cargo_like_utils/

flock.rs

//! File-locking support.
//!
//! This module defines the [`Filesystem`] type which is an abstraction over a
//! filesystem, ensuring that access to the filesystem is only done through
//! coordinated locks.
//!
//! The [`FileLock`] type represents a locked file, and provides access to the
//! file.

use std::fs::{File, OpenOptions};
use std::io;
use std::io::{Read, Seek, SeekFrom, Write};
use std::path::{Display, Path, PathBuf};

use anyhow::Context as _;
use sys::*;
use crate::shell::{Shell, style};

/// A locked file.
///
/// This provides access to file while holding a lock on the file. This type
/// implements the [`Read`], [`Write`], and [`Seek`] traits to provide access
/// to the underlying file.
///
/// Locks are either shared (multiple processes can access the file) or
/// exclusive (only one process can access the file).
///
/// This type is created via methods on the [`Filesystem`] type.
///
/// When this value is dropped, the lock will be released.
#[derive(Debug)]
pub struct FileLock {
    f: Option<File>,
    path: PathBuf,
}

impl FileLock {
    /// Returns the underlying file handle of this lock.
    pub fn file(&self) -> &File {
        self.f.as_ref().unwrap()
    }

    /// Returns the underlying path that this lock points to.
    ///
    /// Note that special care must be taken to ensure that the path is not
    /// referenced outside the lifetime of this lock.
    pub fn path(&self) -> &Path {
        &self.path
    }

    /// Returns the parent path containing this file
    pub fn parent(&self) -> &Path {
        self.path.parent().unwrap()
    }

    /// Removes all sibling files to this locked file.
    ///
    /// This can be useful if a directory is locked with a sentinel file but it
    /// needs to be cleared out as it may be corrupt.
    pub fn remove_siblings(&self) -> anyhow::Result<()> {
        let path = self.path();
        for entry in path.parent().unwrap().read_dir()? {
            let entry = entry?;
            if Some(&entry.file_name()[..]) == path.file_name() {
                continue;
            }
            let kind = entry.file_type()?;
            if kind.is_dir() {
                fs_err::remove_dir_all(entry.path())?;
            } else {
                fs_err::remove_file(entry.path())?;
            }
        }
        Ok(())
    }
}

impl Read for FileLock {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        self.file().read(buf)
    }
}

impl Seek for FileLock {
    fn seek(&mut self, to: SeekFrom) -> io::Result<u64> {
        self.file().seek(to)
    }
}

impl Write for FileLock {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        self.file().write(buf)
    }

    fn flush(&mut self) -> io::Result<()> {
        self.file().flush()
    }
}

impl Drop for FileLock {
    fn drop(&mut self) {
        if let Some(f) = self.f.take() {
            if let Err(e) = unlock(&f) {
                tracing::warn!("failed to release lock: {e:?}");
            }
        }
    }
}

/// A "filesystem" is intended to be a globally shared, hence locked, resource
/// in Cargo.
///
/// The `Path` of a filesystem cannot be learned unless it's done in a locked
/// fashion, and otherwise functions on this structure are prepared to handle
/// concurrent invocations across multiple instances of Cargo.
///
/// The methods on `Filesystem` that open files return a [`FileLock`] which
/// holds the lock, and that type provides methods for accessing the
/// underlying file.
///
/// If the blocking methods (like [`Filesystem::open_ro_shared`]) detect that
/// they will block, then they will display a message to the user letting them
/// know it is blocked. There are non-blocking variants starting with the
/// `try_` prefix like [`Filesystem::try_open_ro_shared_create`].
///
/// The behavior of locks acquired by the `Filesystem` depend on the operating
/// system. On unix-like system, they are advisory using [`flock`], and thus
/// not enforced against processes which do not try to acquire the lock. On
/// Windows, they are mandatory using [`LockFileEx`], enforced against all
/// processes.
///
/// This **does not** guarantee that a lock is acquired. In some cases, for
/// example on filesystems that don't support locking, it will return a
/// [`FileLock`] even though the filesystem lock was not acquired. This is
/// intended to provide a graceful fallback instead of refusing to work.
/// Usually there aren't multiple processes accessing the same resource. In
/// that case, it is the user's responsibility to not run concurrent
/// processes.
///
/// [`flock`]: https://linux.die.net/man/2/flock
/// [`LockFileEx`]: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-lockfileex
#[derive(Clone, Debug)]
pub struct Filesystem {
    root: PathBuf,
}

impl Filesystem {
    /// Creates a new filesystem to be rooted at the given path.
    pub fn new(path: PathBuf) -> Filesystem {
        Filesystem { root: path }
    }

    /// Like `Path::join`, creates a new filesystem rooted at this filesystem
    /// joined with the given path.
    pub fn join<T: AsRef<Path>>(&self, other: T) -> Filesystem {
        Filesystem::new(self.root.join(other))
    }

    /// Like `Path::push`, pushes a new path component onto this filesystem.
    pub fn push<T: AsRef<Path>>(&mut self, other: T) {
        self.root.push(other);
    }

    /// Consumes this filesystem and returns the underlying `PathBuf`.
    ///
    /// Note that this is a relatively dangerous operation and should be used
    /// with great caution!.
    pub fn into_path_unlocked(self) -> PathBuf {
        self.root
    }

    /// Returns the underlying `Path`.
    ///
    /// Note that this is a relatively dangerous operation and should be used
    /// with great caution!.
    pub fn as_path_unlocked(&self) -> &Path {
        &self.root
    }

    /// Creates the directory pointed to by this filesystem.
    ///
    /// Handles errors where other Cargo processes are also attempting to
    /// concurrently create this directory.
    pub fn create_dir(&self) -> anyhow::Result<()> {
        fs_err::create_dir_all(&self.root).map_err(anyhow::Error::from)
    }

    /// Returns an adaptor that can be used to print the path of this
    /// filesystem.
    pub fn display(&self) -> Display<'_> {
        self.root.display()
    }

    /// Opens read-write exclusive access to a file, returning the locked
    /// version of a file.
    ///
    /// This function will create a file at `path` if it doesn't already exist
    /// (including intermediate directories), and then it will acquire an
    /// exclusive lock on `path`. If the process must block waiting for the
    /// lock, the `msg` is printed to the [`Shell`].
    ///
    /// The returned file can be accessed to look at the path and also has
    /// read/write access to the underlying file.
    pub fn open_rw_exclusive_create<P>(
        &self,
        path: P,
        shell: &mut Shell,
        msg: &str,
    ) -> anyhow::Result<FileLock>
        where
            P: AsRef<Path>,
    {
        let mut opts = OpenOptions::new();
        opts.read(true).write(true).truncate(true).create(true);
        let (path, f) = self.open(path.as_ref(), &opts, true)?;
        acquire(shell, msg, &path, &|| try_lock_exclusive(&f), &|| {
            lock_exclusive(&f)
        })?;
        Ok(FileLock { f: Some(f), path })
    }

    /// A non-blocking version of [`Filesystem::open_rw_exclusive_create`].
    ///
    /// Returns `None` if the operation would block due to another process
    /// holding the lock.
    pub fn try_open_rw_exclusive_create<P: AsRef<Path>>(
        &self,
        path: P,
    ) -> anyhow::Result<Option<FileLock>> {
        let mut opts = OpenOptions::new();
        opts.read(true).write(true).truncate(true).create(true);
        let (path, f) = self.open(path.as_ref(), &opts, true)?;
        if try_acquire(&path, &|| try_lock_exclusive(&f))? {
            Ok(Some(FileLock { f: Some(f), path }))
        } else {
            Ok(None)
        }
    }

    /// Opens read-only shared access to a file, returning the locked version of a file.
    ///
    /// This function will fail if `path` doesn't already exist, but if it does
    /// then it will acquire a shared lock on `path`. If the process must block
    /// waiting for the lock, the `msg` is printed to [`Shell`].
    ///
    /// The returned file can be accessed to look at the path and also has read
    /// access to the underlying file. Any writes to the file will return an
    /// error.
    pub fn open_ro_shared<P>(&self, path: P, shell: &mut Shell, msg: &str) -> anyhow::Result<FileLock>
        where
            P: AsRef<Path>,
    {
        let (path, f) = self.open(path.as_ref(), &OpenOptions::new().read(true), false)?;
        acquire(shell, msg, &path, &|| try_lock_shared(&f), &|| {
            lock_shared(&f)
        })?;
        Ok(FileLock { f: Some(f), path })
    }

    /// Opens read-only shared access to a file, returning the locked version of a file.
    ///
    /// Compared to [`Filesystem::open_ro_shared`], this will create the file
    /// (and any directories in the parent) if the file does not already
    /// exist.
    pub fn open_ro_shared_create<P: AsRef<Path>>(
        &self,
        path: P,
        shell: &mut Shell,
        msg: &str,
    ) -> anyhow::Result<FileLock> {
        let mut opts = OpenOptions::new();
        opts.read(true).write(true).create(true);
        let (path, f) = self.open(path.as_ref(), &opts, true)?;
        acquire(shell, msg, &path, &|| try_lock_shared(&f), &|| {
            lock_shared(&f)
        })?;
        Ok(FileLock { f: Some(f), path })
    }

    /// A non-blocking version of [`Filesystem::open_ro_shared_create`].
    ///
    /// Returns `None` if the operation would block due to another process
    /// holding the lock.
    pub fn try_open_ro_shared_create<P: AsRef<Path>>(
        &self,
        path: P,
    ) -> anyhow::Result<Option<FileLock>> {
        let mut opts = OpenOptions::new();
        opts.read(true).write(true).create(true);
        let (path, f) = self.open(path.as_ref(), &opts, true)?;
        if try_acquire(&path, &|| try_lock_shared(&f))? {
            Ok(Some(FileLock { f: Some(f), path }))
        } else {
            Ok(None)
        }
    }

    fn open(
        &self,
        path: &Path,
        opts: &OpenOptions,
        create: bool,
    ) -> anyhow::Result<(PathBuf, File)> {
        let path = self.root.join(path);
        let f = opts
            .open(&path)
            .or_else(|e| {
                // If we were requested to create this file, and there was a
                // NotFound error, then that was likely due to missing
                // intermediate directories. Try creating them and try again.
                if e.kind() == io::ErrorKind::NotFound && create {
                    fs_err::create_dir_all(path.parent().unwrap())?;
                    Ok(opts.open(&path)?)
                } else {
                    Err(anyhow::Error::from(e))
                }
            })
            .with_context(|| format!("failed to open: {}", path.display()))?;
        Ok((path, f))
    }
}

impl PartialEq<Path> for Filesystem {
    fn eq(&self, other: &Path) -> bool {
        self.root == other
    }
}

impl PartialEq<Filesystem> for Path {
    fn eq(&self, other: &Filesystem) -> bool {
        self == other.root
    }
}

fn try_acquire(path: &Path, lock_try: &dyn Fn() -> io::Result<()>) -> anyhow::Result<bool> {
    // File locking on Unix is currently implemented via `flock`, which is known
    // to be broken on NFS. We could in theory just ignore errors that happen on
    // NFS, but apparently the failure mode [1] for `flock` on NFS is **blocking
    // forever**, even if the "non-blocking" flag is passed!
    //
    // As a result, we just skip all file locks entirely on NFS mounts. That
    // should avoid calling any `flock` functions at all, and it wouldn't work
    // there anyway.
    //
    // [1]: https://github.com/rust-lang/cargo/issues/2615
    if is_on_nfs_mount(path) {
        tracing::debug!("{path:?} appears to be an NFS mount, not trying to lock");
        return Ok(true);
    }

    match lock_try() {
        Ok(()) => return Ok(true),

        // In addition to ignoring NFS which is commonly not working we also
        // just ignore locking on filesystems that look like they don't
        // implement file locking.
        Err(e) if error_unsupported(&e) => return Ok(true),

        Err(e) => {
            if !error_contended(&e) {
                let e = anyhow::Error::from(e);
                let cx = format!("failed to lock file: {}", path.display());
                return Err(e.context(cx));
            }
        }
    }
    Ok(false)
}

/// Acquires a lock on a file in a "nice" manner.
///
/// Almost all long-running blocking actions in Cargo have a status message
/// associated with them as we're not sure how long they'll take. Whenever a
/// conflicted file lock happens, this is the case (we're not sure when the lock
/// will be released).
///
/// This function will acquire the lock on a `path`, printing out a nice message
/// to the console if we have to wait for it. It will first attempt to use `try`
/// to acquire a lock on the crate, and in the case of contention it will emit a
/// status message based on `msg` to [`Shell`], and then use `block` to
/// block waiting to acquire a lock.
///
/// Returns an error if the lock could not be acquired or if any error other
/// than a contention error happens.
fn acquire(
    shell: &mut Shell,
    msg: &str,
    path: &Path,
    lock_try: &dyn Fn() -> io::Result<()>,
    lock_block: &dyn Fn() -> io::Result<()>,
) -> anyhow::Result<()> {
    if try_acquire(path, lock_try)? {
        return Ok(());
    }
    let msg = format!("waiting for file lock on {}", msg);
    shell
        .status_with_color("Blocking", &msg, &style::NOTE)?;

    lock_block().with_context(|| format!("failed to lock file: {}", path.display()))?;
    Ok(())
}

#[cfg(all(target_os = "linux", not(target_env = "musl")))]
fn is_on_nfs_mount(path: &Path) -> bool {
    use std::ffi::CString;
    use std::mem;
    use std::os::unix::prelude::*;

    let Ok(path) = CString::new(path.as_os_str().as_bytes()) else {
        return false;
    };

    unsafe {
        let mut buf: libc::statfs = mem::zeroed();
        let r = libc::statfs(path.as_ptr(), &mut buf);

        r == 0 && buf.f_type as u32 == libc::NFS_SUPER_MAGIC as u32
    }
}

#[cfg(any(not(target_os = "linux"), target_env = "musl"))]
fn is_on_nfs_mount(_path: &Path) -> bool {
    false
}

#[cfg(unix)]
mod sys {
    use std::fs::File;
    use std::io::{Error, Result};
    use std::os::unix::io::AsRawFd;

    pub(super) fn lock_shared(file: &File) -> Result<()> {
        flock(file, libc::LOCK_SH)
    }

    pub(super) fn lock_exclusive(file: &File) -> Result<()> {
        flock(file, libc::LOCK_EX)
    }

    pub(super) fn try_lock_shared(file: &File) -> Result<()> {
        flock(file, libc::LOCK_SH | libc::LOCK_NB)
    }

    pub(super) fn try_lock_exclusive(file: &File) -> Result<()> {
        flock(file, libc::LOCK_EX | libc::LOCK_NB)
    }

    pub(super) fn unlock(file: &File) -> Result<()> {
        flock(file, libc::LOCK_UN)
    }

    pub(super) fn error_contended(err: &Error) -> bool {
        err.raw_os_error().map_or(false, |x| x == libc::EWOULDBLOCK)
    }

    pub(super) fn error_unsupported(err: &Error) -> bool {
        match err.raw_os_error() {
            // Unfortunately, depending on the target, these may or may not be the same.
            // For targets in which they are the same, the duplicate pattern causes a warning.
            #[allow(unreachable_patterns)]
            Some(libc::ENOTSUP | libc::EOPNOTSUPP) => true,
            Some(libc::ENOSYS) => true,
            _ => false,
        }
    }

    #[cfg(not(target_os = "solaris"))]
    fn flock(file: &File, flag: libc::c_int) -> Result<()> {
        let ret = unsafe { libc::flock(file.as_raw_fd(), flag) };
        if ret < 0 {
            Err(Error::last_os_error())
        } else {
            Ok(())
        }
    }

    #[cfg(target_os = "solaris")]
    fn flock(file: &File, flag: libc::c_int) -> Result<()> {
        // Solaris lacks flock(), so try to emulate using fcntl()
        let mut flock = libc::flock {
            l_type: 0,
            l_whence: 0,
            l_start: 0,
            l_len: 0,
            l_sysid: 0,
            l_pid: 0,
            l_pad: [0, 0, 0, 0],
        };
        flock.l_type = if flag & libc::LOCK_UN != 0 {
            libc::F_UNLCK
        } else if flag & libc::LOCK_EX != 0 {
            libc::F_WRLCK
        } else if flag & libc::LOCK_SH != 0 {
            libc::F_RDLCK
        } else {
            panic!("unexpected flock() operation")
        };

        let mut cmd = libc::F_SETLKW;
        if (flag & libc::LOCK_NB) != 0 {
            cmd = libc::F_SETLK;
        }

        let ret = unsafe { libc::fcntl(file.as_raw_fd(), cmd, &flock) };

        if ret < 0 {
            Err(Error::last_os_error())
        } else {
            Ok(())
        }
    }
}

#[cfg(windows)]
mod sys {
    use std::fs::File;
    use std::io::{Error, Result};
    use std::mem;
    use std::os::windows::io::AsRawHandle;

    use windows_sys::Win32::Foundation::HANDLE;
    use windows_sys::Win32::Foundation::{ERROR_INVALID_FUNCTION, ERROR_LOCK_VIOLATION};
    use windows_sys::Win32::Storage::FileSystem::{
        LockFileEx, UnlockFile, LOCKFILE_EXCLUSIVE_LOCK, LOCKFILE_FAIL_IMMEDIATELY,
    };

    pub(super) fn lock_shared(file: &File) -> Result<()> {
        lock_file(file, 0)
    }

    pub(super) fn lock_exclusive(file: &File) -> Result<()> {
        lock_file(file, LOCKFILE_EXCLUSIVE_LOCK)
    }

    pub(super) fn try_lock_shared(file: &File) -> Result<()> {
        lock_file(file, LOCKFILE_FAIL_IMMEDIATELY)
    }

    pub(super) fn try_lock_exclusive(file: &File) -> Result<()> {
        lock_file(file, LOCKFILE_EXCLUSIVE_LOCK | LOCKFILE_FAIL_IMMEDIATELY)
    }

    pub(super) fn error_contended(err: &Error) -> bool {
        err.raw_os_error()
            .map_or(false, |x| x == ERROR_LOCK_VIOLATION as i32)
    }

    pub(super) fn error_unsupported(err: &Error) -> bool {
        err.raw_os_error()
            .map_or(false, |x| x == ERROR_INVALID_FUNCTION as i32)
    }

    pub(super) fn unlock(file: &File) -> Result<()> {
        unsafe {
            let ret = UnlockFile(file.as_raw_handle() as HANDLE, 0, 0, !0, !0);
            if ret == 0 {
                Err(Error::last_os_error())
            } else {
                Ok(())
            }
        }
    }

    fn lock_file(file: &File, flags: u32) -> Result<()> {
        unsafe {
            let mut overlapped = mem::zeroed();
            let ret = LockFileEx(
                file.as_raw_handle() as HANDLE,
                flags,
                0,
                !0,
                !0,
                &mut overlapped,
            );
            if ret == 0 {
                Err(Error::last_os_error())
            } else {
                Ok(())
            }
        }
    }
}