Plain POSIX files: remove commit on close (fs.rs)

Staging every write until close broke tempfile, git, and SQLite, which
expect normal files. Actors that need an atomic publish write a temporary
file and rename it; the SDK has write_atomic for that.

sdk/src/lib.rs

@@ -77,6 +77,15 @@ pub fn reply(message: &Message, body: Value) -> String {
     format!("{}/{id}", me())
 }
 
+/// Write a file so that readers see either the old or the new content,
+/// never half of it: write a temporary file next to `path`, then rename it.
+pub fn write_atomic(path: impl AsRef<std::path::Path>, contents: impl AsRef<[u8]>) -> std::io::Result<()> {
+    let path = path.as_ref();
+    let tmp = path.with_file_name(format!(".{}.tmp-{}", path.file_name().unwrap_or_default().to_string_lossy(), next_id()));
+    std::fs::write(&tmp, contents)?;
+    std::fs::rename(&tmp, path)
+}
+
 /// Publish a message to this actor's subscribers and return its full ID.
 pub fn publish(body: Value) -> String {
     let id = next_id();
@@ -201,6 +210,18 @@ mod tests {
         assert_eq!(message.kind(), "");
     }
 
+    #[test]
+    fn write_atomic_replaces_the_file() {
+        let dir = std::env::temp_dir().join(format!("sausage-actor-{}", next_id()));
+        std::fs::create_dir_all(&dir).unwrap();
+        let path = dir.join("state.json");
+        write_atomic(&path, "old").unwrap();
+        write_atomic(&path, "new").unwrap();
+        assert_eq!(std::fs::read_to_string(&path).unwrap(), "new");
+        assert_eq!(std::fs::read_dir(&dir).unwrap().count(), 1);
+        std::fs::remove_dir_all(dir).unwrap();
+    }
+
     #[test]
     fn generated_ids_differ() {
         assert_ne!(next_id(), next_id());