Refactor datafile path handling

disk/src/index.rs

@@ -0,0 +1,26 @@
+use err::Error;
+use netpod::log::*;
+use netpod::{ChannelConfig, Nanos, Node};
+use tokio::fs::OpenOptions;
+use tokio::io::ErrorKind;
+
+pub async fn find_start_pos_for_config(
+    ts: Nanos,
+    channel_config: &ChannelConfig,
+    node: &Node,
+) -> Result<Option<u64>, Error> {
+    let index_path = super::paths::index_path(ts, channel_config, node)?;
+    let ret = match OpenOptions::new().open(&index_path).await {
+        Ok(_file) => {
+            info!("opened index file");
+            error!("???????????????    TODO   search index for start");
+            err::todoval::<u32>();
+            None
+        }
+        Err(e) => match e.kind() {
+            ErrorKind::NotFound => None,
+            _ => Err(e)?,
+        },
+    };
+    Ok(ret)
+}