Struct atomic_write_file::AtomicWriteFile

pub struct AtomicWriteFile { /* private fields */ }

A file whose contents become visible to users only after the file is committed.

An AtomicWriteFile is a file that is assigned to a path, but whose contents won’t appear at that path until the file is committed. If AtomicWriteFile is used to open a file that already exists, the contents of the existing file will remain available until the AtomicWriteFile is committed. During that time, the AtomicWriteFile may be used to write new contents, but these new contents won’t be visible until after the file is committed.

Internally, AtomicWriteFile is implemented by initally opening a temporary file, and then renaming the temporary file to its final path on commit. See the module-level documentation for more details about the implementation.

An AtomicWriteFile is automatically discarded when it goes out of scope (when it gets dropped). Any error that occurs on drop is ignored. For this reason, if the file should not be committed, it is highly recommended that AtomicWriteFile is discarded explicitly using the discard() method, which allows callers to detect errors on cleanup. See committing or discarding changes below for more information.

Opening an AtomicWriteFile

There are two ways to obtain an AtomicWriteFile struct:

• AtomicWriteFile::open()
• OpenOptions::open()

The first method opens a file at the specified path with some default options. The second method using OpenOptions allows configuring how the file is opened.

Compatibility with std::fs::File

AtomicWriteFile implements the same methods and traits of std::fs::File, and aims to be as much compatible with File as possible. In fact, AtomicWriteFile can be dereferenced into a File struct: this means that you can use all methods provided by File directly on an AtomicWriteFile (just like you can use all of str methods on a String).

A reference to the wrapped File struct may also be explicitly obtained using as_file() and as_file_mut().

Committing or discarding changes

AtomicWriteFile provides two additional methods that are not provided by File: commit() and discard(). These methods can be called to save the new contents to the file path, or to destroy the new contents and leave the original file (if any) unchaged, respectively.

Changes are automatically discarded also when AtomicWriteFile is dropped. Therefore calling discard() is not mandatory, but it is highly recommended because the Drop implementation ignores all errors.

Cloning

Cloning a AtomicWriteFile is not possible, because this would result in ambiguity and race conditions when committing the file and its clones. It is however possible to clone the underlaying File struct using try_clone(). Writes to this cloned File however won’t be atomic after the AtomicWriteFile is committed.

Examples

Opening a file, writing new contents, and committing the changes:

use std::io::Write;
use atomic_write_file::AtomicWriteFile;

let mut file = AtomicWriteFile::open("foo.txt")?; // if "foo.txt" already exists, it is not
                                                  // initially truncated or deleted
writeln!(file, "hello")?; // "hello" is written to a temporary location; "foo.txt" (if it
                          // exists) keeps its old contents after this write

file.commit()?; // only now "foo.txt" gets swapped with the new contents ("hello")

Implementations

impl AtomicWriteFile

pub fn open<P: AsRef<Path>>(path: P) -> Result<AtomicWriteFile>

Opens an atomically-written file at path.

See OpenOptions for more details.

Examples

use atomic_write_file::AtomicWriteFile;
let file = AtomicWriteFile::open("foo.txt");

pub fn options() -> OpenOptions

Creates a new OpenOptions with default options.

This is equivalent to OpenOptions::new(), but allows for more readable code.

Examples

use atomic_write_file::AtomicWriteFile;
let file = AtomicWriteFile::options().read(true).open("foo.txt");

pub fn as_file(&self) -> &File

Returns a reference to the underlaying File struct.

The returned reference can be used to inspect or manipulate the contents and metadata of this AtomicWriteFile.

Examples

use std::os::fd::AsRawFd;
use atomic_write_file::AtomicWriteFile;

let file = AtomicWriteFile::open("foo.txt")?;
assert_eq!(file.as_raw_fd(), file.as_file().as_raw_fd());

pub fn as_file_mut(&mut self) -> &mut File

Returns a mutable reference to the underlaying File struct.

The returned reference can be used to inspect or manipulate the contents and metadata of this AtomicWriteFile.

Examples

use std::io::Write;
use atomic_write_file::AtomicWriteFile;

let mut file = AtomicWriteFile::open("foo.txt")?;
writeln!(file.as_file_mut(), "hello")?;

pub fn directory(&self) -> Option<Directory<'_>>

Returns a borrowed reference to the directory containing the file.

This method allows you to obtain the directory file descriptor, without having to open it through a call to open(2). This method is guaranteed to make no system calls.

The returned struct supports only two operations:

• conversion to a borrowed directory file descriptor through AsFd::as_fd()
• conversion to a raw directory file descriptor through AsRawFd::as_raw_fd()

This method will return a result only if the platform supports directory file descriptors, and if the AtomicWriteFile implementation makes use of them. In all other cases, this method returns None.

Examples

use std::os::fd::AsFd;
use atomic_write_file::AtomicWriteFile;

let file = AtomicWriteFile::open("foo.txt")?;
if let Some(dir) = file.directory() {
    let borrowed_fd = dir.as_fd();
    println!("directory fd: {:?}", borrowed_fd);
}

pub fn commit(self) -> Result<()>

Saves the contents of this file to its path.

After calling commit(), the AtomicWriteFile is consumed and can no longer be used. Clones of the underlaying File may still be used after calling commit(), although any write from that point onwards will no longer be atomic.

See the documentation for AtomicWriteFile and the module-level documentation for details about the internal implementation of commit(), as well as platform-specific details.

This method is automatically called when AtomicWriteFile is dropped, although in that case any error produced by commit() is ignored.

See also AtomicWriteFile::discard().

Examples

use std::io::Write;
use atomic_write_file::AtomicWriteFile;

let file = AtomicWriteFile::open("foo.txt")?;
writeln!(&file, "hello")?;
file.commit()?;

pub fn discard(self) -> Result<()>

Discard the contents of this file, and leave its path unchanged.

After calling discard(), the AtomicWriteFile is consumed and can no longer be used. Clones of the underlaying File may still be used after calling discard().

See also AtomicWriteFile::commit().

Examples

use std::io::Write;
use atomic_write_file::AtomicWriteFile;

let file = AtomicWriteFile::open("foo.txt")?;
writeln!(&file, "hello")?;
file.discard()?;

Methods from Deref<Target = File>

pub fn sync_all(&self) -> Result<(), Error>

Attempts to sync all OS-internal file content and metadata to disk.

This function will attempt to ensure that all in-memory data reaches the filesystem before returning.

This can be used to handle errors that would otherwise only be caught when the File is closed, as dropping a File will ignore all errors. Note, however, that sync_all is generally more expensive than closing a file by dropping it, because the latter is not required to block until the data has been written to the filesystem.

If synchronizing the metadata is not required, use sync_data instead.

Examples

use std::fs::File;
use std::io::prelude::*;

fn main() -> std::io::Result<()> {
    let mut f = File::create("foo.txt")?;
    f.write_all(b"Hello, world!")?;

    f.sync_all()?;
    Ok(())
}

pub fn sync_data(&self) -> Result<(), Error>

This function is similar to sync_all, except that it might not synchronize file metadata to the filesystem.

This is intended for use cases that must synchronize content, but don’t need the metadata on disk. The goal of this method is to reduce disk operations.

Note that some platforms may simply implement this in terms of sync_all.

Examples

use std::fs::File;
use std::io::prelude::*;

fn main() -> std::io::Result<()> {
    let mut f = File::create("foo.txt")?;
    f.write_all(b"Hello, world!")?;

    f.sync_data()?;
    Ok(())
}

pub fn set_len(&self, size: u64) -> Result<(), Error>

Truncates or extends the underlying file, updating the size of this file to become size.

If the size is less than the current file’s size, then the file will be shrunk. If it is greater than the current file’s size, then the file will be extended to size and have all of the intermediate data filled in with 0s.

The file’s cursor isn’t changed. In particular, if the cursor was at the end and the file is shrunk using this operation, the cursor will now be past the end.

Errors

This function will return an error if the file is not opened for writing. Also, std::io::ErrorKind::InvalidInput will be returned if the desired length would cause an overflow due to the implementation specifics.

Examples

use std::fs::File;

fn main() -> std::io::Result<()> {
    let mut f = File::create("foo.txt")?;
    f.set_len(10)?;
    Ok(())
}

Note that this method alters the content of the underlying file, even though it takes &self rather than &mut self.

pub fn metadata(&self) -> Result<Metadata, Error>

Queries metadata about the underlying file.

Examples

use std::fs::File;

fn main() -> std::io::Result<()> {
    let mut f = File::open("foo.txt")?;
    let metadata = f.metadata()?;
    Ok(())
}

pub fn try_clone(&self) -> Result<File, Error>

Creates a new File instance that shares the same underlying file handle as the existing File instance. Reads, writes, and seeks will affect both File instances simultaneously.

Examples

Creates two handles for a file named foo.txt:

use std::fs::File;

fn main() -> std::io::Result<()> {
    let mut file = File::open("foo.txt")?;
    let file_copy = file.try_clone()?;
    Ok(())
}

Assuming there’s a file named foo.txt with contents abcdef\n, create two handles, seek one of them, and read the remaining bytes from the other handle:

use std::fs::File;
use std::io::SeekFrom;
use std::io::prelude::*;

fn main() -> std::io::Result<()> {
    let mut file = File::open("foo.txt")?;
    let mut file_copy = file.try_clone()?;

    file.seek(SeekFrom::Start(3))?;

    let mut contents = vec![];
    file_copy.read_to_end(&mut contents)?;
    assert_eq!(contents, b"def\n");
    Ok(())
}

pub fn set_permissions(&self, perm: Permissions) -> Result<(), Error>

Changes the permissions on the underlying file.

Platform-specific behavior

This function currently corresponds to the fchmod function on Unix and the SetFileInformationByHandle function on Windows. Note that, this may change in the future.

Errors

This function will return an error if the user lacks permission change attributes on the underlying file. It may also return an error in other os-specific unspecified cases.

Examples

fn main() -> std::io::Result<()> {
    use std::fs::File;

    let file = File::open("foo.txt")?;
    let mut perms = file.metadata()?.permissions();
    perms.set_readonly(true);
    file.set_permissions(perms)?;
    Ok(())
}

Note that this method alters the permissions of the underlying file, even though it takes &self rather than &mut self.

pub fn set_times(&self, times: FileTimes) -> Result<(), Error>

Changes the timestamps of the underlying file.

Platform-specific behavior

This function currently corresponds to the futimens function on Unix (falling back to futimes on macOS before 10.13) and the SetFileTime function on Windows. Note that this may change in the future.

Errors

This function will return an error if the user lacks permission to change timestamps on the underlying file. It may also return an error in other os-specific unspecified cases.

This function may return an error if the operating system lacks support to change one or more of the timestamps set in the FileTimes structure.

Examples

fn main() -> std::io::Result<()> {
    use std::fs::{self, File, FileTimes};

    let src = fs::metadata("src")?;
    let dest = File::options().write(true).open("dest")?;
    let times = FileTimes::new()
        .set_accessed(src.accessed()?)
        .set_modified(src.modified()?);
    dest.set_times(times)?;
    Ok(())
}

pub fn set_modified(&self, time: SystemTime) -> Result<(), Error>

Changes the modification time of the underlying file.

This is an alias for set_times(FileTimes::new().set_modified(time)).

Trait Implementations

impl AsFd for AtomicWriteFile

fn as_fd(&self) -> BorrowedFd<'_>

Borrows the file descriptor.

impl AsRawFd for AtomicWriteFile

fn as_raw_fd(&self) -> RawFd

Extracts the raw file descriptor.

impl Debug for AtomicWriteFile

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Deref for AtomicWriteFile

type Target = File

The resulting type after dereferencing.

fn deref(&self) -> &Self::Target

Dereferences the value.

impl DerefMut for AtomicWriteFile

fn deref_mut(&mut self) -> &mut Self::Target

Mutably dereferences the value.

impl Drop for AtomicWriteFile

fn drop(&mut self)

Executes the destructor for this type.

impl FileExt for AtomicWriteFile

fn read_at(&self, buf: &mut [u8], offset: u64) -> Result<usize>

Reads a number of bytes starting from a given offset.

fn write_at(&self, buf: &[u8], offset: u64) -> Result<usize>

Writes a number of bytes starting from a given offset.

fn read_exact_at(&self, buf: &mut [u8], offset: u64) -> Result<()>

Reads the exact number of bytes required to fill buf from the given offset.

fn write_all_at(&self, buf: &[u8], offset: u64) -> Result<()>

Attempts to write an entire buffer starting from a given offset.

fn read_vectored_at( &self, bufs: &mut [IoSliceMut<'_>], offset: u64, ) -> Result<usize, Error>

🔬This is a nightly-only experimental API. (unix_file_vectored_at)

Like read_at, except that it reads into a slice of buffers.

fn write_vectored_at( &self, bufs: &[IoSlice<'_>], offset: u64, ) -> Result<usize, Error>

🔬This is a nightly-only experimental API. (unix_file_vectored_at)

Like write_at, except that it writes from a slice of buffers.

impl Read for &AtomicWriteFile

fn read(&mut self, buf: &mut [u8]) -> Result<usize>

Pull some bytes from this source into the specified buffer, returning how many bytes were read.

fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>

Like read, except that it reads into a slice of buffers.

fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize>

Reads all bytes until EOF in this source, placing them into buf.

fn read_to_string(&mut self, buf: &mut String) -> Result<usize>

Reads all bytes until EOF in this source, appending them to buf.

fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>

Reads the exact number of bytes required to fill buf.

fn is_read_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector)

Determines if this Reader has an efficient read_vectored implementation.

fn read_buf(&mut self, buf: BorrowedCursor<'_>) -> Result<(), Error>

🔬This is a nightly-only experimental API. (read_buf)

Pull some bytes from this source into the specified buffer.

fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<(), Error>

🔬This is a nightly-only experimental API. (read_buf)

Reads the exact number of bytes required to fill cursor.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Creates a “by reference” adaptor for this instance of Read.

fn bytes(self) -> Bytes<Self>

where Self: Sized,

Transforms this Read instance to an Iterator over its bytes.

fn chain<R>(self, next: R) -> Chain<Self, R>

where R: Read, Self: Sized,

Creates an adapter which will chain this stream with another.

fn take(self, limit: u64) -> Take<Self>

where Self: Sized,

Creates an adapter which will read at most limit bytes from it.

impl Read for AtomicWriteFile

fn read(&mut self, buf: &mut [u8]) -> Result<usize>

Pull some bytes from this source into the specified buffer, returning how many bytes were read.

fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>

Like read, except that it reads into a slice of buffers.

fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize>

Reads all bytes until EOF in this source, placing them into buf.

fn read_to_string(&mut self, buf: &mut String) -> Result<usize>

Reads all bytes until EOF in this source, appending them to buf.

fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>

Reads the exact number of bytes required to fill buf.

fn is_read_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector)

Determines if this Reader has an efficient read_vectored implementation.

fn read_buf(&mut self, buf: BorrowedCursor<'_>) -> Result<(), Error>

🔬This is a nightly-only experimental API. (read_buf)

Pull some bytes from this source into the specified buffer.

fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<(), Error>

🔬This is a nightly-only experimental API. (read_buf)

Reads the exact number of bytes required to fill cursor.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Creates a “by reference” adaptor for this instance of Read.

fn bytes(self) -> Bytes<Self>

where Self: Sized,

Transforms this Read instance to an Iterator over its bytes.

fn chain<R>(self, next: R) -> Chain<Self, R>

where R: Read, Self: Sized,

Creates an adapter which will chain this stream with another.

fn take(self, limit: u64) -> Take<Self>

where Self: Sized,

Creates an adapter which will read at most limit bytes from it.

impl Seek for &AtomicWriteFile

fn seek(&mut self, pos: SeekFrom) -> Result<u64>

Seek to an offset, in bytes, in a stream.

fn rewind(&mut self) -> Result<()>

Rewind to the beginning of a stream.

fn stream_position(&mut self) -> Result<u64>

Returns the current seek position from the start of the stream.

fn stream_len(&mut self) -> Result<u64, Error>

🔬This is a nightly-only experimental API. (seek_stream_len)

Returns the length of this stream (in bytes).

fn seek_relative(&mut self, offset: i64) -> Result<(), Error>

Seeks relative to the current position.

impl Seek for AtomicWriteFile

fn seek(&mut self, pos: SeekFrom) -> Result<u64>

Seek to an offset, in bytes, in a stream.

fn rewind(&mut self) -> Result<()>

Rewind to the beginning of a stream.

fn stream_position(&mut self) -> Result<u64>

Returns the current seek position from the start of the stream.

fn stream_len(&mut self) -> Result<u64, Error>

🔬This is a nightly-only experimental API. (seek_stream_len)

Returns the length of this stream (in bytes).

fn seek_relative(&mut self, offset: i64) -> Result<(), Error>

Seeks relative to the current position.

impl Write for &AtomicWriteFile

fn write(&mut self, buf: &[u8]) -> Result<usize>

Writes a buffer into this writer, returning how many bytes were written.

fn flush(&mut self) -> Result<()>

Flushes this output stream, ensuring that all intermediately buffered contents reach their destination.

fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>

Like write, except that it writes from a slice of buffers.

fn write_all(&mut self, buf: &[u8]) -> Result<()>

Attempts to write an entire buffer into this writer.

fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<()>

Writes a formatted string into this writer, returning any error encountered.

fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector)

Determines if this Writer has an efficient write_vectored implementation.

fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<(), Error>

🔬This is a nightly-only experimental API. (write_all_vectored)

Attempts to write multiple buffers into this writer.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Creates a “by reference” adapter for this instance of Write.

impl Write for AtomicWriteFile

fn write(&mut self, buf: &[u8]) -> Result<usize>

Writes a buffer into this writer, returning how many bytes were written.

fn flush(&mut self) -> Result<()>

Flushes this output stream, ensuring that all intermediately buffered contents reach their destination.

fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>

Like write, except that it writes from a slice of buffers.

fn write_all(&mut self, buf: &[u8]) -> Result<()>

Attempts to write an entire buffer into this writer.

fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<()>

Writes a formatted string into this writer, returning any error encountered.

fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector)

Determines if this Writer has an efficient write_vectored implementation.

fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<(), Error>

🔬This is a nightly-only experimental API. (write_all_vectored)

Attempts to write multiple buffers into this writer.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Creates a “by reference” adapter for this instance of Write.