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#![doc(html_root_url = "https://docs.rs/fcntl/0.1.0")] //! Wrapper around [fcntl (2)](https://www.man7.org/linux/man-pages/man2/fcntl.2.html) and convenience methods to make //! interacting with it easier. Currently only supports commands related to Advisory record locking. // // // TODO: Instead of exposing `libc::flock` we should implement our own flock which implements // - `From<flock>` // - `Into<flock>` // - Methods from `FlockOperations` without the need for the extra trait // - Other common traits (`Eq`, `PartialEq`, `Ord`, `PartialOrd`, `Hash`, `Debug`, `Display`, `Default`) use libc::{ __errno_location, fcntl as libc_fcntl, }; use std::{ convert::TryInto, error::Error, fmt::{self, Display}, os::unix::io::AsRawFd, }; // re-exports pub use libc::{ c_int, c_short, flock, }; /// Allowed types for the `arg` parameter for the `fcntl` syscall. #[derive(Copy, Clone)] pub enum FcntlArg { Flock(flock), } /// Allowed commands (`cmd` parameter) for the `fcntl` syscall. #[derive(Copy, Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)] pub enum FcntlCmd { /// F_SETLK, SetLock, /// F_SETLKW SetLockWait, /// F_GETLK GetLock, } /// Error type which functions of this crate will return. #[derive(Copy, Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)] pub enum FcntlError { /// The requested FcntlCmd is not yet handled by our implementation CommandNotImplemented(FcntlCmd), /// The syscall returned the respective error (which may be `None`, if the errno lookup fails) Errno(Option<c_int>), /// An `crate`-internal error occured. If you get this error variant, please report this as a bug! Internal, /// The enum variant of `arg` does not match the expected variant for the requested `cmd`. No operation was /// performed. InvalidArgForCmd, } /// Defines which types of lock can be set onto files. #[derive(Copy, Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)] pub enum FcntlLockType { /// Wrapper for `F_RDLCK` Read, /// Wrapper for `F_WRLCK` Write, } /// This trait is used to define functions wich directly operate on `flock`. /// ```rust /// use libc::flock; /// use fcntl::{FcntlLockType, FlockOperations}; /// /// let flock = flock::default().with_locktype(FcntlLockType::Write); /// ``` pub trait FlockOperations { /// Since we don't have control over `libc::flock` we cannot `impl Default for libc::flock` fn default() -> Self; /// Sets `l_type` to the given value using the builder pattern. This is mostly intended for internal use. Where /// possible, it is recommended to use other methods which alter `l_type` (e.g. `with_locktype`). fn with_l_type(self, l_type: c_short) -> Self; /// Sets the lock type (`l_type`) to the appropriate value for the given `FcntlLockType`, using the builder pattern. fn with_locktype(self, locktype: FcntlLockType) -> Self; } /// Calls `fcntl` with the given `cmd` and `arg`. On success, the structure passed to the `arg` parameter is returned /// as returned by the kernel. /// **Note**: Where possible convenience wrappers (such as `is_file_locked`, `lock_file`, etc.) should be used as they /// correctly interpret possible return values of the syscall. /// /// Currently supported `cmd`s: /// - `FcntlCmd::GetLock` /// - `FcntlCmd::SetLock` /// /// # Errors /// /// In case of an error (syscall returned an error, invalid `arg` provided, `cmd` not supported, etc.) an appropriate /// value is returned. pub fn fcntl<'a, RF>(fd: &'a RF, cmd: FcntlCmd, arg: FcntlArg) -> Result<FcntlArg, FcntlError> where RF: AsRawFd { let fd = fd.as_raw_fd(); // different commands require different types of `arg` match cmd { FcntlCmd::GetLock | FcntlCmd::SetLock /*| FcntlCmd::SetLockWait*/ => { match arg { FcntlArg::Flock(flock) => { let mut flock = flock; let rv = unsafe { libc_fcntl(fd, cmd.into(), &mut flock) }; if rv == 0 { Ok(FcntlArg::Flock(flock)) } else { let errno_ptr = unsafe { __errno_location() }; let errno = if errno_ptr.is_null() { None } else { // *should* be safe here as we checked against NULL pointer.. Some(unsafe {*errno_ptr}) }; Err(FcntlError::Errno(errno)) } } _ => Err(FcntlError::InvalidArgForCmd), } } FcntlCmd::SetLockWait => Err(FcntlError::CommandNotImplemented(FcntlCmd::SetLockWait)), } } /// Checks whether the given file is locked. /// /// The caller is responsible that `fd` was opened with the appropriate parameters, as stated by /// [fcntl (2)](https://www.man7.org/linux/man-pages/man2/fcntl.2.html): /// > In order to place a read lock, `fd` must be open for reading. In order to place a write lock, `fd` must be open /// for writing. To place both types of lock, open a file read-write. /// /// ```rust /// use std::fs::OpenOptions; /// use fcntl::is_file_locked; /// # let file_name = "README.md"; /// /// let file = OpenOptions::new().read(true).open(file_name).unwrap(); /// match is_file_locked(&file, None) { /// Ok(true) => println!("File is currently locked"), /// Ok(false) => println!("File is not locked"), /// Err(err) => println!("Error: {:?}", err), /// } /// ``` pub fn is_file_locked<'a, RF>(fd: &'a RF, flock: Option<flock>) -> Result<bool, FcntlError> where RF: AsRawFd { let arg = match flock { Some(flock) => FcntlArg::Flock(flock), None => FcntlArg::Flock(libc::flock::default()) }; match fcntl(fd, FcntlCmd::GetLock, arg) { Ok(FcntlArg::Flock(result)) => { // We need to convert from c_int into c_short. F_UNLCK is defined with value 2, so this should never panic. // To be extra safe, we have a test case for that ;) Ok(result.l_type != libc::F_UNLCK.try_into().unwrap()) } Ok(_) => Err(FcntlError::Internal), Err(err) => Err(err), } } /// Locks the given file (using `FcntlCmd::SetLock`). If `flock` is `None` all parameters of the flock structure ( /// `l_whence`, `l_start`, `l_len`, `l_pid`) will be set to 0. `locktype` controls the `l_type` parameter. When it is /// `None`, `FcntlLockType::Read` is used. `flock.l_type` will be overwritten in all cases to avoid passing an invalid /// parameter to the syscall. /// /// The caller is responsible that `fd` was opened with the appropriate parameters, as stated by `fcntl 2`: /// > In order to place a read lock, `fd` must be open for reading. In order to place a write lock, `fd` must be open /// for writing. To place both types of lock, open a file read-write. /// /// ```rust /// use std::fs::OpenOptions; /// use fcntl::{FcntlLockType, lock_file}; /// # let file_name = "README.md"; /// /// let file = OpenOptions::new().read(true).write(true).open(file_name).unwrap(); /// match lock_file(&file, None, Some(FcntlLockType::Write)) { /// Ok(true) => println!("Lock acuired!"), /// Ok(false) => println!("Could not acquire lock!"), /// Err(err) => println!("Error: {:?}", err), /// } /// ``` pub fn lock_file<'a, RF>(fd: &'a RF, flock: Option<flock>, locktype: Option<FcntlLockType>) -> Result<bool, FcntlError> where RF: AsRawFd { let locktype = locktype.unwrap_or(FcntlLockType::Read); let arg = match flock { Some(flock) => FcntlArg::Flock(flock.with_locktype(locktype)), None => FcntlArg::Flock(libc::flock::default().with_locktype(locktype)), }; match fcntl(fd, FcntlCmd::SetLock, arg) { // Locking was successful Ok(FcntlArg::Flock(_result)) => Ok(true), // This should not happen, unless we have a bug.. Ok(_) => Err(FcntlError::Internal), // "If a conflicting lock is held by another process, this call returns -1 and sets errno to EACCES or EAGAIN." Err(FcntlError::Errno(Some(libc::EACCES))) | Err(FcntlError::Errno(Some(libc::EAGAIN))) => Ok(false), // Everything else is also an error Err(err) => Err(err), } } /// Releases the lock on the given file (using `FcntlCmd::SetLock`). If `flock` is `None` all parameters of the flock /// structure (`l_whence`, `l_start`, `l_len`, `l_pid`) will be set to 0. `flock.l_type` will be set to `libc::F_UNLCK` /// regardless of its original value. /// /// ```rust /// use std::fs::OpenOptions; /// use fcntl::unlock_file; /// # let file_name = "README.md"; /// /// let file = OpenOptions::new().read(true).open(file_name).unwrap(); /// match unlock_file(&file, None) { /// Ok(true) => println!("Lock successfully released"), /// Ok(false) => println!("Falied to release lock"), /// Err(err) => println!("Error: {:?}", err), /// } /// ``` pub fn unlock_file<'a, RF>(fd: &'a RF, flock: Option<flock>) -> Result<bool, FcntlError> where RF: AsRawFd { let arg = match flock { // unrwap is safe here Some(flock) => FcntlArg::Flock(flock.with_l_type(libc::F_UNLCK.try_into().unwrap())), // unwrap is safe here None => FcntlArg::Flock(libc::flock::default().with_l_type(libc::F_UNLCK.try_into().unwrap(),)), }; match fcntl(fd, FcntlCmd::SetLock, arg) { // Unlocking was successful Ok(FcntlArg::Flock(_result)) => Ok(true), // This should not happen, unless we have a bug.. Ok(_) => Err(FcntlError::Internal), // "If a conflicting lock is held by another process, this call returns -1 and sets errno to EACCES or EAGAIN." //Err(FcntlError::Errno(Some(libc::EACCES))) | Err(FcntlError::Errno(Some(libc::EAGAIN))) => Ok(false), // Everything else is also an error Err(err) => Err(err), } } impl FlockOperations for flock { /// Sets all fields to 0. fn default() -> Self { flock { l_type: 0, l_whence: 0, l_start: 0, l_len: 0, l_pid: 0, } } fn with_l_type(mut self, l_type: c_short) -> Self { self.l_type = l_type; self } fn with_locktype(mut self, locktype: FcntlLockType) -> Self { self.l_type = locktype.into(); self } } impl Display for FcntlError { fn fmt(&self, ff: &mut fmt::Formatter<'_>) -> fmt::Result { match self { Self::CommandNotImplemented(cmd) => write!(ff, "{:?} is not implemented for this operation", cmd), Self::Errno(Some(errno)) => write!(ff, "syscall returned unknown or unexpected value: {}", errno), Self::Errno(None) => write!(ff, "syscall returned error but we could not retrieve errno"), Self::Internal => write!(ff, "we encountered an internal error. Please report this as a bug (fcntl)!"), Self::InvalidArgForCmd => write!(ff, "the provided arg parameter is invalid for the requested cmd"), } } } impl Error for FcntlError {} impl From<FcntlCmd> for c_int { fn from(cmd: FcntlCmd) -> c_int { match cmd { FcntlCmd::GetLock => libc::F_GETLK, FcntlCmd::SetLock => libc::F_SETLK, FcntlCmd::SetLockWait => libc::F_SETLKW, } } } impl From<FcntlLockType> for c_short { fn from(locktype: FcntlLockType) -> c_short { match locktype { // These should never panic as their values are hardcoded in the kernel with low enough values FcntlLockType::Read => libc::F_RDLCK.try_into().unwrap(), FcntlLockType::Write => libc::F_WRLCK.try_into().unwrap(), } } } #[cfg(test)] mod tests { use super::*; use std::fs::{OpenOptions, remove_file}; const LOCK_FILE_NAME: &str = "./test-work-dir/lock-test"; #[test] fn check_cmd_conversion() { let pairs = vec![ (FcntlCmd::GetLock, libc::F_GETLK), (FcntlCmd::SetLock, libc::F_SETLK), (FcntlCmd::SetLockWait, libc::F_SETLKW), ]; for (cmd, check) in pairs.into_iter() { assert_eq!(libc::c_int::from(cmd), check); } } #[test] fn ensure_conversions_dont_panic() { let _: libc::c_short = libc::F_UNLCK.try_into().unwrap(); } #[test] fn check_file_locking_simple() { // nested so that `file` goes out of scope before we delete the file again let result = { let file = OpenOptions::new().write(true).create(true).open(LOCK_FILE_NAME).unwrap(); is_file_locked(&file, None) }; // cleanup let _ = remove_file(LOCK_FILE_NAME); // final assertion assert_eq!(result, Ok(false)); } #[test] fn lock_file_simple() { // nested so that `file` goes out of scope before we delete the file again // create the file { let file = OpenOptions::new().write(true).create(true).open(LOCK_FILE_NAME).unwrap(); } let file = OpenOptions::new().read(true).open(LOCK_FILE_NAME).unwrap(); let result_is_file_locked_before = is_file_locked(&file, None); let result_lock_file_read = lock_file(&file, None, None); // We would need to test this with a separate process... //let result_is_file_locked_after = is_file_locked(&file, None); let result_unlock_after = unlock_file(&file, None); // cleanup let _ = remove_file(LOCK_FILE_NAME); // final assertions assert_eq!(result_is_file_locked_before, Ok(false), "Verify that file was unlocked"); assert_eq!(result_lock_file_read, Ok(true), "Lock file"); //assert_eq!(result_is_file_locked_after, Ok(true)); assert_eq!(result_unlock_after, Ok(true), "Unlock file"); } }