fix: ocamlformat-rpc initialization

if [get_process] is alled concurrently, the ivar might be filled twice.

Fixes #1115

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

<!-- ps-id: d5c77719-29a5-4b46-92e8-c09f6453ba56 -->

CHANGES.md

@@ -2,6 +2,9 @@
 
 ## Fixes
 
+- Fix initilization of `ocamlformat-rpc` in some edge cases when ocamlformat is
+  initialized concurrently (#1132)
+
 - Kill unnecessary `$ dune ocaml-merlin` with SIGTERM rather than SIGKILL
   (#1124)
 

ocaml-lsp-server/src/ocamlformat_rpc.ml

@@ -136,12 +136,18 @@ type state =
 
 type t = state ref
 
+let maybe_fill ivar x =
+  let* v = Fiber.Ivar.peek ivar in
+  match v with
+  | Some _ -> Fiber.return ()
+  | None -> Fiber.Ivar.fill ivar x
+
 let get_process t =
   match !t with
   | Running p -> Fiber.return @@ Ok p
   | Disabled | Stopped -> Fiber.return @@ Error `No_process
   | Waiting_for_init { ask_init; wait_init } -> (
-    let* () = Fiber.Ivar.fill ask_init () in
+    let* () = maybe_fill ask_init () in
     let+ () = Fiber.Ivar.read wait_init in
     match !t with
     | Running p -> Ok p
@@ -189,12 +195,6 @@ let create_state () =
 
 let create () = ref (if Sys.win32 then Disabled else create_state ())
 
-let maybe_fill ivar x =
-  let* v = Fiber.Ivar.peek ivar in
-  match v with
-  | Some _ -> Fiber.return ()
-  | None -> Fiber.Ivar.fill ivar x
-
 let stop t =
   match !t with
   | Disabled | Stopped -> Fiber.return ()