fix(backup): work with temporary file while writing json metadata file

CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased](https://git.rustybever.be/Chewing_Bever/alex/src/branch/dev)
 
+## Fixed
+
+* Fix bug where JSON metadata file can be corrupted if crash occurs while
+  writing (data is now written to a temporary file before atomically renaming)
+
 ## [0.4.1](https://git.rustybever.be/Chewing_Bever/alex/src/tag/0.4.1)
 
 ### Changed

backup/src/manager/mod.rs

@@ -122,10 +122,24 @@ where
     }
 
     /// Write the in-memory state to disk.
+    ///
+    /// The state is first written to a temporary file before being (atomically, depending on the
+    /// file system) renamed to the final path.
     pub fn save(&self) -> io::Result<()> {
-        let json_file = File::create(self.backup_dir.join(Self::METADATA_FILE))?;
+        let dest_path = self.backup_dir.join(Self::METADATA_FILE);
+
+        let dest_ext = dest_path
+            .extension()
+            .map(|ext| ext.to_string_lossy().to_string())
+            .unwrap_or(String::new());
+        let temp_path = dest_path.with_extension(format!("{dest_ext}.temp"));
+
+        let json_file = File::create(&temp_path)?;
         serde_json::to_writer(json_file, &self.chains)?;
 
+        // Rename temp file to the destination path after writing was successful
+        std::fs::rename(temp_path, dest_path)?;
+
         Ok(())
     }