1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131

//! Provides low-level support operations for file locking on UNIX platforms.
use libc::{fcntl, off_t, F_RDLCK, F_SETLK, F_SETLKW, F_UNLCK, F_WRLCK, SEEK_SET};

use std::fs::File;
use std::io::{self, Error, ErrorKind};
use std::ops::Deref;
use std::os::raw::c_short;
use std::os::unix::io::AsRawFd;

use crate::{FileGuard, Lock};

/// Acquires and releases a file lock.
///
/// # Safety
///
/// When used to unlock, this does not guarantee that an exclusive lock is
/// already held.
pub unsafe fn raw_file_lock(
    f: &File,
    lock: Option<Lock>,
    off: usize,
    len: usize,
    wait: bool,
) -> io::Result<()> {
    if len == 0 {
        return Err(ErrorKind::InvalidInput.into());
    }

    let op = match wait {
        true => F_SETLKW,
        false => F_SETLK,
    };

    let lock = libc::flock {
        l_start: off as off_t,
        l_len: len as off_t,
        l_pid: 0,
        l_type: match lock {
            Some(Lock::Shared) => F_RDLCK as c_short,
            Some(Lock::Exclusive) => F_WRLCK as c_short,
            None => F_UNLCK as c_short,
        },
        l_whence: SEEK_SET as c_short,
        #[cfg(any(target_os = "freebsd", target_os = "solaris", target_os = "illumos"))]
        l_sysid: 0,
        #[cfg(any(target_os = "solaris", target_os = "illumos"))]
        l_pad: [0; 4],
    };

    loop {
        let rc = fcntl(f.as_raw_fd(), op, &lock);
        if rc == -1 {
            let err = Error::last_os_error();
            if err.kind() != ErrorKind::Interrupted {
                break Err(err);
            }
        } else {
            break Ok(());
        }
    }
}

/// Downgrades a file lock from exclusive to shared.
///
/// # Safety
///
/// This does not guarantee that an exclusive lock is already held.
pub unsafe fn raw_file_downgrade(f: &File, off: usize, len: usize) -> io::Result<()> {
    raw_file_lock(f, Some(Lock::Shared), off, len, false)
}

/// UNIX-specific extensions to [`FileGuard`].
///
/// [`FileGuard`]: ../../struct.FileGuard.html
pub trait FileGuardExt {
    /// Upgrades a lock from [`Shared`] to [`Exclusive`].
    ///
    /// If the currently held lock is already [`Exclusive`], no change is made
    /// and the method succeeds.
    ///
    /// [`Shared`]: ../../enum.Lock.html#variant.Shared
    /// [`Exclusive`]: ../../enum.Lock.html#variant.Exclusive
    fn upgrade(&mut self) -> io::Result<()>;

    /// Attempts to upgrade a lock from [`Shared`] to [`Exclusive`].
    ///
    /// If the currently held lock is already [`Exclusive`], no change is made
    /// and the method succeeds. If the upgrade cannot be obtained without
    /// blocking, an `Error` of kind `ErrorKind::WouldBlock` is returned.
    ///
    /// [`Shared`]: ../../enum.Lock.html#variant.Shared
    /// [`Exclusive`]: ../../enum.Lock.html#variant.Exclusive
    fn try_upgrade(&mut self) -> io::Result<()>;
}

impl<T> FileGuardExt for FileGuard<T>
where
    T: Deref<Target = File>,
{
    fn upgrade(&mut self) -> io::Result<()> {
        if self.is_shared() {
            unsafe {
                raw_file_lock(
                    &self.file,
                    Some(Lock::Exclusive),
                    self.offset,
                    self.len,
                    true,
                )?;
            }
            self.lock = Lock::Exclusive;
        }
        Ok(())
    }

    fn try_upgrade(&mut self) -> io::Result<()> {
        if self.is_shared() {
            unsafe {
                raw_file_lock(
                    &self.file,
                    Some(Lock::Exclusive),
                    self.offset,
                    self.len,
                    false,
                )?;
            }
            self.lock = Lock::Exclusive;
        }
        Ok(())
    }
}