Use Swift System to implement NonBlockingFileIO

[simonjbeaumont]

Motivation:

#1957 suggests we make use of Swift System for the NonBlockingFileIO implementation which would also reduce its dependency on NIOPosix.

Modifications:

• Add dependency on Swift System package.
• Replace implementation of NonBlockingFileIO with System's FileDescriptor-based implementation.

To do:

• Currently this PR depends on an unmerged PR in swift-system, this needs to be merged and then we need to depend on a released version.
• Decide if we want to expose the withUnsafeSystemFileDescriptor function (probably not initially?)

[simonjbeaumont]

@Lukasa any thoughts on this one?

[weissi]

@compnerd / @milseman Does swift-system have FileDescriptor on Windows and all other platforms?

[simonjbeaumont]

It does have FileDescriptor on Windows but, now you mention it, we explicitly opted to not include resize() on Windows. See apple/swift-system#82.

[compnerd]

That is easily accomplished with SetFilePointerEx and SetEndOfFile; you will need _get_osfhandle to convert the file descriptor to the appropriate handle.

[simonjbeaumont]

Thanks @compnerd. I guess that means that this (NIO) PR is probably blocked on a follow-up PR to swift-system to make sure that Windows has a FileDescriptor.resize(to:) implementation.

[simonjbeaumont]

Here's the PR for a Windows implementation of FileDescriptor.resize(to:): apple/swift-system#89

[1proprogrammerchant]

seems the commits are out of date to the branch

[simonjbeaumont]

seems the commits are out of date to the branch

Yeah, this one died on the vine. And the addition of ftruncate to swift-system for Windows got reverted because it broke the build and we don't have any CI on swift-system for Windows so I'm a bit hesitant to add it again.

[Lukasa]

We’ve deprecated NonBlockingFileIO.