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//! Advisory lock provides simple and convenient API for using file locks. //! //! These are called advisory because they don't prevent other processes from //! accessing the files directly, bypassing the locks. //! However, if multiple processes agree on acquiring file locks, they should //! work as expected. //! //! The main entity of the crate is [`AdvisoryFileLock`] which is effectively //! a [`RwLock`] but for [`File`]. //! //! Example: //! ``` //! use std::fs::File; //! use advisory_lock::{AdvisoryFileLock, FileLockMode, FileLockError}; //! # //! # //! // Create the file and obtain its exclusive advisory lock //! let exclusive_file = File::create("foo.txt").unwrap(); //! exclusive_file.lock(FileLockMode::Exclusive)?; //! //! let shared_file = File::open("foo.txt")?; //! //! // Try to acquire the lock in non-blocking way //! assert!(matches!(shared_file.try_lock(FileLockMode::Shared), Err(FileLockError::AlreadyLocked))); //! //! exclusive_file.unlock()?; //! //! shared_file.try_lock(FileLockMode::Shared).expect("Works, because the exclusive lock was released"); //! //! let shared_file_2 = File::open("foo.txt")?; //! //! shared_file_2.lock(FileLockMode::Shared).expect("Should be fine to have multiple shared locks"); //! //! // Nope, now we have to wait until all shared locks are released... //! assert!(matches!(exclusive_file.try_lock(FileLockMode::Exclusive), Err(FileLockError::AlreadyLocked))); //! //! // We can unlock them explicitly and handle the potential error //! shared_file.unlock()?; //! // Or drop the lock, such that we `log::error!()` if it happens and discard it //! drop(shared_file_2); //! //! exclusive_file.lock(FileLockMode::Exclusive).expect("All other locks should have been released"); //! # //! # std::fs::remove_file("foo.txt")?; //! # Ok::<(), Box<dyn std::error::Error>>(()) //! ``` //! //! [`AdvisoryFileLock`]: struct.AdvisoryFileLock.html //! [`RwLock`]: https://doc.rust-lang.org/stable/std/sync/struct.RwLock.html //! [`File`]: https://doc.rust-lang.org/stable/std/fs/struct.File.html use std::{fmt, io}; #[cfg(windows)] mod windows; #[cfg(unix)] mod unix; /// An enumeration of possible errors which can occur while trying to acquire a lock. #[derive(Debug)] pub enum FileLockError { /// The file is already locked by other process. AlreadyLocked, /// The error occurred during I/O operations. Io(io::Error), } impl fmt::Display for FileLockError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { match self { FileLockError::AlreadyLocked => f.write_str("the file is already locked"), FileLockError::Io(err) => write!(f, "I/O error: {}", err), } } } impl std::error::Error for FileLockError {} /// An enumeration of types which represents how to acquire an advisory lock. #[derive(Copy, Clone, Eq, PartialEq, Debug)] pub enum FileLockMode { /// Obtain an exclusive file lock. Exclusive, /// Obtain a shared file lock. Shared, } /// An advisory lock for files. /// /// An advisory lock provides a mutual-exclusion mechanism among processes which explicitly /// acquires and releases the lock. Processes that are not aware of the lock will ignore it. /// /// `AdvisoryFileLock` provides following features: /// - Blocking or non-blocking operations. /// - Shared or exclusive modes. /// - All operations are thread-safe. /// /// ## Notes /// /// `AdvisoryFileLock` has following limitations: /// - Locks are allowed only on files, but not directories. pub trait AdvisoryFileLock { /// Acquire the advisory file lock. /// /// `lock` is blocking; it will block the current thread until it succeeds or errors. fn lock(&self, file_lock_mode: FileLockMode) -> Result<(), FileLockError>; /// Try to acquire the advisory file lock. /// /// `try_lock` returns immediately. fn try_lock(&self, file_lock_mode: FileLockMode) -> Result<(), FileLockError>; /// Unlock this advisory file lock. fn unlock(&self) -> Result<(), FileLockError>; } #[cfg(test)] mod tests { use super::*; use std::env::temp_dir; use std::fs::File; #[test] fn simple_shared_lock() { let mut test_file = temp_dir(); test_file.push("shared_lock"); File::create(&test_file).unwrap(); { let f1 = File::open(&test_file).unwrap(); f1.lock(FileLockMode::Shared).unwrap(); let f2 = File::open(&test_file).unwrap(); f2.lock(FileLockMode::Shared).unwrap(); } std::fs::remove_file(&test_file).unwrap(); } #[test] fn simple_exclusive_lock() { let mut test_file = temp_dir(); test_file.push("exclusive_lock"); File::create(&test_file).unwrap(); { let f1 = File::open(&test_file).unwrap(); f1.lock(FileLockMode::Exclusive).unwrap(); let f2 = File::open(&test_file).unwrap(); assert!(f2.try_lock(FileLockMode::Exclusive).is_err()); } std::fs::remove_file(&test_file).unwrap(); } #[test] fn simple_shared_exclusive_lock() { let mut test_file = temp_dir(); test_file.push("shared_exclusive_lock"); File::create(&test_file).unwrap(); { let f1 = File::open(&test_file).unwrap(); f1.lock(FileLockMode::Shared).unwrap(); let f2 = File::open(&test_file).unwrap(); assert!(matches!( f2.try_lock(FileLockMode::Exclusive), Err(FileLockError::AlreadyLocked) )); } std::fs::remove_file(&test_file).unwrap(); } #[test] fn simple_exclusive_shared_lock() { let mut test_file = temp_dir(); test_file.push("exclusive_shared_lock"); File::create(&test_file).unwrap(); { let f1 = File::open(&test_file).unwrap(); f1.lock(FileLockMode::Exclusive).unwrap(); let f2 = File::open(&test_file).unwrap(); assert!(f2.try_lock(FileLockMode::Shared).is_err()); } std::fs::remove_file(&test_file).unwrap(); } }