diff --git a/lib_eio/buf_write.ml b/lib_eio/buf_write.ml
index cf5e290e..b35e79bd 100644
--- a/lib_eio/buf_write.ml
+++ b/lib_eio/buf_write.ml
@@ -515,21 +515,16 @@ let copy t flow =
   try aux ()
   with End_of_file -> ()
 
-let with_flow ?(initial_size=0x1000) flow fn =
-  Switch.run ~name:"Buf_write.with_flow" @@ fun sw ->
+let of_flow ~sw ?(initial_size=0x1000) flow =
   let t = create ~sw initial_size in
+  Switch.on_release sw (fun () -> close t);
   Fiber.fork ~sw (fun () -> copy t flow);
-  match fn t with
-  | x ->
-    close t;
-    x
-  | exception ex ->
-    close t;
-    (* Raising the exception will cancel the writer thread, so do a flush first.
-       We don't want to flush if cancelled, but in that case the switch will
-       end the writer thread itself (and [flush] will raise). *)
-    flush t;
-    raise ex
+  t
+
+let with_flow ?initial_size flow fn =
+  Switch.run ~name:"Buf_write.with_flow" @@ fun sw ->
+  let t = of_flow ~sw ?initial_size flow in
+  fn t
 
 let rec serialize t writev =
   match await_batch t with