Provide lib_config for spawning Unix processes

src/ocaml/driver/pparse.ml

@@ -46,7 +46,7 @@ let merlin_system_command =
     windows_merlin_system_command
   else
     fun cmd ~cwd ->
-      Sys.command (Printf.sprintf "cd %s && %s" (Filename.quote cwd) cmd)
+      !Std.System_command.unix ~cmd:cmd ~cwd
 
 let ppx_commandline cmd fn_in fn_out =
   Printf.sprintf "%s %s %s%s"

src/utils/lib_config.ml

@@ -7,4 +7,9 @@ let program_name () = !program_name
 module Json = struct
   let set_pretty_to_string f =
     Std.Json.pretty_to_string := f
-end
+end
+
+module System_command = struct
+  let set_unix f =
+    Std.System_command.unix := f
+end

src/utils/lib_config.mli

@@ -15,3 +15,10 @@ module Json : sig
         is [Yojson.Basic.pretty_to_string].  *)
     val set_pretty_to_string : (Std.json -> string) -> unit
 end
+
+(** Merlin spawns child processes for preprocessors (pp and ppx), which can be
+    customized via [System_command] *)
+module System_command : sig
+  (** [set_unix] sets an implementation for Unix systems. *)
+  val set_unix : (cmd:string -> cwd:string -> int) -> unit
+end

src/utils/std.ml

@@ -57,6 +57,12 @@ module Json = struct
         A common one is `Yojson.Basic.pretty_to_string`."
 end
 
+module System_command = struct
+  let unix = ref @@
+    fun ~cmd ~cwd ->
+       Sys.command (Printf.sprintf "cd %s && %s" (Filename.quote cwd) cmd)
+end
+
 module Hashtbl = struct
   include Hashtbl