Add abstraction for atomic file I/O

1 files changed, 72 insertions, 0 deletions

diff --git a/mullvad-api/src/fs.rs b/mullvad-api/src/fs.rs
new file mode 100644
index 0000000000..d643563c9a
--- /dev/null
+++ b/mullvad-api/src/fs.rs
@@ -0,0 +1,72 @@
+use std::{
+    ops::{Deref, DerefMut},
+    path::{Path, PathBuf},
+};
+use talpid_types::ErrorExt;
+use tokio::{fs, io};
+
+/// Stores content in a temporary file before moving it to the
+/// final destination, ensuring that consumers of the file never
+/// end up with partial content. Must be moved with `finalize`.
+pub struct AtomicFile {
+    file: Option<fs::File>,
+    temp_path: PathBuf,
+    target_path: PathBuf,
+}
+
+impl AtomicFile {
+    pub async fn new(target_path: PathBuf) -> io::Result<Self> {
+        let temp_path = target_path.with_file_name(uuid::Uuid::new_v4().to_string());
+        Ok(Self {
+            file: Some(fs::File::create(&temp_path).await?),
+            temp_path,
+            target_path,
+        })
+    }
+
+    /// Flushes and moves the file to `self.target_path`, replacing it if it exists.
+    pub async fn finalize(mut self) -> io::Result<()> {
+        let result = async {
+            let file = self.file.take().unwrap();
+            file.sync_all().await?;
+            let std_file = file.into_std().await;
+            let _ = tokio::task::spawn_blocking(move || drop(std_file)).await;
+            fs::rename(&self.temp_path, &self.target_path).await
+        }
+        .await;
+        if result.is_err() {
+            let _ = tokio::task::spawn_blocking(move || try_remove_file(&self.temp_path)).await;
+        }
+        result
+    }
+}
+
+impl Drop for AtomicFile {
+    fn drop(&mut self) {
+        if self.file.is_some() {
+            log::error!("{} was not finalized", self.target_path.display());
+            try_remove_file(&self.temp_path);
+        }
+    }
+}
+
+fn try_remove_file(temp_path: &Path) {
+    if let Err(error) = std::fs::remove_file(temp_path) {
+        let msg = format!("Failed to delete temp file: {}", temp_path.display());
+        log::warn!("{}", error.display_chain_with_msg(&msg));
+    }
+}
+
+impl Deref for AtomicFile {
+    type Target = fs::File;
+
+    fn deref(&self) -> &Self::Target {
+        self.file.as_ref().unwrap()
+    }
+}
+
+impl DerefMut for AtomicFile {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.file.as_mut().unwrap()
+    }
+}