Remove program path resolution and document use of execve

lib_eio_linux/eio_linux.ml

@@ -959,22 +959,8 @@ module Low_level = struct
           let status = FD.use_exn "wait" t.process pidfd_wait in
           Promise.resolve r status;
           status
-
-      let resolve_program ~paths prog =
-        if not (Filename.is_implicit prog) then Some prog
-        else
-        let exists path =
-          let p = Filename.concat path prog in
-          if Sys.file_exists p then Some p else None
-        in
-        List.find_map exists paths
 
       let spawn ?env ~cwd ~stdin ~stdout ~stderr ~sw prog argv =
-        let paths = Option.map (fun v -> String.split_on_char ':' v) (Sys.getenv_opt "PATH") |> Option.value ~default:[ "/usr/bin"; "/usr/local/bin" ] in
-        let prog = match resolve_program ~paths prog with
-          | Some prog -> prog
-          | None -> raise (Eio.Fs.err (Eio.Fs.Not_found (Eio_unix.Unix_error (Unix.ENOENT, "", ""))))
-        in
         FD.use_exn "spawn_stdin" stdin @@ fun stdin ->
         FD.use_exn "spawn_stdout" stdout @@ fun stdout ->
         FD.use_exn "spawn_stderr" stderr @@ fun stderr ->

lib_eio_linux/eio_linux.mli

@@ -279,7 +279,8 @@ module Low_level : sig
       string list ->
       t
     (** Spawns a subprocess. If the process has not finished when the switch is released, the process
-        will be sent [Sys.sigkill]. *)
+        will be sent [Sys.sigkill]. [spawn] uses [execve] which means the program will {e not} be searched
+        for in the [PATH] environment variable. *)
 
     val wait : t -> Unix.process_status
     (** [wait t] waits for the process [t] to exit. This blocks the fiber until the process

lib_eio_linux/tests/process.md

@@ -23,7 +23,21 @@ let read_all fd =
     | exception End_of_file -> Buffer.contents acc_buffer
   in read ()
 
+let resolve_program prog =
+  let paths = Option.map (fun v -> String.split_on_char ':' v) (Sys.getenv_opt "PATH") |> Option.value ~default:[ "/usr/bin"; "/usr/local/bin" ] in
+  if not (Filename.is_implicit prog) then Some prog
+  else
+  let exists path =
+    let p = Filename.concat path prog in
+    if Sys.file_exists p then Some p else None
+  in
+  List.find_map exists paths
+
 let exec ?stdin ?stdout ?stderr ?cwd ~env ~sw prog args = 
+  let prog = match resolve_program prog with
+    | Some prog -> prog
+    | None -> raise (Eio.Fs.err (Eio.Fs.Not_found (Eio_unix.Unix_error (Unix.ENOENT, "", ""))))
+  in
   let stdin = Option.value ~default:(Eio_linux.get_fd env#stdin) stdin in
   let stdout = Option.value ~default:(Eio_linux.get_fd env#stdout) stdout in
   let stderr = Option.value ~default:(Eio_linux.get_fd env#stderr) stderr in