Replace crate `tempfile` with custom code - Fixes #169 (#181)

- Write our own `atomically_write_file` function instead of using
  `tempfile`.

- Save data to `tedge.toml.tmp` and then atomically renaming it to the
  final destination `tedge.toml`.

- By storing the temporary file in the same directory as the destination
  file we get around issues with cross-device link errors (EXDEV), which
  occur when the tempfile and the destination file would be on different
  file systems (`/tmp` on FreeBSD is a separate file system from `/home` or
  `/`).

- Only use POSIX functionality explosed by Rust's `std`, so it should
  really be cross-platform.

- Tested on FreeBSD 12.2

Fixes bug: #169

3 files changed, 37 insertions, 13 deletions

diff --git a/tedge_config/Cargo.toml b/tedge_config/Cargo.toml
index 311c1f09..601d0d27 100644
--- a/tedge_config/Cargo.toml
+++ b/tedge_config/Cargo.toml
@@ -8,7 +8,7 @@ edition = "2018"
 toml = "0.5"
 thiserror = "1.0"
 serde = { version = "1.0", features = ["derive"] }
-tempfile = "3.2"
 
 [dev-dependencies]
 assert_matches = "1.4"
+tempfile = "3.2"
diff --git a/tedge_config/src/tedge_config_location.rs b/tedge_config/src/tedge_config_location.rs
index 1ca2ff28..a6f9a844 100644
--- a/tedge_config/src/tedge_config_location.rs
+++ b/tedge_config/src/tedge_config_location.rs
@@ -2,6 +2,7 @@ use std::path::{Path, PathBuf};
 
 const DEFAULT_ETC_PATH: &str = "/etc";
 const TEDGE_CONFIG_FILE: &str = "tedge.toml";
+const TEDGE_CONFIG_FILE_TMP: &str = "tedge.toml.tmp";
 
 /// Information about where `tedge.toml` is located.
 ///
@@ -48,6 +49,10 @@ impl TEdgeConfigLocation {
     pub fn tedge_config_file_path(&self) -> &Path {
         &self.tedge_config_file_path
     }
+
+    pub fn temporary_tedge_config_file_path(&self) -> impl AsRef<Path> {
+        self.tedge_config_root_path.join(TEDGE_CONFIG_FILE_TMP)
+    }
 }
 
 #[test]
diff --git a/tedge_config/src/tedge_config_repository.rs b/tedge_config/src/tedge_config_repository.rs
index c7fdd2b2..59df9a7a 100644
--- a/tedge_config/src/tedge_config_repository.rs
+++ b/tedge_config/src/tedge_config_repository.rs
@@ -1,7 +1,7 @@
 use crate::*;
+use std::fs;
 use std::io::Write;
-use std::path::PathBuf;
-use tempfile::NamedTempFile;
+use std::path::{Path, PathBuf};
 
 /// TEdgeConfigRepository is resposible for loading and storing TEdgeConfig entities.
 ///
@@ -25,23 +25,42 @@ impl ConfigRepository<TEdgeConfig> for TEdgeConfigRepository {
         Ok(config)
     }
 
-    // XXX: Explicitly set the file permissions in this function and file ownership!
+    // TODO: Explicitly set the file permissions in this function and file ownership!
     fn store(&self, config: TEdgeConfig) -> Result<(), TEdgeConfigError> {
         let toml = toml::to_string_pretty(&config.data)?;
-        let mut file = NamedTempFile::new()?;
-        file.write_all(toml.as_bytes())?;
 
-        // Create $HOME/.tedge or /etc/tedge directory if it does not exist
+        // Create `$HOME/.tedge` or `/etc/tedge` directory in case it does not exist yet
         if !self.config_location.tedge_config_root_path.exists() {
-            let () = std::fs::create_dir(self.config_location.tedge_config_root_path())?;
-            // XXX: Correctly assign permissions
+            let () = fs::create_dir(self.config_location.tedge_config_root_path())?;
         }
 
-        match file.persist(self.config_location.tedge_config_file_path()) {
-            Ok(_) => Ok(()),
-            Err(err) => Err(err.error.into()),
-        }
+        let () = atomically_write_file(
+            self.config_location.temporary_tedge_config_file_path(),
+            self.config_location.tedge_config_file_path(),
+            toml.as_bytes(),
+        )?;
+        Ok(())
+    }
+}
+
+fn atomically_write_file(
+    tempfile: impl AsRef<Path>,
+    dest: impl AsRef<Path>,
+    content: &[u8],
+) -> std::io::Result<()> {
+    let mut file = fs::OpenOptions::new()
+        .write(true)
+        .create_new(true)
+        .open(tempfile.as_ref())?;
+    if let Err(err) = file.write_all(content) {
+        let _ = fs::remove_file(tempfile);
+        return Err(err);
+    }
+    if let Err(err) = fs::rename(tempfile.as_ref(), dest) {
+        let _ = fs::remove_file(tempfile);
+        return Err(err);
     }
+    Ok(())
 }
 
 impl TEdgeConfigRepository {