Function safe_text_patch 

pub fn safe_text_patch(
    path: &Path,
    content: &str,
    expected_hash: Option<&str>,
    session_id: Option<&str>,
    create_parents: bool,
    overwrite: bool,
) -> Result<SafeTextPatchOutcome, HostlibError>

Atomic compare-and-swap-style text write.

Reads the current bytes at path through the staged-fs overlay (when a session is active) so concurrent agent edits see each other’s pending writes. If expected_hash is supplied and differs from the observed sha256:HEX, returns SafeTextPatchResult::StaleBase without mutating any state. On a hash match the post-image is written through the same overlay path, keeping the read and the write atomic with respect to other staged-fs consumers in the same process.

Atomicity:

• When a session is in staged mode, the read, hash check, and write all happen under a single acquisition of the sessions mutex, so a sibling thread cannot stage a write into the window between the pre-image snapshot and the commit.
• When the call routes through disk (no active session, or session in immediate mode), the write goes through an atomic rename-into-place so partial-write tearing is impossible. Cross-process races are intentionally out of scope — the staged-fs overlay is the collision-rejection layer.