Fix resumed copy truncation

src/bin/dosh-client.rs

@@ -2498,11 +2498,15 @@ fn download_file(
         0
     };
     let mut file = if offset > 0 {
-        fs::OpenOptions::new()
+        let file = fs::OpenOptions::new()
             .read(true)
+            .write(true)
             .append(true)
             .open(local)
-            .with_context(|| format!("open {}", local.display()))?
+            .with_context(|| format!("open {}", local.display()))?;
+        file.set_len(offset)
+            .with_context(|| format!("truncate {}", local.display()))?;
+        file
     } else {
         fs::OpenOptions::new()
             .create(true)

src/bin/dosh-server.rs

@@ -2667,10 +2667,13 @@ async fn handle_file_request(
             let (file, temp_path, written, atomic) = if resume && final_path.exists() {
                 let mut file = fs::OpenOptions::new()
                     .read(true)
+                    .write(true)
                     .append(true)
                     .open(&final_path)
                     .with_context(|| format!("open {}", final_path.display()))?;
                 let written = file.metadata()?.len().min(size);
+                file.set_len(written)
+                    .with_context(|| format!("truncate {}", final_path.display()))?;
                 if written > 0 {
                     let mut prefix = fs::File::open(&final_path)?;
                     hash_exact_prefix(&mut prefix, written, &mut hasher)?;