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//! Filesystem-backed [`CoordinationLock`] (POSIX advisory `flock`, unix).
//!
//! One lock file per `key` under `root`. The flock call is blocking, so it is
//! offloaded via [`tokio::task::spawn_blocking`].
use std::{fs::OpenOptions, io, os::fd::AsRawFd, path::PathBuf, time::Duration};
use async_trait::async_trait;
use crate::traits::{CoordinationLock, LockError, LockGuard};
/// Filesystem-backed advisory lock. One lock file per `key` under `root`.
pub struct FileLock {
root: PathBuf,
}
impl FileLock {
#[must_use]
pub fn new(root: impl Into<PathBuf>) -> Self {
Self { root: root.into() }
}
}
// FileGuard carries only the locked fd: the sentinel's path used to ride
// along for the release-time unlink, but malkuth-R1-10 kept the file as a
// zero-byte sentinel instead (an unlink-then-create race could mint a
// double-holder) — the path has been dead state ever since, so it stays
// out of the guard entirely.
struct FileGuard {
file: Option<std::fs::File>,
}
#[async_trait]
impl LockGuard for FileGuard {
async fn release(&mut self) {
if let Some(file) = self.file.take() {
// SAFETY: fd is valid and owned by `file`.
unsafe {
libc::flock(file.as_raw_fd(), libc::LOCK_UN);
}
// Audit malkuth-R1-10: DO NOT remove the lock file here —
// an unlink-then-create race lets a waiter holding an fd to
// the old inode acquire a lock on a file nobody else can
// see (classic double-holder). The file is a zero-byte
// sentinel; leaving it costs nothing and keeps every
// waiter's fd pointing at the same inode.
}
}
}
impl Drop for FileGuard {
fn drop(&mut self) {
// Best-effort release if `release()` wasn't called explicitly.
if let Some(file) = self.file.take() {
unsafe {
libc::flock(file.as_raw_fd(), libc::LOCK_UN);
}
// Same rationale as release(): never unlink the sentinel.
}
}
}
#[async_trait]
impl CoordinationLock for FileLock {
/// Acquire an exclusive `flock` on the lock file for `key` with a single
/// non-blocking `LOCK_EX | LOCK_NB` try.
///
/// # Semantics
///
/// - The lock lives until [`LockGuard::release`], the guard's `Drop`
/// best-effort fallback, or process exit (the kernel releases a `flock`
/// when the process or its file descriptor goes away).
/// - `lease` is **ignored** — there is no TTL and no automatic expiry.
///
/// On contention this returns [`LockError::Contended`] immediately; it
/// never queues or waits.
async fn acquire(&self, key: &str, _lease: Duration) -> Result<Box<dyn LockGuard>, LockError> {
let root = self.root.clone();
let path = root.join(sanitize(key));
let key_owned = key.to_owned();
let result = tokio::task::spawn_blocking(move || -> Result<FileGuard, LockError> {
std::fs::create_dir_all(&root)?;
let file = OpenOptions::new()
.create(true)
.write(true)
.truncate(false)
.open(&path)?;
let fd = file.as_raw_fd();
// LOCK_EX | LOCK_NB: non-blocking exclusive advisory lock.
// SAFETY: fd is a valid open file descriptor.
let r = unsafe { libc::flock(fd, libc::LOCK_EX | libc::LOCK_NB) };
if r != 0 {
let err = io::Error::last_os_error();
// EAGAIN / EWOULDBLOCK ⇒ contended; anything else ⇒ io error.
if matches!(err.raw_os_error(), Some(libc::EAGAIN)) {
return Err(LockError::Contended(format!(
"flock on '{key_owned}' is held by another live process"
)));
}
return Err(LockError::Io(err));
}
Ok(FileGuard { file: Some(file) })
})
.await
.map_err(|e| LockError::Io(io::Error::other(format!("blocking task failed: {e}"))))?;
match result {
Ok(g) => Ok(Box::new(g)),
Err(e) => Err(e),
}
}
}
fn sanitize(key: &str) -> String {
let mut out = String::with_capacity(key.len());
for c in key.chars() {
if c.is_ascii_alphanumeric() || c == '-' || c == '_' {
out.push(c);
} else {
out.push('_');
}
}
if out.is_empty() {
out.push_str("default");
}
out.push_str(".lock");
out
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn sanitize_basic() {
assert_eq!(sanitize("hello"), "hello.lock");
assert_eq!(sanitize(""), "default.lock");
assert_eq!(sanitize("a/b c"), "a_b_c.lock");
}
}