200 lines
9.2 KiB
Rust
200 lines
9.2 KiB
Rust
//! Safe saving: a file is written to a temporary file beside it, then renamed over it, so a failed
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//! or interrupted write never leaves a damaged file behind (the old one stays as it was).
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//! [`write_new_with`] creates a file the same way without ever replacing one.
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use std::fs::{File, OpenOptions};
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use std::io::{self, Write as _};
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use std::path::{Path, PathBuf};
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use std::sync::atomic::{AtomicU64, Ordering};
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/// Write `bytes` to `path` atomically (see the module docs).
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pub fn write_atomic(path: &Path, bytes: &[u8]) -> io::Result<()> {
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write_atomic_with(path, |f| f.write_all(bytes))
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}
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/// Write `path` atomically with `fill`, which writes the content into the temporary file. When
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/// anything fails (`fill`, flushing, the rename) the temporary file is removed and `path` is left
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/// untouched.
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pub fn write_atomic_with(path: &Path, fill: impl FnOnce(&mut File) -> io::Result<()>) -> io::Result<()> {
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let target = resolve_link(path)?;
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let tmp = temp_path(&target)?;
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let mut f = OpenOptions::new().write(true).create_new(true).open(&tmp)?;
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let filled = fill(&mut f).and_then(|()| f.sync_all());
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drop(f);
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filled.and_then(|()| keep_permissions(&target, &tmp)).and_then(|()| std::fs::rename(&tmp, &target)).inspect_err(|_| {
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// Best effort: the temporary file is all there is to clean up.
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let _ = std::fs::remove_file(&tmp);
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})
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}
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/// Create `path` with the content `fill` writes, never replacing a file or a link of that name. The
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/// content goes to a temporary file beside it, which is then linked in under `path`
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/// (`std::fs::hard_link`, which fails with `AlreadyExists` when the name is taken, also when it was
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/// taken meanwhile). The file appears whole or not at all. Where hard links fail for another
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/// reason, `path` is created with `create_new` and the content copied in; a failed copy removes
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/// it. The temporary file is always removed.
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pub fn write_new_with(path: &Path, fill: impl FnOnce(&mut File) -> io::Result<()>) -> io::Result<()> {
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write_new_linking(path, fill, |tmp, path| std::fs::hard_link(tmp, path))
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}
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/// [`write_new_with`], with `link` linking the temporary file in under `path`.
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fn write_new_linking(
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path: &Path,
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fill: impl FnOnce(&mut File) -> io::Result<()>,
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link: impl FnOnce(&Path, &Path) -> io::Result<()>,
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) -> io::Result<()> {
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let tmp = temp_path(path)?;
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let mut f = OpenOptions::new().write(true).create_new(true).open(&tmp)?;
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let filled = fill(&mut f).and_then(|()| f.sync_all());
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drop(f);
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let placed = filled.and_then(|()| match link(&tmp, path) {
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Err(e) if e.kind() != io::ErrorKind::AlreadyExists => copy_new(&tmp, path),
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linked => linked,
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});
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// Best effort: the temporary file is all there is to clean up.
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let _ = std::fs::remove_file(&tmp);
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placed
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}
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/// Create `path` (never replacing anything) with the content of the file `from`; a failed copy
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/// removes it again.
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fn copy_new(from: &Path, path: &Path) -> io::Result<()> {
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let mut to = OpenOptions::new().write(true).create_new(true).open(path)?;
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let copied = File::open(from).and_then(|mut f| io::copy(&mut f, &mut to)).and_then(|_| to.sync_all());
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drop(to);
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if copied.is_err() {
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// Best effort: the part copied is all there is to clean up.
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let _ = std::fs::remove_file(path);
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}
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copied
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}
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/// A symbolic link keeps pointing at its file: write the file it names.
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fn resolve_link(path: &Path) -> io::Result<PathBuf> {
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match std::fs::symlink_metadata(path) {
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Ok(m) if m.file_type().is_symlink() => std::fs::canonicalize(path),
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_ => Ok(path.to_path_buf()),
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}
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}
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/// A fresh hidden name beside `target` (the same folder, so the rename stays on one volume).
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fn temp_path(target: &Path) -> io::Result<PathBuf> {
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static NEXT: AtomicU64 = AtomicU64::new(0);
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let name = target.file_name().ok_or_else(|| io::Error::new(io::ErrorKind::InvalidInput, format!("{}: not a file path", target.display())))?;
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let n = NEXT.fetch_add(1, Ordering::Relaxed);
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Ok(target.with_file_name(format!(".{}.{}-{n}.tmp", name.to_string_lossy(), std::process::id())))
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}
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/// The replacement keeps the replaced file's permissions (Unix; Windows files have none to keep
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/// beyond read-only, which makes the save fail as writing in place would).
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#[cfg(unix)]
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fn keep_permissions(target: &Path, tmp: &Path) -> io::Result<()> {
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match std::fs::metadata(target) {
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Ok(m) => std::fs::set_permissions(tmp, m.permissions()),
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// A new file: the default permissions.
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Err(_) => Ok(()),
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}
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}
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#[cfg(not(unix))]
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fn keep_permissions(_: &Path, _: &Path) -> io::Result<()> {
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Ok(())
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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fn dir(name: &str) -> PathBuf {
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let d = std::env::temp_dir().join(format!("vc-atomic-{name}-{}", std::process::id()));
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let _ = std::fs::remove_dir_all(&d);
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std::fs::create_dir_all(&d).unwrap();
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d
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}
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/// The other files in `d` besides `keep`.
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fn leftovers(d: &Path, keep: &str) -> Vec<String> {
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std::fs::read_dir(d).unwrap().map(|e| e.unwrap().file_name().to_string_lossy().to_string()).filter(|n| n != keep).collect()
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}
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#[test]
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fn the_original_file_survives_a_failed_write() {
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let d = dir("fail");
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let path = d.join("doc.vectorcraft");
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std::fs::write(&path, b"original").unwrap();
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let r = write_atomic_with(&path, |f| {
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f.write_all(b"half a fi")?;
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Err(io::Error::other("disk full"))
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});
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assert!(r.is_err());
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assert_eq!(std::fs::read(&path).unwrap(), b"original");
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assert!(leftovers(&d, "doc.vectorcraft").is_empty(), "the temporary file is removed");
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write_atomic(&path, b"new").unwrap();
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assert_eq!(std::fs::read(&path).unwrap(), b"new");
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assert!(leftovers(&d, "doc.vectorcraft").is_empty());
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// A folder that doesn't exist: an error, nothing written.
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assert!(write_atomic(&d.join("missing").join("x.vectorcraft"), b"x").is_err());
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let _ = std::fs::remove_dir_all(&d);
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}
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#[test]
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fn write_new_never_replaces_what_is_there() {
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let d = dir("new");
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let path = d.join("a.otf");
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write_new_with(&path, |f| f.write_all(b"first")).unwrap();
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assert_eq!(std::fs::read(&path).unwrap(), b"first");
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assert!(leftovers(&d, "a.otf").is_empty(), "the temporary file is removed");
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let r = write_new_with(&path, |f| f.write_all(b"second"));
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assert_eq!(r.map_err(|e| e.kind()), Err(io::ErrorKind::AlreadyExists));
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assert_eq!(std::fs::read(&path).unwrap(), b"first");
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assert!(leftovers(&d, "a.otf").is_empty());
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// A failed write leaves nothing.
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let failed = d.join("b.otf");
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assert!(write_new_with(&failed, |f| f.write_all(b"half").and(Err(io::Error::other("disk full")))).is_err());
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assert!(!failed.exists() && leftovers(&d, "a.otf").is_empty());
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let _ = std::fs::remove_dir_all(&d);
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}
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/// Where hard links fail (a file system without them), the content is copied in, and a taken
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/// name stays as it was. A link that fails because the name is taken copies nothing.
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#[test]
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fn write_new_copies_where_hard_links_fail() {
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let d = dir("new-copy");
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let unsupported = |_: &Path, _: &Path| Err(io::Error::from(io::ErrorKind::Unsupported));
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let path = d.join("c.otf");
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write_new_linking(&path, |f| f.write_all(b"copied"), unsupported).unwrap();
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assert_eq!(std::fs::read(&path).unwrap(), b"copied");
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assert!(leftovers(&d, "c.otf").is_empty(), "the temporary file is removed");
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let r = write_new_linking(&path, |f| f.write_all(b"second"), unsupported);
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assert_eq!(r.map_err(|e| e.kind()), Err(io::ErrorKind::AlreadyExists));
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assert_eq!(std::fs::read(&path).unwrap(), b"copied");
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let taken = |_: &Path, _: &Path| Err(io::Error::from(io::ErrorKind::AlreadyExists));
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let other = d.join("d.otf");
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let r = write_new_linking(&other, |f| f.write_all(b"other"), taken);
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assert_eq!(r.map_err(|e| e.kind()), Err(io::ErrorKind::AlreadyExists));
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assert!(!other.exists() && leftovers(&d, "c.otf").is_empty());
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let _ = std::fs::remove_dir_all(&d);
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}
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/// A link of that name, even one to nothing, is never replaced or written through.
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#[cfg(unix)]
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#[test]
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fn write_new_leaves_links_alone() {
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let d = dir("new-links");
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let target = d.join("target.otf");
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std::fs::write(&target, b"target").unwrap();
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std::os::unix::fs::symlink(&target, d.join("link.otf")).unwrap();
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std::os::unix::fs::symlink(d.join("nothing.otf"), d.join("dangling.otf")).unwrap();
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for name in ["link.otf", "dangling.otf"] {
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let r = write_new_with(&d.join(name), |f| f.write_all(b"new"));
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assert_eq!(r.map_err(|e| e.kind()), Err(io::ErrorKind::AlreadyExists), "{name}");
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}
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assert_eq!(std::fs::read(&target).unwrap(), b"target");
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assert!(!d.join("nothing.otf").exists());
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let mut names: Vec<String> = std::fs::read_dir(&d).unwrap().map(|e| e.unwrap().file_name().to_string_lossy().to_string()).collect();
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names.sort();
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assert_eq!(names, ["dangling.otf", "link.otf", "target.otf"], "no temporary file is left");
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let _ = std::fs::remove_dir_all(&d);
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}
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}
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