Add Buf_write.printf

lib_eio/buf_write.ml

@@ -163,6 +163,8 @@ type t =
   ; mutable bytes_written  : int        (* Total written bytes. Wraps. *)
   ; mutable state          : state
   ; mutable wake_writer    : unit -> unit
+  ; mutable is_formatting  : bool
+  ; mutable formatter      : Format.formatter option
   }
 (* Invariant: [write_pos >= scheduled_pos] *)
 
@@ -377,6 +379,8 @@ let of_buffer ?sw buffer =
           ; bytes_written   = 0
           ; state           = Active
           ; wake_writer     = ignore
+          ; is_formatting   = false
+          ; formatter       = None
           }
   in
   begin match sw with
@@ -416,6 +420,30 @@ let flush t =
     Promise.await_exn p
   )
 
+  let get_formatter = function
+  | { formatter = Some x; _ } -> x
+  | ({ formatter = None; _ } as t) ->
+    let formatter = 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 t.is_formatting then flush t)
+    in
+    t.formatter <- Some formatter;
+    formatter
+
+  let printf t =
+    let ppf = get_formatter t in
+    t.is_formatting <- true;
+    Format.kfprintf (fun ppf ->
+      assert t.is_formatting;
+      t.is_formatting <- false;
+      Format.pp_print_flush ppf ()
+    ) ppf
+
 let rec shift_buffers t written =
   match Buffers.dequeue_exn t.scheduled with
   | { Cstruct.len; _ } as iovec ->

lib_eio/buf_write.mli

@@ -46,7 +46,7 @@
     or application-specific output APIs.
 
     A Buf_write serializer manages an internal buffer and a queue of output
-    buffers. The output bufferes may be a sub range of the serializer's
+    buffers. The output buffers may be a sub range of the serializer's
     internal buffer or one that is user-provided. Buffered writes such as
     {!string}, {!char}, {!cstruct}, etc., copy the source bytes into the
     serializer's internal buffer. Unbuffered writes are done with
@@ -123,6 +123,18 @@ val cstruct : t -> Cstruct.t -> unit
     It is safe to modify [cs] after this call returns.
     For large cstructs, it may be more efficient to use {!schedule_cstruct}. *)
 
+val printf : t -> ('a, Format.formatter, unit) format -> 'a
+(** [printf t fmt x y z] formats the arguments according to the format string [fmt].
+    It supports all formatting and pretty-printing features of the Format module.
+    Accordingly, explicit flushes using [@.] or [%!] must perform a full (blocking) flush
+    so consider using [Fiber.fork] in such cases. *)
+
+val get_formatter : t -> Format.formatter
+(** [get_formatter t] returns the underlying formatter used by calls to [printf].
+    This function is useful to mutate formatter settings by calling, for example,
+    [Format.pp_set_margin] or [Format.pp_set_geometry].
+    Note that these formatter settings only affect calls to [printf]. *)
+
 val write_gen
   :  t
   -> blit:('a -> src_off:int -> Cstruct.buffer -> dst_off:int -> len:int -> unit)

tests/buf_write.md

@@ -146,6 +146,44 @@ With pausing
 - : unit = ()
 ```
 
+## Formatting
+
+```ocaml
+# Eio_mock.Backend.run @@ fun () ->
+  let f t =
+    let formatter = Write.get_formatter t in
+    Write.string t "Hello";
+    Format.pp_set_geometry formatter ~max_indent:4 ~margin:10;
+    (*
+      "@ "      breakable space
+      "@[<v 6>" open vertical box, indentation: 6 (overriden by our geometry settings)
+      "%s"      print string
+      "@ "      breakable space
+      "%i"      print int
+      "@."      print newline + explicit flush
+      "%a"      print arbitrary type
+      "@]"      close box
+      "@ "      breakable space
+    *)
+    Write.printf t "@ @[<v 6>%s@ %i@.%a@]@ "
+      "This is a test" 123
+      Eio.Net.Sockaddr.pp (`Tcp (Eio.Net.Ipaddr.V6.loopback, 8080));
+
+    Write.string t "-> Not from printf <-";
+    Write.printf t "@.Ok back to %s@." "printf";
+    Write.string t "Goodbye"
+  in
+  Write.with_flow flow f;;
++flow: wrote "Hello\n"
++            "This is a test\n"
++            "    123\n"
++flow: wrote "tcp:[::1]:8080\n"
++            "-> Not from printf <-\n"
++flow: wrote "Ok back to printf\n"
++flow: wrote "Goodbye"
+- : unit = ()
+```
+
 ## Flushing
 
 ```ocaml