fslock_guard/

lib.rs

#![cfg_attr(docsrs, feature(doc_auto_cfg, 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)]
#![warn(clippy::cognitive_complexity)]
#![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)]
#![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_duration_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
//! <!-- @@ end lint list maintained by maint/add_warning @@ -->

use std::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 locked [`fslock::LockFile`].
    ///
    /// This `LockFile` instance will remain locked for as long as this
    /// LockFileGuard exists.
    locked: fslock::LockFile,
}

impl LockFileGuard {
    /// 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, fslock::Error>
    where
        P: AsRef<Path>,
    {
        let path = path.as_ref();
        loop {
            let mut lockfile = fslock::LockFile::open(path)?;
            lockfile.lock()?;

            if os::lockfile_has_path(&lockfile, path)? {
                return Ok(Self { locked: lockfile });
            }
        }
    }

    /// Try to construct a new [`LockFileGuard`] representing a lock we hold on
    /// the file `path`.
    ///
    /// Does not block; returns Ok(None) if somebody else holds the lock.
    pub fn try_lock<P>(path: P) -> Result<Option<Self>, fslock::Error>
    where
        P: AsRef<Path>,
    {
        let path = path.as_ref();
        let mut lockfile = fslock::LockFile::open(path)?;
        if lockfile.try_lock()? && os::lockfile_has_path(&lockfile, path)? {
            return Ok(Some(Self { locked: lockfile }));
        }
        Ok(None)
    }

    /// 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>
    where
        P: AsRef<Path>,
    {
        let path = path.as_ref();
        if os::lockfile_has_path(&self.locked, path)? {
            std::fs::remove_file(path)
        } else {
            Err(std::io::Error::other(MismatchedPathError {}))
        }
    }
}

/// 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 {}

// Note: This requires AsFd and AsHandle implementations for `LockFile`.
//  See https://github.com/brunoczim/fslock/pull/15
// This is why we are using fslock-arti-fork in place of fslock.

/// 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::fd::AsFd, 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: &fslock::LockFile, path: &Path) -> std::io::Result<bool> {
        let m1 = std::fs::metadata(path)?;
        // TODO: This does an unnecessary dup().
        let f_dup = File::from(lf.as_fd().try_clone_to_owned()?);
        let m2 = f_dup.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 `fslock` 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 `fslock` 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 `fslock` opened it with
///   FILE_SHARE_DELETE.
/// * 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)]
mod os {
    use std::{fs::File, mem::MaybeUninit, os::windows::io::AsRawHandle, path::Path};
    use winapi::um::fileapi::{GetFileInformationByHandle, BY_HANDLE_FILE_INFORMATION as Info};

    /// Return true if `lf` currently exists with the given `path`, and false otherwise.
    pub(crate) fn lockfile_has_path(lf: &fslock::LockFile, path: &Path) -> std::io::Result<bool> {
        let mut m1: MaybeUninit<Info> = MaybeUninit::uninit();
        let mut m2: MaybeUninit<Info> = MaybeUninit::uninit();

        let f2 = File::open(path)?;

        let (i1, i2) = unsafe {
            if GetFileInformationByHandle(lf.as_raw_handle() as _, m1.as_mut_ptr()) == 0 {
                return Err(std::io::Error::last_os_error());
            }
            if GetFileInformationByHandle(f2.as_raw_handle() as _, m2.as_mut_ptr()) == 0 {
                return Err(std::io::Error::last_os_error());
            }
            (m1.assume_init(), m2.assume_init())
        };

        // 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.nFileIndexHigh == i2.nFileIndexHigh
            && i1.nFileIndexLow == i2.nFileIndexLow
            && i1.dwVolumeSerialNumber == i2.dwVolumeSerialNumber)
    }
}

#[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_duration_subtraction)]
    #![allow(clippy::useless_vec)]
    #![allow(clippy::needless_pass_by_value)]
    //! <!-- @@ end test lint list maintained by maint/add_warning @@ -->

    use crate::LockFileGuard;
    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);
            assert!(file.try_exists().unwrap());
        });
    }

    #[test]
    fn delete_lock_file_if_requested() {
        test_temp_dir!().used_by(|dir| {
            let file = dir.join("file");
            let flock_guard = LockFileGuard::lock(&file).unwrap();
            assert!(file.try_exists().unwrap());
            assert!(flock_guard.delete_lock_file(&file).is_ok());
            assert!(!file.try_exists().unwrap());
        });
    }
}