Add atomic file saving

Adapted from the atomicwrites crate, but with directory fsync disabled
for performance.

Cargo.lock

@@ -383,11 +383,13 @@ dependencies = [
  "json",
  "mime",
  "mime-db",
+ "nix",
  "once_cell",
  "pest",
  "pest_derive",
  "serde",
  "shlex",
+ "tempfile",
  "thiserror",
  "url",
  "xdg",
@@ -567,9 +569,9 @@ dependencies = [
 
 [[package]]
 name = "libc"
-version = "0.2.94"
+version = "0.2.96"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "18794a8ad5b29321f790b55d93dfba91e125cb1a9edbd4f8e3150acc771c1a5e"
+checksum = "5600b4e6efc5421841a2138a6b082e07fe12f9aaa12783d50e5d13325b26b4fc"
 
 [[package]]
 name = "log"
@@ -598,6 +600,15 @@ version = "2.3.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0ee1c47aaa256ecabcaea351eae4a9b01ef39ed810004e298d2511ed284b1525"
 
+[[package]]
+name = "memoffset"
+version = "0.6.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "59accc507f1338036a0477ef61afdae33cde60840f4dfe481319ce3ad116ddf9"
+dependencies = [
+ "autocfg",
+]
+
 [[package]]
 name = "mime"
 version = "0.3.16"
@@ -655,6 +666,19 @@ dependencies = [
  "tempfile",
 ]
 
+[[package]]
+name = "nix"
+version = "0.21.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5c3728fec49d363a50a8828a190b379a446cc5cf085c06259bbbeb34447e4ec7"
+dependencies = [
+ "bitflags",
+ "cc",
+ "cfg-if 1.0.0",
+ "libc",
+ "memoffset",
+]
+
 [[package]]
 name = "nom"
 version = "5.1.2"

Cargo.toml

@@ -27,6 +27,8 @@ xdg-mime = "0.3.3"
 freedesktop_entry_parser = "1.1.1"
 once_cell = "1.7.2"
 aho-corasick = "0.7.15"
+tempfile = "3.2.0"
+nix = "0.21.0"
 
 [profile.release]
 opt-level=3

src/apps/user.rs

@@ -1,3 +1,6 @@
+use crate::common::atomic_save::{
+    AtomicFile, AtomicSaveError, Durability, OverwriteBehavior,
+};
 use crate::{apps::SystemApps, common::Handler, Error, Result, CONFIG};
 use mime::Mime;
 use once_cell::sync::Lazy;
@@ -179,31 +182,37 @@ impl MimeApps {
         use itertools::Itertools;
         use std::io::{prelude::*, BufWriter};
 
-        let f = std::fs::OpenOptions::new()
-            .read(true)
-            .create(true)
-            .write(true)
-            .truncate(true)
-            .open(Self::path()?)?;
-        let mut writer = BufWriter::new(f);
+        let af = AtomicFile::new(
+            &Self::path()?,
+            OverwriteBehavior::AllowOverwrite,
+            Durability::DontSyncDir,
+        );
+        af.write(|f| -> Result<()> {
+            let mut writer = BufWriter::new(f);
 
-        writer.write_all(b"[Added Associations]\n")?;
-        for (k, v) in self.added_associations.iter().sorted() {
-            writer.write_all(k.essence_str().as_ref())?;
-            writer.write_all(b"=")?;
-            writer.write_all(v.iter().join(";").as_ref())?;
-            writer.write_all(b";\n")?;
-        }
+            writer.write_all(b"[Added Associations]\n")?;
+            for (k, v) in self.added_associations.iter().sorted() {
+                writer.write_all(k.essence_str().as_ref())?;
+                writer.write_all(b"=")?;
+                writer.write_all(v.iter().join(";").as_ref())?;
+                writer.write_all(b";\n")?;
+            }
 
-        writer.write_all(b"\n[Default Applications]\n")?;
-        for (k, v) in self.default_apps.iter().sorted() {
-            writer.write_all(k.essence_str().as_ref())?;
-            writer.write_all(b"=")?;
-            writer.write_all(v.iter().join(";").as_ref())?;
-            writer.write_all(b";\n")?;
-        }
+            writer.write_all(b"\n[Default Applications]\n")?;
+            for (k, v) in self.default_apps.iter().sorted() {
+                writer.write_all(k.essence_str().as_ref())?;
+                writer.write_all(b"=")?;
+                writer.write_all(v.iter().join(";").as_ref())?;
+                writer.write_all(b";\n")?;
+            }
 
-        writer.flush()?;
+            writer.flush()?;
+            Ok(())
+        })
+        .map_err(|e| match e {
+            AtomicSaveError::Internal(e) => Error::Io(e),
+            AtomicSaveError::User(e) => e,
+        })?;
         Ok(())
     }
     pub fn print(&self, detailed: bool) -> Result<()> {

src/common/atomic_save.rs

@@ -0,0 +1,249 @@
+//! Adapted from https://github.com/untitaker/rust-atomicwrites/blob/master/src/lib.rs.
+
+use std::error::Error as ErrorTrait;
+use std::fmt;
+use std::fs;
+use std::io;
+use std::path;
+
+pub use OverwriteBehavior::{AllowOverwrite, DisallowOverwrite};
+
+/// Whether to allow overwriting if the target file exists.
+#[derive(Clone, Copy)]
+pub enum OverwriteBehavior {
+    /// Overwrite files silently.
+    AllowOverwrite,
+
+    /// Don't overwrite files. `AtomicFile.write` will raise errors for such conditions only after
+    /// you've already written your data.
+    DisallowOverwrite,
+}
+
+/// Whether to ensure durability after a system crash (guaranteed to contain the new data).
+/// Regardless of the option you pick, the atomic write will be consistent after a crash
+/// (will never contain partially-written data).
+#[derive(Clone, Copy)]
+pub enum Durability {
+    /// Faster, ensures either old or new file contents (but not half-written data)
+    /// will be present after system crash.
+    DontSyncDir,
+    /// Slower, ensures new file contents will be present after system crash.
+    /// Not possible on Windows.
+    SyncDir,
+}
+
+/// Represents an error raised by `AtomicFile.write`.
+#[derive(Debug)]
+pub enum AtomicSaveError<E> {
+    /// The error originated in the library itself, while it was either creating a temporary file
+    /// or moving the file into place.
+    Internal(io::Error),
+    /// The error originated in the user-supplied callback.
+    User(E),
+}
+
+/// If your callback returns a `std::io::Error`, you can unwrap this type to `std::io::Error`.
+impl From<AtomicSaveError<io::Error>> for io::Error {
+    fn from(e: AtomicSaveError<io::Error>) -> Self {
+        match e {
+            AtomicSaveError::Internal(x) => x,
+            AtomicSaveError::User(x) => x,
+        }
+    }
+}
+
+impl<E: fmt::Display> fmt::Display for AtomicSaveError<E> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            AtomicSaveError::Internal(ref e) => e.fmt(f),
+            AtomicSaveError::User(ref e) => e.fmt(f),
+        }
+    }
+}
+
+impl<E: ErrorTrait> ErrorTrait for AtomicSaveError<E> {
+    fn cause(&self) -> Option<&dyn ErrorTrait> {
+        match *self {
+            AtomicSaveError::Internal(ref e) => Some(e),
+            AtomicSaveError::User(ref e) => Some(e),
+        }
+    }
+}
+
+fn safe_parent(p: &path::Path) -> Option<&path::Path> {
+    match p.parent() {
+        None => None,
+        Some(x) if x.as_os_str().len() == 0 => Some(&path::Path::new(".")),
+        x => x,
+    }
+}
+
+/// Create a file and write to it atomically, in a callback.
+pub struct AtomicFile {
+    /// Path to the final file that is atomically written.
+    path: path::PathBuf,
+    overwrite: OverwriteBehavior,
+    durability: Durability,
+    /// Directory to which to write the temporary subdirectories.
+    tmpdir: path::PathBuf,
+}
+
+impl AtomicFile {
+    /// Helper for writing to the file at `path` atomically, in write-only mode.
+    ///
+    /// If `OverwriteBehaviour::DisallowOverwrite` is given,
+    /// an `Error::Internal` containing an `std::io::ErrorKind::AlreadyExists`
+    /// will be returned from `self.write(...)` if the file exists.
+    ///
+    /// The temporary file is written to a temporary subdirectory in `.`, to ensure
+    /// it’s on the same filesystem (so that the move is atomic).
+    pub fn new(
+        p: &path::Path,
+        overwrite: OverwriteBehavior,
+        durability: Durability,
+    ) -> Self {
+        AtomicFile::new_with_tmpdir(
+            p,
+            overwrite,
+            durability,
+            safe_parent(p).unwrap_or(path::Path::new(".")),
+        )
+    }
+
+    /// Like `AtomicFile::new`, but the temporary file is written to a temporary subdirectory in `tmpdir`.
+    ///
+    /// TODO: does `tmpdir` have to exist?
+    pub fn new_with_tmpdir(
+        path: &path::Path,
+        overwrite: OverwriteBehavior,
+        durability: Durability,
+        tmpdir: &path::Path,
+    ) -> Self {
+        AtomicFile {
+            path: path.to_path_buf(),
+            overwrite,
+            durability,
+            tmpdir: tmpdir.to_path_buf(),
+        }
+    }
+
+    /// Move the file to `self.path()`. Not exposed!
+    fn commit(self, tmppath: &path::Path) -> io::Result<()> {
+        match self.overwrite {
+            AllowOverwrite => {
+                replace_atomic(tmppath, self.path(), self.durability)
+            }
+            DisallowOverwrite => {
+                move_atomic(tmppath, self.path(), self.durability)
+            }
+        }
+    }
+
+    /// Get the target filepath.
+    pub fn path(&self) -> &path::Path {
+        &self.path
+    }
+
+    /// Open a temporary file, call `f` on it (which is supposed to write to it), then move the
+    /// file atomically to `self.path`.
+    ///
+    /// The temporary file is written to a randomized temporary subdirectory with prefix `.atomicwrite`.
+    pub fn write<T, E, F>(self, f: F) -> Result<T, AtomicSaveError<E>>
+    where
+        F: FnOnce(&mut fs::File) -> Result<T, E>,
+    {
+        let tmpdir = tempfile::Builder::new()
+            .prefix(".atomicwrite")
+            .tempdir_in(&self.tmpdir)
+            .map_err(AtomicSaveError::Internal)?;
+
+        let tmppath = tmpdir.path().join("tmpfile.tmp");
+        let rv = {
+            let mut tmpfile = fs::File::create(&tmppath)
+                .map_err(AtomicSaveError::Internal)?;
+            let r = f(&mut tmpfile).map_err(AtomicSaveError::User)?;
+            tmpfile.sync_all().map_err(AtomicSaveError::Internal)?;
+            r
+        };
+        self.commit(&tmppath).map_err(AtomicSaveError::Internal)?;
+        Ok(rv)
+    }
+}
+
+mod imp {
+    use super::{safe_parent, Durability};
+
+    use std::os::unix::io::AsRawFd;
+    use std::{fs, io, path};
+
+    fn fsync<T: AsRawFd>(f: T) -> io::Result<()> {
+        match nix::unistd::fsync(f.as_raw_fd()) {
+            Ok(()) => Ok(()),
+            Err(nix::Error::Sys(errno)) => Err(errno.into()),
+            Err(nix::Error::InvalidPath) => {
+                Err(io::Error::new(io::ErrorKind::Other, "invalid path"))
+            }
+            Err(nix::Error::InvalidUtf8) => {
+                Err(io::Error::new(io::ErrorKind::Other, "invalid utf-8"))
+            }
+            Err(nix::Error::UnsupportedOperation) => Err(io::Error::new(
+                io::ErrorKind::Other,
+                "unsupported operation",
+            )),
+        }
+    }
+
+    fn fsync_dir(x: &path::Path) -> io::Result<()> {
+        let f = fs::File::open(x)?;
+        fsync(f)
+    }
+
+    /// Move `src` to `dst`. If `dst` exists, it will be silently overwritten.
+    ///
+    /// Both paths must reside on the same filesystem for the operation to be atomic.
+    pub fn replace_atomic(
+        src: &path::Path,
+        dst: &path::Path,
+        durability: Durability,
+    ) -> io::Result<()> {
+        fs::rename(src, dst)?;
+
+        match durability {
+            Durability::SyncDir => {
+                let dst_directory = safe_parent(dst).unwrap();
+                fsync_dir(dst_directory)?;
+            }
+            Durability::DontSyncDir => {}
+        }
+
+        Ok(())
+    }
+
+    /// Move `src` to `dst`. An error will be returned if `dst` exists.
+    ///
+    /// Both paths must reside on the same filesystem for the operation to be atomic.
+    pub fn move_atomic(
+        src: &path::Path,
+        dst: &path::Path,
+        durability: Durability,
+    ) -> io::Result<()> {
+        fs::hard_link(src, dst)?;
+        fs::remove_file(src)?;
+
+        match durability {
+            Durability::SyncDir => {
+                let src_directory = safe_parent(src).unwrap();
+                let dst_directory = safe_parent(dst).unwrap();
+                fsync_dir(dst_directory)?;
+                if src_directory != dst_directory {
+                    fsync_dir(src_directory)?;
+                }
+            }
+            Durability::DontSyncDir => {}
+        }
+
+        Ok(())
+    }
+}
+
+use imp::{move_atomic, replace_atomic};

src/common/mod.rs

@@ -1,3 +1,4 @@
+pub mod atomic_save;
 mod db;
 mod desktop_entry;
 mod handler;