renameat2 for atomic IO

[juancarlospaco]

• Use renameat2 with RENAME_EXCHANGE for Atomic IO.
• renameat2 is the same as renameat but allows additional arguments.
• RENAME_EXCHANGE is a constant int flag for renameat2 to atomically exchange an old path with a new path.
• It is faster because is a single swap operation, but thats not the point.
• It is safer because the 2 paths do not exist at the same time, instead swap atomically.
• Tiny diff, because is used by std/os only.
• Alternatively can be a new Nim symbol, swapDir ?.

Cons

• renameat2 and RENAME_EXCHANGE is Linux only, but thats easy to detect in Nim.
• MUSL does NOT have renameat2 nor RENAME_EXCHANGE, but thats easy to detect in Nim.

Solution: Fallback to current implementation iff MUSL or non-Linux.

Links

• https://github.com/nim-lang/Nim/blob/devel/lib/pure/os.nim#L1123
• https://stackoverflow.com/a/27862160
• https://man7.org/linux/man-pages/man2/rename.2.html
• https://lists.gnu.org/archive/html/coreutils/2021-05/msg00030.html
• https://gist.github.com/eatnumber1/f97ac7dad7b1f5a9721f
• https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/namei.c?id=7bb91e06730140a693611e51a4a9636152448bd3#n4474

Sample

#include <fcntl.h>
#include <stdio.h>
#include <sys/syscall.h>
#include <unistd.h>

/* MUSL Fallback */
#ifndef RENAME_EXCHANGE
#define RENAME_EXCHANGE 2
#endif

int main(int argc, char** argv) {
    /* MUSL Fallback */
    if (syscall(SYS_renameat2, AT_FDCWD, argv[1], AT_FDCWD, argv[2], RENAME_EXCHANGE)) {
        perror(NULL);
        return 1;
    } else {
        return 0;
    }
}

$ mkdir old
$ mkdir new
$ gcc -o moveit moveit.c
$ ./moveit "old" "new"

You can use strace ./moveit "old" "new" or similar to see differences with rename API.

/cc @timotheecour