update gitignore

.gitignore

@@ -3,3 +3,4 @@
 cache
 flamegraph*.svg
 perf.data*
+dhat.out.*

src/cache/fs.rs

@@ -6,12 +6,13 @@ use once_cell::sync::Lazy;
 use serde::Deserialize;
 use std::collections::HashMap;
 use std::fmt::Display;
+use std::io::SeekFrom;
 use std::num::NonZeroU64;
 use std::path::{Path, PathBuf};
 use std::pin::Pin;
 use std::task::{Context, Poll};
 use tokio::fs::{create_dir_all, remove_file, File};
-use tokio::io::{AsyncRead, AsyncWriteExt, ReadBuf};
+use tokio::io::{AsyncRead, AsyncSeekExt, AsyncWriteExt, ReadBuf};
 use tokio::sync::mpsc::UnboundedSender;
 use tokio::sync::watch::{channel, Receiver};
 use tokio::sync::RwLock;
@@ -85,14 +86,15 @@ pub async fn write_file(
         file
     };
 
+    let metadata = serde_json::to_string(&metadata).unwrap();
+    let metadata_size = metadata.len();
     // need owned variant because async lifetime
     let path_buf = path.to_path_buf();
     tokio::spawn(async move {
         let path_buf = path_buf; // moves path buf into async
         let mut errored = false;
         let mut bytes_written: u64 = 0;
-        file.write_all(serde_json::to_string(&metadata).unwrap().as_bytes())
+        file.write_all(&metadata.as_bytes()).await?;
-            .await?;
         while let Some(bytes) = byte_stream.next().await {
             if let Ok(mut bytes) = bytes {
                 loop {
@@ -149,7 +151,7 @@ pub async fn write_file(
     });
 
     Ok(CacheStream::Concurrent(
-        ConcurrentFsStream::new(path, WatchStream::new(rx)).await?,
+        ConcurrentFsStream::new(path, metadata_size, WatchStream::new(rx)).await?,
     ))
 }
 
@@ -163,11 +165,12 @@ pub struct ConcurrentFsStream {
 impl ConcurrentFsStream {
     async fn new(
         path: &Path,
+        seek: usize,
         receiver: WatchStream<WritingStatus>,
     ) -> Result<Self, std::io::Error> {
-        File::open(path)
+        let mut file = File::open(path).await?;
-            .await
+        file.seek(SeekFrom::Start(seek as u64)).await?;
-            .map(|file| Self::from_file(file, receiver))
+        Ok(Self::from_file(file, receiver))
     }
 
     fn from_file(file: File, receiver: WatchStream<WritingStatus>) -> Self {