filelocks

Platform-native advisory file locking for Rust — blocking and non-blocking, shared and exclusive, with an OFD/LockFileEx backend for network shares.

Features

• Shared (read) locks — multiple holders allowed simultaneously
• Exclusive (write) locks — mutual exclusion among cooperating lock users
• Blocking — park the thread until the lock is free
• Non-blocking — return Err(Error::WouldBlock) immediately
• Two backends — flock for local Unix files, OFD/LockFileEx for network shares
• Upgrade / downgrade — change lock kind without re-opening the file
• RAII guard — lock released automatically on drop
• Unsafe code confined to the two sys/ back-end modules

Platform / backend matrix

Backend	Linux / Android	Apple platforms	illumos	Other Unix	Windows
Flock	flock(2)	flock(2)	flock(2)	flock(2)	LockFileEx
Fcntl	OFD locks F_OFD_SETLK(W)	OFD locks F_OFD_SETLK(W)	OFD locks F_OFD_SETLK(W)	Error::Unsupported	LockFileEx

Use Fcntl for files that may reside on NFS or CIFS/SMB mounts. On Unix targets without open-file-description lock support, this crate returns Error::Unsupported instead of falling back to POSIX record locks.

⚠ Why no POSIX fallback? POSIX locks are per-process. Closing any file descriptor to the same inode from the same process can release all POSIX locks that process holds on that file. That cannot faithfully model this crate's per-guard RAII semantics.

📌 flock vs fcntl are independent: the Linux kernel tracks them in separate tables. An active flock lock does not prevent a fcntl lock from being acquired by a different file description (and vice versa). Never mix the two backends across cooperating processes.

Usage

# Cargo.toml
[dependencies]
filelocks = "0.1"

Local files (default flock backend)

use std::fs::OpenOptions;
use filelocks::{FileLock, LockKind, LockMode, Error};

let file = OpenOptions::new()
    .read(true).write(true).create(true)
    .open("my.lock")?;

match FileLock::lock(file, LockKind::Exclusive, LockMode::NonBlocking) {
    Ok(guard) => { /* critical section — guard dropped = lock released */ }
    Err(Error::WouldBlock) => { /* retry or bail */ }
    Err(e) => return Err(e.into()),
}

Network mounts — NFS / CIFS (fcntl backend)

use std::fs::OpenOptions;
use filelocks::{LockBackend, LockKind, LockMode, LockOptions};

let file = OpenOptions::new()
    .read(true).write(true).create(true)
    .open("/mnt/nfs/share/my.lock")?;

let guard = LockOptions::new()
    .backend(LockBackend::Fcntl)   // OFD locks on supported Unix, LockFileEx on Windows
    .lock(file, LockKind::Exclusive, LockMode::Blocking)?;

// guard released on drop

Upgrade / downgrade

let mut guard = LockOptions::new()
    .backend(LockBackend::Fcntl)
    .lock(file, LockKind::Shared, LockMode::Blocking)?;

// promote to exclusive (non-blocking — may return WouldBlock)
guard.upgrade(LockMode::NonBlocking)?;

// demote back to shared
guard.downgrade()?;

API overview

// Acquisition
FileLock::lock(file, kind, mode)           → Result<FileLock>  (Flock backend)
LockOptions::new().backend(b).lock(…)      → Result<FileLock>  (any backend)

// Inspection
guard.kind()     → LockKind
guard.backend()  → LockBackend
guard.file()     → &File
guard.file_mut() → &mut File

// Lock management
guard.upgrade(mode)   → Result<()>
guard.downgrade()     → Result<()>
guard.unlock()        → Result<File, (File, Error)>   (explicit unlock)
guard.into_file()     → File                           (no explicit unlock)

Error handling

use filelocks::Error;

match result {
    Err(Error::WouldBlock) => { /* lock contended — retry or skip */ }
    Err(Error::Unsupported(reason)) => { /* backend unavailable on this target */ }
    Err(Error::Io(e))      => { /* OS-level failure */ }
    Ok(guard)              => { /* success */ }
}

License

MIT OR Apache-2.0