Function safe_write_json_file

src/agents/mod.rs:591–657

Write a JSON value to a file via atomic rename. The caller is responsible for creating the backup via [`backup_config_file`] before loading the config. Pass the backup path here so that it can be mentioned in error messages and used for restore if the rename somehow leaves the target in a bad state. # Strategy 1. Serialize → validate → write to a **new** sibling file (`.new`). The original file

(
    path: &Path,
    value: &serde_json::Value,
    backup: Option<&Path>,
)

///
/// In every error case the original file remains intact.
pub fn safe_write_json_file(
    path: &Path,
    value: &serde_json::Value,
    backup: Option<&Path>,
) -> Result<()> {
    // 1. Serialize
    let pretty = serde_json::to_string_pretty(value).map_err(|e| TraceDecayError::Config {
        message: format!("failed to serialize JSON for {}: {e}", path.display()),
    })?;

    // 2. Re-parse to verify the serialized output is valid JSON
    if serde_json::from_str::<serde_json::Value>(&pretty).is_err() {
        return Err(TraceDecayError::Config {
            message: format!(
                "internal error: serialized JSON for {} failed re-parse validation.\n  \
                 This is a bug in tracedecay — please report it.",
                path.display()
            ),
        });
    }

    // 3. Ensure parent dir
    if let Some(parent) = path.parent() {
        std::fs::create_dir_all(parent).map_err(|e| TraceDecayError::Config {
            message: format!("cannot create directory {}: {e}", parent.display()),
        })?;
    }

    // 4. Write to a NEW sibling file — the original is never opened for
    //    writing, so an interrupted write or crash only affects the .new file.
    let content = format!("{pretty}\n");
    let new_path = PathBuf::from(format!("{}.new", path.display()));
    if let Err(e) = std::fs::write(&new_path, &content) {
        std::fs::remove_file(&new_path).ok(); // clean up partial write
        return Err(TraceDecayError::Config {
            message: format!(
                "failed to write new config file {}: {e}",
                new_path.display()
            ),
        });
    }

    // 5. Atomic rename: new → original.
    //    On POSIX, rename(2) atomically replaces the target.
    //    If this fails the original file is still intact.
    if let Err(e) = std::fs::rename(&new_path, path) {
        std::fs::remove_file(&new_path).ok(); // clean up
        let hint = if let Some(b) = backup {
            format!(
                "\n  Backup is at: {}\n  \
                 The original file was NOT modified.",
                b.display()
            )
        } else {
            "\n  The original file was NOT modified.".to_string()
        };
        return Err(TraceDecayError::Config {
            message: format!(