Launch process without stdin by default

Introduce a `null` input/output for processes. This can be used for
redirecting to /dev/null or running a process without stdin.

Change the default for Process.run to not read anything from stdin.
Previously, processes would inherit stdin from dune. That seems like a
bad default.

Signed-off-by: Rudi Grinberg <[email protected]>

src/dune/process.ml

@@ -96,6 +96,26 @@ module Io = struct
     let channel = lazy (channel_of_descr (Lazy.force fd) mode) in
     { kind = File fn; mode; fd; channel; status = Close_after_exec }
 
+  let null =
+    let fname =
+      lazy
+        (Path.of_string
+           ( if Sys.win32 then
+             "nul"
+           else
+             "/dev/null" ))
+    in
+    let empty = lazy (Temp.create File ~prefix:"empty." ~suffix:".dune") in
+    let f (type a) (mode : a mode) : a t =
+      let (lazy fname) =
+        match mode with
+        | Out -> fname
+        | In -> empty
+      in
+      file fname mode
+    in
+    f
+
   let flush : type a. a t -> unit =
    fun t ->
     if Lazy.is_val t.channel then
@@ -425,7 +445,7 @@ module Exit_status = struct
 end
 
 let run_internal ?dir ?(stdout_to = Io.stdout) ?(stderr_to = Io.stderr)
-    ?(stdin_from = Io.stdin) ~env ~purpose fail_mode prog args =
+    ?(stdin_from = Io.null In) ~env ~purpose fail_mode prog args =
   Scheduler.with_job_slot (fun () ->
       let display = (Config.t ()).display in
       let dir =

src/dune/process.mli

@@ -28,6 +28,8 @@ module Io : sig
 
   val stdin : input t
 
+  val null : 'a mode -> 'a t
+
   (** Return a buffered channel for this output. The channel is created lazily. *)
   val out_channel : output t -> out_channel