Function read_changed_file

src/sessions/source.rs:381–413

`ContentHash`** reader for full-file-rewrite JSON. Detects a change via `(content_hash64, mtime)` versus the stored cursor and, on change, returns the whole file so the caller can re-derive every message with deterministic ids. Idempotent upserts make re-adding unchanged messages a no-op. Returns `None` when the file cannot be read or is unchanged since the last run.

(path: &Path, prev: StoredCursor)

/// a no-op. Returns `None` when the file cannot be read or is unchanged since
/// the last run.
pub fn read_changed_file(path: &Path, prev: StoredCursor) -> Option<ChangedFile> {
    let meta = match std::fs::metadata(path) {
        Ok(meta) => meta,
        Err(error) => {
            log_source_skip(path, "stat transcript file", &error);
            return None;
        }
    };
    let mtime = file_mtime_secs(&meta);
    let contents = match std::fs::read_to_string(path) {
        Ok(contents) => contents,
        Err(error) => {
            log_source_skip(path, "read transcript file", &error);
            return None;
        }
    };
    let hash = content_hash64(&contents);

    // Unchanged since last run (we have read it before and neither content hash
    // nor mtime moved) -> nothing to do.
    if prev.position == hash && prev.mtime == mtime && (prev.position != 0 || prev.mtime != 0) {
        return None;
    }

    Some(ChangedFile {
        contents,
        new_cursor: StoredCursor {
            position: hash,
            mtime,
            file_id: 0,
        },
    })
}

/// Like [`read_changed_file`], but treats `primary` as changed when either its
/// own content hash moves or a companion sidecar file's hash moves. The stored