Reuse Buf_write formatter

Author: SGrondin

lib_eio/buf_write.ml

@@ -163,6 +163,7 @@ type t =
   ; mutable bytes_written  : int        (* Total written bytes. Wraps. *)
   ; mutable state          : state
   ; mutable wake_writer    : unit -> unit
+  ; mutable printf         : (Format.formatter * bool ref) option
   }
 (* Invariant: [write_pos >= scheduled_pos] *)
 
@@ -377,6 +378,7 @@ let of_buffer ?sw buffer =
           ; bytes_written   = 0
           ; state           = Active
           ; wake_writer     = ignore
+          ; printf          = None
           }
   in
   begin match sw with
@@ -422,19 +424,26 @@ let make_formatter t =
     (fun () -> flush t)
 
 let printf t =
-  let is_formatting = ref true in
-  let ppf =
-    Format.make_formatter
-      (fun buf off len -> write_gen t buf ~off ~len ~blit:Bigstringaf.blit_from_string)
-      (fun () ->
-         (* As per the Format module manual, an explicit flush writes to the
-            output channel and ensures that "all pending text is displayed"
-            and "these explicit flush calls [...] could dramatically impact efficiency".
-            Therefore it is clear that we need to call `flush t` instead of `flush_buffer t`. *)
-         if !is_formatting then flush t)
+  let ppf, is_formatting =
+    match t.printf with
+    | Some x -> x
+    | None ->
+      let is_formatting = ref true in
+      let ppf =
+        Format.make_formatter
+          (fun buf off len -> write_gen t buf ~off ~len ~blit:Bigstringaf.blit_from_string)
+          (fun () ->
+             (* As per the Format module manual, an explicit flush writes to the
+                output channel and ensures that "all pending text is displayed"
+                and "these explicit flush calls [...] could dramatically impact efficiency".
+                Therefore it is clear that we need to call `flush t` instead of `flush_buffer t`. *)
+             if !is_formatting then flush t)
+      in
+      t.printf <- Some (ppf, is_formatting);
+      ppf, is_formatting
   in
   Format.kfprintf (fun ppf ->
-      assert !is_formatting;
+      if not !is_formatting then raise (Sys_error "Buf_write.printf: invalid concurrent access");
       (* Ensure that [ppf]'s internal buffer is flushed to [t], but without flushing [t] itself: *)
       is_formatting := false;
       Format.pp_print_flush ppf ()