rust/hg-core/src/lock.rs
author Pierre-Yves David <pierre-yves.david@octobus.net>
Thu, 16 Mar 2023 20:37:11 +0100
branchstable
changeset 50311 e2ba2234bf1c
parent 49930 e98fd81bb151
child 51868 db7dbe6f7bb2
permissions -rw-r--r--
revlog: update the split + transaction test We add section, increase the amount of comments and simplify some of the constructs. We are about to build more on top this tests so lets do a small cleanup first.
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//! Filesystem-based locks for local repositories
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use crate::errors::HgError;
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use crate::errors::HgResultExt;
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use crate::vfs::Vfs;
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use std::io;
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use std::io::ErrorKind;
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#[derive(derive_more::From)]
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pub enum LockError {
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    AlreadyHeld,
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    #[from]
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    Other(HgError),
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}
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/// Try to call `f` with the lock acquired, without waiting.
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///
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/// If the lock is aready held, `f` is not called and `LockError::AlreadyHeld`
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/// is returned. `LockError::Io` is returned for any unexpected I/O error
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/// accessing the lock file, including for removing it after `f` was called.
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/// The return value of `f` is dropped in that case. If all is successful, the
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/// return value of `f` is forwarded.
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pub fn try_with_lock_no_wait<R>(
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    hg_vfs: Vfs,
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    lock_filename: &str,
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    f: impl FnOnce() -> R,
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) -> Result<R, LockError> {
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    let our_lock_data = &*OUR_LOCK_DATA;
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    for _retry in 0..5 {
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        match make_lock(hg_vfs, lock_filename, our_lock_data) {
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            Ok(()) => {
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                let result = f();
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                unlock(hg_vfs, lock_filename)?;
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                return Ok(result);
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            }
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            Err(HgError::IoError { error, .. })
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                if error.kind() == ErrorKind::AlreadyExists =>
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            {
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                let lock_data = read_lock(hg_vfs, lock_filename)?;
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                if lock_data.is_none() {
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                    // Lock was apparently just released, retry acquiring it
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                    continue;
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                }
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                if !lock_should_be_broken(&lock_data) {
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                    return Err(LockError::AlreadyHeld);
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                }
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                // The lock file is left over from a process not running
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                // anymore. Break it, but with another lock to
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                // avoid a race.
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                break_lock(hg_vfs, lock_filename)?;
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                // Retry acquiring
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            }
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            Err(error) => Err(error)?,
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        }
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    }
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    Err(LockError::AlreadyHeld)
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}
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fn break_lock(hg_vfs: Vfs, lock_filename: &str) -> Result<(), LockError> {
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    try_with_lock_no_wait(hg_vfs, &format!("{}.break", lock_filename), || {
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        // Check again in case some other process broke and
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        // acquired the lock in the meantime
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        let lock_data = read_lock(hg_vfs, lock_filename)?;
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        if !lock_should_be_broken(&lock_data) {
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            return Err(LockError::AlreadyHeld);
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        }
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        Ok(hg_vfs.remove_file(lock_filename)?)
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    })?
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}
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#[cfg(unix)]
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fn make_lock(
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    hg_vfs: Vfs,
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    lock_filename: &str,
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    data: &str,
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) -> Result<(), HgError> {
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    // Use a symbolic link because creating it is atomic.
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    // The link’s "target" contains data not representing any path.
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    let fake_symlink_target = data;
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    hg_vfs.create_symlink(lock_filename, fake_symlink_target)
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}
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fn read_lock(
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    hg_vfs: Vfs,
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    lock_filename: &str,
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) -> Result<Option<String>, HgError> {
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    let link_target =
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        hg_vfs.read_link(lock_filename).io_not_found_as_none()?;
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    if let Some(target) = link_target {
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        let data = target
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            .into_os_string()
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            .into_string()
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            .map_err(|_| HgError::corrupted("non-UTF-8 lock data"))?;
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        Ok(Some(data))
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    } else {
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        Ok(None)
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    }
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}
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fn unlock(hg_vfs: Vfs, lock_filename: &str) -> Result<(), HgError> {
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    hg_vfs.remove_file(lock_filename)
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}
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/// Return whether the process that is/was holding the lock is known not to be
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/// running anymore.
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fn lock_should_be_broken(data: &Option<String>) -> bool {
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    (|| -> Option<bool> {
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        let (prefix, pid) = data.as_ref()?.split_once(':')?;
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        if prefix != *LOCK_PREFIX {
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            return Some(false);
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        }
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        let process_is_running;
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        #[cfg(unix)]
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        {
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            let pid: libc::pid_t = pid.parse().ok()?;
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            unsafe {
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                let signal = 0; // Test if we could send a signal, without sending
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                let result = libc::kill(pid, signal);
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                if result == 0 {
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                    process_is_running = true
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                } else {
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                    let errno =
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                        io::Error::last_os_error().raw_os_error().unwrap();
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                    process_is_running = errno != libc::ESRCH
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                }
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            }
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        }
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        Some(!process_is_running)
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    })()
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    .unwrap_or(false)
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}
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lazy_static::lazy_static! {
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    /// A string which is used to differentiate pid namespaces
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    ///
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    /// It's useful to detect "dead" processes and remove stale locks with
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    /// confidence. Typically it's just hostname. On modern linux, we include an
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    /// extra Linux-specific pid namespace identifier.
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    static ref LOCK_PREFIX: String = {
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        // Note: this must match the behavior of `_getlockprefix` in `mercurial/lock.py`
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        /// Same as https://github.com/python/cpython/blob/v3.10.0/Modules/socketmodule.c#L5414
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        const BUFFER_SIZE: usize = 1024;
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        // This cast is *needed* for platforms with signed chars
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        #[allow(clippy::unnecessary_cast)]
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        let mut buffer = [0 as libc::c_char; BUFFER_SIZE];
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        let hostname_bytes = unsafe {
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            let result = libc::gethostname(buffer.as_mut_ptr(), BUFFER_SIZE);
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            if result != 0 {
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                panic!("gethostname: {}", io::Error::last_os_error())
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            }
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            std::ffi::CStr::from_ptr(buffer.as_mut_ptr()).to_bytes()
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        };
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        let hostname =
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            std::str::from_utf8(hostname_bytes).expect("non-UTF-8 hostname");
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        #[cfg(target_os = "linux")]
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        {
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            use std::os::linux::fs::MetadataExt;
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            match std::fs::metadata("/proc/self/ns/pid") {
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                Ok(meta) => {
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                    return format!("{}/{:x}", hostname, meta.st_ino())
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                }
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                Err(error) => {
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                    // TODO: match on `error.kind()` when `NotADirectory`
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                    // is available on all supported Rust versions:
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                    // https://github.com/rust-lang/rust/issues/86442
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                    use libc::{
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                        ENOENT, // ErrorKind::NotFound
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                        ENOTDIR, // ErrorKind::NotADirectory
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                        EACCES, // ErrorKind::PermissionDenied
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                    };
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                    match error.raw_os_error() {
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                        Some(ENOENT) | Some(ENOTDIR) | Some(EACCES) => {}
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                        _ => panic!("stat /proc/self/ns/pid: {}", error),
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                    }
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                }
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            }
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        }
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        hostname.to_owned()
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    };
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    static ref OUR_LOCK_DATA: String = format!("{}:{}", &*LOCK_PREFIX, std::process::id());
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}