Module atomic 

Atomic symlink swap primitives and helpers.

This module implements a TOCTOU-safe sequence using directory handles: open_dir_nofollow(parent) -> symlinkat(tmp) -> renameat(tmp, final) -> fsync(dirfd).

Test override knobs:

• SWITCHYARD_FORCE_EXDEV=1 — simulate a cross-filesystem rename error (EXDEV) to exercise degraded fallback paths and telemetry in higher layers.

Functions

atomic_symlink_swap

Atomically swap a symlink target using a temporary file and renameat.

fsync_parent_dir

Fsync the parent directory of path for durability.

open_dir_nofollow

Open a directory with O_DIRECTORY | O_NOFOLLOW for atomic operations.