1 files changed, 192 insertions, 0 deletions
diff --git a/src/fs.rs b/src/fs.rs
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+++ b/src/fs.rs
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+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    fs::{
+        remove_file,
+        rename,
+        File,
+    },
+    io::{
+        self,
+        Read,
+        Seek,
+        Write,
+    },
+    path::{
+        Path,
+        PathBuf,
+    },
+};
+
+/// A [`File`] which is protected by a git-style lock file
+///
+/// When a [`LockedFile`] is created, a lock file named after its path with
+/// suffix ".lock" is created with `O_EXCL`. That is, if the lock file already
+/// exists, the operation will fail.
+///
+/// Then, either the lock file (when using [`LockedFile::atomic`]) or the base
+/// file (when using [`LockedFile::in_place`] is opened for writing.
+/// [`LockedFile`] implements [`Write`], [`Read`], and [`Seek`].
+///
+/// When a [`LockedFile`] is dropped, the lock file is unlinked. **NOTE** that
+/// this may leave the lock file in place if the process exits forcefully.
+///
+/// When using [`LockedFile::atomic`], the modified lock file is renamed to the
+/// base file atomically. For this to happen, [`LockedFile::persist`] must be
+/// called explicitly.
+pub struct LockedFile {
+    /// Path to the lock file
+    lock: PathBuf,
+    /// Path to the file being edited
+    path: PathBuf,
+    /// File being edited
+    edit: File,
+    /// Commit mode
+    mode: Commit,
+}
+
+enum Commit {
+    Atomic,
+    InPlace,
+}
+
+impl Drop for LockedFile {
+    fn drop(&mut self) {
+        remove_file(&self.lock).ok();
+    }
+}
+
+impl LockedFile {
+    pub const DEFAULT_PERMISSIONS: u32 = 0o644;
+
+    pub fn atomic<P, M>(path: P, truncate: bool, mode: M) -> io::Result<Self>
+    where
+        P: Into<PathBuf>,
+        M: Into<Option<u32>>,
+    {
+        let path = path.into();
+        let perm = mode.into().unwrap_or(Self::DEFAULT_PERMISSIONS);
+        let lock = path.with_extension("lock");
+        let mut edit = File::options()
+            .read(true)
+            .write(true)
+            .create_new(true)
+            .permissions(perm)
+            .open(&lock)?;
+        if !truncate && path.exists() {
+            std::fs::copy(&path, &lock)?;
+            edit = File::options().read(true).append(true).open(&lock)?;
+        }
+        let mode = Commit::Atomic;
+
+        Ok(Self {
+            lock,
+            path,
+            edit,
+            mode,
+        })
+    }
+
+    pub fn in_place<P, M>(path: P, truncate: bool, mode: M) -> io::Result<Self>
+    where
+        P: Into<PathBuf>,
+        M: Into<Option<u32>>,
+    {
+        let path = path.into();
+        let perm = mode.into().unwrap_or(Self::DEFAULT_PERMISSIONS);
+        let lock = path.with_extension("lock");
+        let _ = File::options()
+            .read(true)
+            .write(true)
+            .create_new(true)
+            .permissions(perm)
+            .open(&lock)?;
+        let edit = File::options()
+            .read(true)
+            .write(true)
+            .truncate(truncate)
+            .create(true)
+            .permissions(perm)
+            .open(&path)?;
+        let mode = Commit::InPlace;
+
+        Ok(Self {
+            lock,
+            path,
+            edit,
+            mode,
+        })
+    }
+
+    /// Reopen the file handle
+    ///
+    /// This is sometimes necessary, eg. when launching an editor to let the
+    /// user modify the file, in which case the file descriptor of the
+    /// handle is invalidated.
+    pub fn reopen(&mut self) -> io::Result<()> {
+        self.edit = File::options()
+            .read(true)
+            .write(true)
+            .open(self.edit_path())?;
+        Ok(())
+    }
+
+    pub fn edit_path(&self) -> &Path {
+        match self.mode {
+            Commit::Atomic => &self.lock,
+            Commit::InPlace => &self.path,
+        }
+    }
+
+    #[allow(unused)]
+    pub fn target_path(&self) -> &Path {
+        &self.path
+    }
+
+    pub fn persist(self) -> io::Result<()> {
+        match self.mode {
+            Commit::Atomic => rename(&self.lock, &self.path),
+            Commit::InPlace => remove_file(&self.lock),
+        }
+    }
+}
+
+impl Read for LockedFile {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        self.edit.read(buf)
+    }
+}
+
+impl Write for LockedFile {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.edit.write(buf)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.edit.flush()
+    }
+}
+
+impl Seek for LockedFile {
+    fn seek(&mut self, pos: io::SeekFrom) -> io::Result<u64> {
+        self.edit.seek(pos)
+    }
+}
+
+pub(crate) trait PermissionsExt {
+    fn permissions(&mut self, mode: u32) -> &mut Self;
+}
+
+impl PermissionsExt for std::fs::OpenOptions {
+    #[cfg(unix)]
+    fn permissions(&mut self, mode: u32) -> &mut Self {
+        use std::os::unix::fs::OpenOptionsExt as _;
+        self.mode(mode)
+    }
+
+    #[cfg(not(unix))]
+    fn permissions(&mut self, mode: u32) -> &mut Self {
+        self
+    }
+}