use closefrom(2) on platforms supports it to reduce race condition window

[michael-yuji]

Use closefrom(2) instead of close(2) in a loop to reduce the window of race condition with other part of the process cloud be polling/processing the file descriptors.

Motivation:

Currently, a loop is used to close all possible file descriptors prior to execve(2) being called. This has the side effect that file descriptors owned by different components of the same process is being closed unexpectedly, which cause issue #8197.

Use closefrom(2) instead of close in a loop to significantly reduce the window between closing the file descriptors and calling execve(2), to minimize the impact.

In #8197, linking a debug build of libdispatch can randomly crash swift-run, due to dispatch catching that the file descriptors it works on no longer being valid, which causes an assertion and crash the process.

Modifications:

Use closefrom(2) instead of close(2) in a loop on platforms supports it.

Result:

Reduce the probability swift-run crash due to race conditions.

[michael-yuji]

@3405691582 I'm making the assumption that closefrom(2) is also available on the OpenBSD Swift toolchain.

[jakepetroules]

@3405691582 I'm making the assumption that closefrom(2) is also available on the OpenBSD Swift toolchain.

Looks like it should be: https://man.openbsd.org/closefrom

[3405691582]

Yes, it's on the platform, but don't we need to import Glibc for that? Or am I missing something?

[jakepetroules]

Ah, that I don't know. Are there any libc imports needed for FreeBSD?

[michael-yuji]

On FreeBSD, no further imports are needed. 🤔

[3405691582]

It's probably fine then. It's been a while for me so I'm guessing it's probably some transitive import?

In the worst case, when I get back to tweaking SPM, can revisit.

[michael-yuji]

@swift-ci please test

[jakepetroules]

@swift-ci please test Windows

[jakepetroules]

Merged at @michael-yuji's request as he does not yet have write access.