diff --git a/lib/build_log.ml b/lib/build_log.ml
index 629ab4db..02b23951 100644
--- a/lib/build_log.ml
+++ b/lib/build_log.ml
@@ -26,45 +26,47 @@ let catch_cancel fn =
     )
 
 let tail ?switch t dst =
+  let rec readonly_tail ch buf =
+    Lwt_io.read_into ch buf 0 max_chunk_size >>= function
+    | 0 -> Lwt_result.return ()
+    | n -> dst (Bytes.sub_string buf 0 n); readonly_tail ch buf
+  in
+
+  let rec open_tail fd cond buf i =
+    match switch with
+    | Some sw when not (Lwt_switch.is_on sw) -> Lwt_result.fail `Cancelled
+    | Some _ | None ->
+      let avail = min (t.len - i) max_chunk_size in
+      if avail > 0 then (
+        Lwt_unix.pread fd ~file_offset:i buf 0 avail >>= fun n ->
+        dst (Bytes.sub_string buf 0 n);
+        open_tail fd cond buf (i + avail)
+      ) else (
+        match t.state with
+        | `Open _ -> Lwt_condition.wait cond >>= fun () -> open_tail fd cond buf i
+        | `Readonly _ | `Empty | `Finished -> Lwt_result.return ()
+      )
+  in
+
+  let interrupt th =
+    catch_cancel @@ fun () ->
+    Lwt_switch.add_hook_or_exec switch (fun () -> Lwt.cancel th; Lwt.return_unit) >>= fun () ->
+    th
+  in
+
   match t.state with
   | `Finished -> invalid_arg "tail: log is finished!"
   | `Readonly path ->
     let flags = [Unix.O_RDONLY; Unix.O_NONBLOCK; Unix.O_CLOEXEC] in
     Lwt_io.(with_file ~mode:input ~flags) path @@ fun ch ->
     let buf = Bytes.create max_chunk_size in
-    let rec aux () =
-      Lwt_io.read_into ch buf 0 max_chunk_size >>= function
-      | 0 -> Lwt_result.return ()
-      | n -> dst (Bytes.sub_string buf 0 n); aux ()
-    in
-    catch_cancel @@ fun () ->
-    let th = aux () in
-    Lwt_switch.add_hook_or_exec switch (fun () -> Lwt.cancel th; Lwt.return_unit) >>= fun () ->
-    th
+    interrupt (readonly_tail ch buf)
   | `Empty -> Lwt_result.return ()
   | `Open (fd, cond) ->
     (* Dup [fd], which can still work after [fd] is closed. *)
     with_dup fd @@ fun fd ->
     let buf = Bytes.create max_chunk_size in
-    let rec aux i =
-      match switch with
-      | Some sw when not (Lwt_switch.is_on sw) -> Lwt_result.fail `Cancelled
-      | _ ->
-        let avail = min (t.len - i) max_chunk_size in
-        if avail > 0 then (
-          Lwt_unix.pread fd ~file_offset:i buf 0 avail >>= fun n ->
-          dst (Bytes.sub_string buf 0 n);
-          aux (i + avail)
-        ) else (
-          match t.state with
-          | `Open _ -> Lwt_condition.wait cond >>= fun () -> aux i
-          | _ -> Lwt_result.return ()
-        )
-    in
-    catch_cancel @@ fun () ->
-    let th = aux 0 in
-    Lwt_switch.add_hook_or_exec switch (fun () -> Lwt.cancel th; Lwt.return_unit) >>= fun () ->
-    th
+    interrupt (open_tail fd cond buf 0)
 
 let create path =
   Lwt_unix.openfile path Lwt_unix.[O_CREAT; O_TRUNC; O_RDWR; O_CLOEXEC] 0o666 >|= fun fd ->
@@ -114,7 +116,7 @@ let empty = {
 }
 
 let copy ~src ~dst =
-  let buf = Bytes.create 4096 in
+  let buf = Bytes.create max_chunk_size in
   let rec aux () =
     Lwt_unix.read src buf 0 (Bytes.length buf) >>= function
     | 0 -> Lwt.return_unit