least, not via dup2 and fcntl). Furthermore, one of the principles in this project is to avoid providing APIs that are not actually implemented on the platform.

[Susurrus]

The previous implementation of dup3 was buggy because it was vulnerable to a race condition. I would argue that it should have never been implemented for OSX (or at least, not via dup2 and fcntl). Furthermore, one of the principles in this project is to avoid providing APIs that are not actually implemented on the platform.

The CHANGELOG should also be viewed as everything someone needs to read to allow them to upgrade. So if a user was using it on OS X, and we broke their code during an upgrade, there should be a part under the Removed section of the CHANGELOG stating a) what we removed and b) why we did so. Could you add a line there for this?

[koutheir]

Could you add a line there for this?

For dup3, I wrote two change log entries:

• Under Changed, to mention the direct system call, instead of emulation, and
• Under Removed, to inform OSX users of the removal.

[Susurrus]

CHANGELOG entries look good, thanks for adding those!

[Susurrus]

There are Travis failures that need to be addressed. Besides that this looks good, though it should be squashed into 2 commits:

• Add fchown
• Modify dup3

After those are addressed, this is ready for merging.