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//! File-based locking using [`flock(2)`](https://linux.die.net/man/2/flock).
//!
//! You must start with an [`UnlockedFile`].
//!
//! ```rust
//! let lock_dir = tempfile::tempdir()?;
//! # use pgdo::lock::UnlockedFile;
//! let mut lock = UnlockedFile::try_from(lock_dir.path().join("foo").as_path())?;
//! let lock = lock.lock_shared()?;
//! let lock = lock.lock_exclusive()?;
//! let lock = lock.unlock()?;
//! # Ok::<(), std::io::Error>(())
//! ```
//!
//! Dropping a [`LockedFileShared`] or [`LockedFileExclusive`] will ordinarily
//! drop the underlying `flock`-based lock by virtue of dropping the [`File`]
//! they each wrap. However, if the file descriptor was duplicated prior to
//! creating the initial [`UnlockedFile`], the lock will persist as long as that
//! descriptor remains valid.

// Ignore deprecation warnings regarding `nix::fcntl::flock`.
// See https://github.com/nix-rust/nix/issues/2356.
#![allow(deprecated)]

use std::fs::File;
use std::os::unix::io::AsRawFd;

use either::{Either, Left, Right};
use nix::errno::Errno;
use nix::fcntl::{flock, FlockArg};
use nix::Result;

#[derive(Debug)]
pub struct UnlockedFile(File);
#[derive(Debug)]
pub struct LockedFileShared(File);
#[derive(Debug)]
pub struct LockedFileExclusive(File);

impl From<File> for UnlockedFile {
    fn from(file: File) -> Self {
        Self(file)
    }
}

impl TryFrom<&std::path::Path> for UnlockedFile {
    type Error = std::io::Error;

    fn try_from(path: &std::path::Path) -> std::io::Result<Self> {
        std::fs::OpenOptions::new()
            .append(true)
            .create(true)
            .open(path)
            .map(UnlockedFile)
    }
}

impl TryFrom<&std::path::PathBuf> for UnlockedFile {
    type Error = std::io::Error;

    fn try_from(path: &std::path::PathBuf) -> std::io::Result<Self> {
        Self::try_from(path.as_path())
    }
}

impl UnlockedFile {
    pub fn try_lock_shared(self) -> Result<Either<Self, LockedFileShared>> {
        match flock(self.0.as_raw_fd(), FlockArg::LockSharedNonblock) {
            Ok(()) => Ok(Right(LockedFileShared(self.0))),
            Err(Errno::EAGAIN) => Ok(Left(self)),
            Err(err) => Err(err),
        }
    }

    pub fn lock_shared(self) -> Result<LockedFileShared> {
        flock(self.0.as_raw_fd(), FlockArg::LockShared)?;
        Ok(LockedFileShared(self.0))
    }

    pub fn try_lock_exclusive(self) -> Result<Either<Self, LockedFileExclusive>> {
        match flock(self.0.as_raw_fd(), FlockArg::LockExclusiveNonblock) {
            Ok(()) => Ok(Right(LockedFileExclusive(self.0))),
            Err(Errno::EAGAIN) => Ok(Left(self)),
            Err(err) => Err(err),
        }
    }

    pub fn lock_exclusive(self) -> Result<LockedFileExclusive> {
        flock(self.0.as_raw_fd(), FlockArg::LockExclusive)?;
        Ok(LockedFileExclusive(self.0))
    }
}

impl LockedFileShared {
    pub fn try_lock_exclusive(self) -> Result<Either<Self, LockedFileExclusive>> {
        match flock(self.0.as_raw_fd(), FlockArg::LockExclusiveNonblock) {
            Ok(()) => Ok(Right(LockedFileExclusive(self.0))),
            Err(Errno::EAGAIN) => Ok(Left(self)),
            Err(err) => Err(err),
        }
    }

    pub fn lock_exclusive(self) -> Result<LockedFileExclusive> {
        flock(self.0.as_raw_fd(), FlockArg::LockExclusive)?;
        Ok(LockedFileExclusive(self.0))
    }

    pub fn try_unlock(self) -> Result<Either<Self, UnlockedFile>> {
        match flock(self.0.as_raw_fd(), FlockArg::UnlockNonblock) {
            Ok(()) => Ok(Right(UnlockedFile(self.0))),
            Err(Errno::EAGAIN) => Ok(Left(self)),
            Err(err) => Err(err),
        }
    }

    pub fn unlock(self) -> Result<UnlockedFile> {
        flock(self.0.as_raw_fd(), FlockArg::Unlock)?;
        Ok(UnlockedFile(self.0))
    }
}

impl LockedFileExclusive {
    pub fn try_lock_shared(self) -> Result<Either<Self, LockedFileShared>> {
        match flock(self.0.as_raw_fd(), FlockArg::LockSharedNonblock) {
            Ok(()) => Ok(Right(LockedFileShared(self.0))),
            Err(Errno::EAGAIN) => Ok(Left(self)),
            Err(err) => Err(err),
        }
    }

    pub fn lock_shared(self) -> Result<LockedFileShared> {
        flock(self.0.as_raw_fd(), FlockArg::LockShared)?;
        Ok(LockedFileShared(self.0))
    }

    pub fn try_unlock(self) -> Result<Either<Self, UnlockedFile>> {
        match flock(self.0.as_raw_fd(), FlockArg::UnlockNonblock) {
            Ok(()) => Ok(Right(UnlockedFile(self.0))),
            Err(Errno::EAGAIN) => Ok(Left(self)),
            Err(err) => Err(err),
        }
    }

    pub fn unlock(self) -> Result<UnlockedFile> {
        flock(self.0.as_raw_fd(), FlockArg::Unlock)?;
        Ok(UnlockedFile(self.0))
    }
}