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#![cfg_attr(docsrs, feature(doc_cfg))]
#![doc = include_str!("../README.md")]
// @@ begin lint list maintained by maint/add_warning @@
#![allow(renamed_and_removed_lints)] // @@REMOVE_WHEN(ci_arti_stable)
#![allow(unknown_lints)] // @@REMOVE_WHEN(ci_arti_nightly)
#![warn(missing_docs)]
#![warn(noop_method_call)]
#![warn(unreachable_pub)]
#![warn(clippy::all)]
#![deny(clippy::await_holding_lock)]
#![deny(clippy::cargo_common_metadata)]
#![deny(clippy::cast_lossless)]
#![deny(clippy::checked_conversions)]
#![allow(clippy::cognitive_complexity)] // See arti#2556
#![deny(clippy::debug_assert_with_mut_call)]
#![deny(clippy::exhaustive_enums)]
#![deny(clippy::exhaustive_structs)]
#![deny(clippy::expl_impl_clone_on_copy)]
#![deny(clippy::fallible_impl_from)]
#![deny(clippy::implicit_clone)]
#![deny(clippy::large_stack_arrays)]
#![warn(clippy::manual_ok_or)]
#![deny(clippy::missing_docs_in_private_items)]
#![warn(clippy::needless_borrow)]
#![warn(clippy::needless_pass_by_value)]
#![warn(clippy::option_option)]
#![warn(clippy::or_fun_call)]
#![deny(clippy::print_stderr)]
#![deny(clippy::print_stdout)]
#![warn(clippy::rc_buffer)]
#![deny(clippy::ref_option_ref)]
#![warn(clippy::semicolon_if_nothing_returned)]
#![warn(clippy::trait_duplication_in_bounds)]
#![deny(clippy::unchecked_time_subtraction)]
#![deny(clippy::unnecessary_wraps)]
#![warn(clippy::unseparated_literal_suffix)]
#![deny(clippy::unwrap_used)]
#![deny(clippy::mod_module_files)]
#![allow(clippy::let_unit_value)] // This can reasonably be done for explicitness
#![allow(clippy::uninlined_format_args)]
#![allow(clippy::significant_drop_in_scrutinee)] // arti/-/merge_requests/588/#note_2812945
#![allow(clippy::result_large_err)] // temporary workaround for arti#587
#![allow(clippy::needless_raw_string_hashes)] // complained-about code is fine, often best
#![allow(clippy::needless_lifetimes)] // See arti#1765
#![allow(mismatched_lifetime_syntaxes)] // temporary workaround for arti#2060
#![allow(clippy::collapsible_if)] // See arti#2342
#![deny(clippy::unused_async)]
#![deny(clippy::string_slice)] // See arti#2571
#![allow(recursion_depth_exceeding_limit)] // arti#2715, rust/issues/159228
//! <!-- @@ end lint list maintained by maint/add_warning @@ -->
use std::{fs, path::Path};
/// A lock-file for which we hold the lock.
///
/// So long as this object exists, we hold the lock on this file.
/// When it is dropped, we will release the lock.
/// # Semantics
/// * Only one `LockFileGuard` can exist at one time
/// for any particular `path`.
/// * This applies across all tasks and threads in all programs;
/// other acquisitions of the lock in the same process are prevented.
/// * This applies across even separate machines, if `path` is on a shared filesystem.
/// # Restrictions
/// * **`path` must only be deleted (or renamed) via the APIs in this module**
/// * This restriction applies to all programs on the computer,
/// so for example automatic file cleaning with `find` and `rm` is forbidden.
/// * Cross-filesystem locking is broken on Linux before 2.6.12.
#[derive(Debug)]
pub struct LockFileGuard {
/// A [`File`](fs::File) with its exclusive lock held.
/// This `File` instance will remain locked for as long as this
/// LockFileGuard exists.
locked_file: fs::File,
}
impl LockFileGuard {
/// Try to open `path` with options suitable for using it as a lockfile,
/// creating it as necessary.
fn open<P>(path: P) -> Result<fs::File, std::io::Error>
where
P: AsRef<Path>,
{
fs::OpenOptions::new()
.read(true)
.write(true)
.create(true)
.truncate(false)
.open(&path)
/// Try to construct a new [`LockFileGuard`] representing a lock we hold on
/// the file `path`.
/// Blocks until we can get the lock.
pub fn lock<P>(path: P) -> Result<Self, std::io::Error>
let path = path.as_ref();
loop {
let file = Self::open(path)?;
do_lock(&file)?;
if os::lockfile_has_path(&file, path)? {
return Ok(Self { locked_file: file });
/// Does not block; returns Ok(None) if somebody else holds the lock.
pub fn try_lock<P>(path: P) -> Result<Option<Self>, std::io::Error>
match do_try_lock(&file) {
Ok(()) => {
Ok(Some(Self { locked_file: file }))
} else {
Ok(None)
Err(fs::TryLockError::WouldBlock) => Ok(None),
Err(fs::TryLockError::Error(e)) => Err(e),
/// Try to delete the lock file that we hold.
/// The provided `path` must be the same as was passed to `lock`.
pub fn delete_lock_file<P>(self, path: P) -> Result<(), std::io::Error>
if os::lockfile_has_path(&self.locked_file, path)? {
std::fs::remove_file(path)
Err(std::io::Error::other(MismatchedPathError {}))
impl Drop for LockFileGuard {
// We pro-actively unlock the file rather than relying on drop of the File closing it.
//
// This is necessary on Unix because otherwise the following scenario is possible:
// 0. The process has multiple threads
// 1. Thread A executes fork (eg as part of spawn), and the child gets a copy of the fd,
// 2. Thread B drops the `LockFileGuard` and calls close() on its copy of the fd
// 3. Thread B tries to re-acquire the same lock with try_lock and fails
// 4. Thread A closes the fd (via exec, or otherwise)
// We want to prevent the error in step 3, which arises from a race which is possible
// due to us violating the expected semantics of a guard (namely, that the lock is
// synchronously released when the guard is dropped).
#[allow(clippy::unnecessary_lazy_evaluations)] // we want to write the discarded error type
fn drop(&mut self) {
self.locked_file
.unlock()
// Ignore errors from unlock. There shouldn't be any, but if there are we
// don't have anything sensible we could do with them.
.unwrap_or_else(|_: std::io::Error| ());
/// Try to lock `f`, blocking if need be.
/// On non-android, this just calls [`fs::File::lock`].
#[cfg(not(target_os = "android"))]
fn do_lock(f: &fs::File) -> std::io::Result<()> {
f.lock()
/// Try to lock `f`, without blocking.
/// On non-android, this just calls [`fs::File::try_lock`].
fn do_try_lock(f: &fs::File) -> Result<(), std::fs::TryLockError> {
f.try_lock()
/// On android, we need to use flock manually, since Rust (as of May 2026)
/// always returns "not implemented" for `lock()` and `try_lock()`.
/// See <https://github.com/rust-lang/rust/issues/148325>.
/// Apparently,
/// although there are filesystems (specifically FUSE filesystems)
/// where flock won't work, it will correctly report ENOSYS
/// on those filesystems.
// TODO MSRV ????: we can remove this once Rust supports file locking on Android
// at our MSRV. As of May 2026, https://github.com/rust-lang/rust/pull/157038/
// seems like the likeliest MR for that, but it has not been merged.
#[cfg(target_os = "android")]
use std::os::fd::AsRawFd;
let fd = f.as_raw_fd();
// SAFETY: Since `f` is a file, it has a valid fd.
let success = unsafe { libc::flock(fd, libc::LOCK_EX) } == 0;
if success {
Ok(())
Err(std::io::Error::last_os_error())
// TODO MSRV ????: See 'TODO MSRV' on do_lock above.
let success = unsafe { libc::flock(fd, libc::LOCK_EX | libc::LOCK_NB) } == 0;
let err = std::io::Error::last_os_error();
if err.kind() == std::io::ErrorKind::WouldBlock {
Err(std::fs::TryLockError::WouldBlock)
Err(std::fs::TryLockError::Error(err))
/// An error that we return when the path given to `delete_lock_file` does not
/// match the file we have.
/// Since we wrap this in an `io::Error`, it doesn't need to be public or fancy.
#[derive(thiserror::Error, Debug, Clone)]
#[error("Called delete_lock_file with a mismatched path.")]
struct MismatchedPathError {}
/// Platform module for locking protocol on Unix.
/// ### Locking protocol on Unix
/// The lock is held by an open-file iff:
/// * that open-file holds an `flock` `LOCK_EX` lock; and
/// * the directory entry for `path` refers to the same file as the open-file
/// `path` may only refer to a plain file, or `ENOENT`.
/// If `path` refers to a file,
/// only the lockholder may cause it to no longer refer to that file.
/// In principle the open-file might be shared with subprocesses.
/// Even a naive program can safely and correctly inherit and hold the lock,
/// since the lockholder only needs to not close an fd.
/// However uncontrolled leaking of the fd into other processes is undesirable,
/// as it might cause delays or even deadlocks, if those processes' inheritors live too long.
/// In our Rust implementation we don't support sharing the held lock
/// with subprocesses or different process images (ie across exec);
/// we use `O_CLOEXEC`.
/// #### Locking algorithm
/// 1. open the file with `O_CREAT|O_RDWR`
/// 2. `flock LOCK_EX`
/// 3. `fstat` the open-file and `lstat` the path
/// 4. If the inode and device numbers don't match,
/// close the fd and go back to the start.
/// 5. Now we hold the lock.
/// Proof sketch:
/// If we get to point 5, we see that at point 3, we had the lock.
/// No-one else could cause the conditions to become false
/// in the meantime:
/// no-one else ~~can~~ may make `path` refer to a different file
/// since they don't hold the lock.
/// And, no-one else can `flock` it since the kernel prevents
/// a conflicting lock.
/// So at step 5 we must still hold the lock.
/// #### Unlocking algorithm
/// 1. Close the fd.
/// 2. Now we no longer hold the lock and others can acquire it.
/// This drops the open-file and
/// leaves the lock available for another caller.
/// #### Deletion algorithm
/// 0. The lock must already be held
/// 1. `unlink` the file
/// 2. close the fd
/// 3. Now we no longer hold the lock and others can acquire it.
/// Step 1 atomically falsifies the lock-holding condition.
/// We are allowed to perform it because we hold the lock.
/// Concurrent lockers might open the old file,
/// which we are about to delete.
/// They will acquire their `flock` (locking step 2)
/// after we close (deletion step 2)
/// and then see that they have a stale file.
#[cfg(unix)]
mod os {
use std::{fs::File, os::unix::fs::MetadataExt as _, path::Path};
/// Return true if `lf` currently exists with the given `path`, and false otherwise.
pub(crate) fn lockfile_has_path(lf: &File, path: &Path) -> std::io::Result<bool> {
let m1 = std::fs::metadata(path)?;
let m2 = lf.metadata()?;
Ok(m1.ino() == m2.ino() && m1.dev() == m2.dev())
/// Platform module for locking protocol on Windows.
/// The argument for correctness on Windows proceeds as for Unix, but with a
/// higher degree of uncertainty, since we are not sufficient Windows experts to
/// determine if our assumptions hold.
/// Here we assume as follows:
/// * When `File::open` calls `CreateFileW`, it gets a `HANDLE` to an open file.
/// As we use them, the `HANDLE` behaves
/// similarly to the "fd" in the Unix argument above,
/// and the open file behaves similarly to the "open-file".
/// * We assume that any differences that exist in their behavior do not
/// affect our correctness above.
/// * When `File::lock` calls `LockFileEx`, and it completes successfully,
/// we now have a lock on the file.
/// Only one lock can exist on a file at a time.
/// * When we compare members of `handle.metadata()` and `path.metadata()`,
/// the comparison will return equal if ~~and only if~~
/// the two files are truly the same.
/// * We rely on the property that a file cannot change its file_index while it is
/// open.
/// * Deleting the lock file will actually work, since `File::open` opened it with
/// FILE_SHARE_DELETE. (This is the default according to the documentation
/// for `OpenOptionsExt::share_mode`.)
/// * When we delete the lock file, possibly-asynchronous ("deferred") deletion
/// definitely won't mean that the OS kernel violates our rule that no-one but the lockholder
/// is allowed to delete the file.
/// * The above is true even if someone with read
/// access to the file - eg the human user - opens it without the FILE_SHARE options.
/// * The same is true even if there is a virus scanner.
/// * The same is true even on a remote filesystem.
/// * If someone with read access to the file - eg the human user - opens it for reading
/// without FILE_SHARE options, the algorithm will still work and not fail
/// with a file sharing violation io error.
/// (Or, every program the user might use to randomly peer at files in arti's
/// state directory, including the equivalents of `grep -R` and backup programs,
/// will use suitable FILE_SHARE options.)
/// (If this assumption is false, the consequence is not data loss;
/// rather, arti would fall over. So that would be tolerable if we don't
/// know how to do better, or if doing better is hard.)
#[cfg(windows)]
use std::{fs::File, mem::MaybeUninit, os::windows::io::AsRawHandle, path::Path};
use windows_sys::Win32::{
Foundation::HANDLE,
Storage::FileSystem::{FILE_ID_INFO, FileIdInfo, GetFileInformationByHandleEx},
};
/// Use `GetFileInformationByHandleEx` to return a FILE_ID_INFO data for `f`.
/// `GetFileInformationByHandleEx` is supported in Vista and later, so it
/// should be fine here. Unlike GetFileInformationByHandle, it gives
/// 128-bit identifiers which are supposedly even more unique.
fn get_id_info(f: &File) -> std::io::Result<FILE_ID_INFO> {
let handle = f.as_raw_handle() as HANDLE;
let mut info: MaybeUninit<FILE_ID_INFO> = MaybeUninit::uninit();
let buffersize: u32 = std::mem::size_of::<FILE_ID_INFO>()
.try_into()
.expect("sizeof(FILE_ID_INFO) is ridiculously large");
let info = unsafe {
// SAFETY: Since `size` is the size of info, this will not write to
// uninitialized memory.
let rv = GetFileInformationByHandleEx(
handle,
FileIdInfo,
info.as_mut_ptr() as _,
buffersize,
);
if rv == 0 {
return Err(std::io::Error::last_os_error());
// SAFETY: since rv was nonzero, this value is initialized.
info.assume_init()
Ok(info)
let f2 = File::open(path)?;
// Note: we would like to just use the MetadataExt methods for index and
// volume serial number, but they are currently available only on
// nightly: https://github.com/rust-lang/rust/issues/63010
// If they stabilize at our MSRV, _and_ the file ID is expanded to the
// 128-bit version, we can use them here instead.
let i1 = get_id_info(lf)?;
let i2 = get_id_info(&f2)?;
// This comparison is about the best we can do on Windows,
// though there are caveats.
// See Raymond Chen's writeup at
// https://devblogs.microsoft.com/oldnewthing/20220128-00/?p=106201
// and also see BurntSushi's caveats at
// https://github.com/BurntSushi/same-file/blob/master/src/win.rs
Ok(i1.VolumeSerialNumber == i2.VolumeSerialNumber
&& i1.FileId.Identifier == i2.FileId.Identifier)
/// Non-windows, non-unix implementation for lockfile_has_path.
/// For now, this implementation always reports an error.
/// It exists so that we can build (but not run) on wasm.
#[cfg(all(not(windows), not(unix)))]
use std::path::Path;
pub(crate) fn lockfile_has_path(_lf: &std::fs::File, _path: &Path) -> std::io::Result<bool> {
Err(std::io::Error::other(
"fslock-guard does not support this operating system".to_string(),
))
#[cfg(test)]
mod tests {
// @@ begin test lint list maintained by maint/add_warning @@
#![allow(clippy::bool_assert_comparison)]
#![allow(clippy::clone_on_copy)]
#![allow(clippy::dbg_macro)]
#![allow(clippy::mixed_attributes_style)]
#![allow(clippy::print_stderr)]
#![allow(clippy::print_stdout)]
#![allow(clippy::single_char_pattern)]
#![allow(clippy::unwrap_used)]
#![allow(clippy::unchecked_time_subtraction)]
#![allow(clippy::useless_vec)]
#![allow(clippy::needless_pass_by_value)]
#![allow(clippy::string_slice)] // See arti#2571
//! <!-- @@ end test lint list maintained by maint/add_warning @@ -->
use crate::LockFileGuard;
use std::sync::Arc;
use std::thread;
use test_temp_dir::test_temp_dir;
#[test]
fn keep_lock_file_after_drop() {
test_temp_dir!().used_by(|dir| {
let file = dir.join("file");
let flock_guard = LockFileGuard::lock(&file).unwrap();
assert!(file.try_exists().unwrap());
drop(flock_guard);
});
fn delete_lock_file_if_requested() {
assert!(flock_guard.delete_lock_file(&file).is_ok());
assert!(!file.try_exists().unwrap());
fn tight_loop() {
let tmp = Arc::new(test_temp_dir!());
// We make several threads in case there are any cross-thread interactions
// that we're not aware of. There shouldn't be.
let threads = (0..10)
.map(|i| {
let tmp = tmp.clone();
thread::spawn(move || {
tmp.used_by(|dir| {
let file = dir.join(format!("{i}"));
for _ in 0..1000 {
// Test that the lock is immediately re-requirable after drop.
let lock: LockFileGuard =
LockFileGuard::try_lock(&file).unwrap().unwrap();
drop(lock);
})
.collect::<Vec<_>>();
for t in threads {
t.join().unwrap_or_else(|e| std::panic::resume_unwind(e));
fn fork_leak_fds() {
use std::ffi::c_int;
let tmp = test_temp_dir!();
tmp.used_by(|tmp| {
let file = tmp.join("lock");
let lock = LockFileGuard::lock(&file).unwrap();
let child = unsafe {
// It would be nicer to do this with std's Command, but
// we'd have to use the unsafe pre-exec hook for synchronisation
// and anyway that runs after Command's impl has closed "unwanted" fds.
match libc::fork() {
-1 => panic!("fork failed"),
0 => {
libc::usleep(10_000);
libc::_exit(0);
child => child,
let _lock: LockFileGuard = LockFileGuard::try_lock(&file).unwrap().unwrap();
unsafe {
let mut status: c_int = 0;
let got = libc::waitpid(child, (&mut status) as *mut _, 0);
assert_eq!(got, child);
assert_eq!(status, 0, "{status}");