Do not run compiled programs to extract #define's

Running a program to extract a #define value doesn't work in a cross compilation
environment. Nevertheless, we can extract #define constants by invoking the
preprocessor directly using the -E flag and doing some parsing to extract
values.

As a consequence, we now ignore the link_flags argument. As we're not going to
be linking any executables. We aren't removing the argument altogether since
it's technically a breaking change. The user will instead see a deprecation
warning when ~link_flags is provided.

src/configurator/v1.ml

@@ -263,6 +263,36 @@ let compile_c_prog t ?(c_flags=[]) ?(link_flags=[]) code =
   in
   if ok then Ok exe_fname else Error ()
 
+let pp_c_prog t ?(c_flags=[]) code =
+  let c_flags = "-E" :: c_flags in
+  let dir = t.dest_dir ^/ sprintf "c-test-%d" (gen_id t) in
+  Unix.mkdir dir 0o777;
+  let base = dir ^/ "test" in
+  let c_fname = base ^ ".c" in
+  Io.write_file c_fname code;
+  logf t "preprocessing c program:";
+  List.iter (String.split_lines code) ~f:(logf t " | %s");
+  let run_ok args =
+    try
+      Ok (
+        run_capture_exn t ~dir
+          (String.concat ~sep:" "
+             (t.c_compiler :: List.map args ~f:Filename.quote))
+      )
+    with e ->
+      Error e
+  in
+  if need_to_compile_and_link_separately t then
+    run_ok (c_flags @ ["-I"; t.stdlib_dir; "-c"; c_fname])
+  else
+    run_ok
+      (List.concat
+         [ c_flags
+         ; [ "-I"; t.stdlib_dir
+           ; c_fname
+           ]
+         ])
+
 let c_test t ?c_flags ?link_flags code =
   match compile_c_prog t ?c_flags ?link_flags code with
   | Ok    _ -> true
@@ -284,42 +314,57 @@ module C_define = struct
   end
 
   let import t ?prelude ?c_flags ?link_flags ~includes vars =
+    Option.iter link_flags ~f:(fun link_flags ->
+      Format.eprintf
+        "Configurator.C_define.import: link_flags argument is always ignored@.\
+        ~link_flags:[%a]"
+        (Format.pp_print_list
+           ~pp_sep:(fun fmt () -> Format.fprintf fmt " ;@,")
+           (fun fmt s -> Format.fprintf fmt "%S" s)) link_flags
+    );
     let buf = Buffer.create 1024 in
     let pr fmt = Printf.bprintf buf (fmt ^^ "\n") in
+    let prefix = "--- configurator " in
+    let pr_cfg fmt =
+      Buffer.add_string buf prefix;
+      Printf.bprintf buf (fmt ^^ "\n") in
     let includes = "stdio.h" :: includes in
     List.iter includes ~f:(pr "#include <%s>");
     pr "";
     Option.iter prelude ~f:(pr "%s");
-    pr "int main()";
-    pr "{";
     List.iter vars ~f:(fun (name, (kind : Type.t)) ->
       match kind with
       | Switch ->
         pr {|#if defined(%s)|} name;
-        pr {|  printf("%s=b:true\n");|} name;
+        pr_cfg "%S=b:true" name;
         pr {|#else|};
-        pr {|  printf("%s=b:false\n");|} name;
+        pr_cfg "%S=b:false" name;
         pr {|#endif|}
       | Int ->
-        pr {|  printf("%s=i:%%d\n", %s);|} name name
+        pr_cfg "%S=i:%s" name name;
       | String ->
-        pr {|  printf("%s=s:%%s\n", %s);|} name name);
-    pr "  return 0;";
-    pr "}";
+        pr_cfg "%S=s:%s" name name);
     let code = Buffer.contents buf in
-    match compile_c_prog t ?c_flags ?link_flags code with
-    | Error () -> die "failed to compile program"
-    | Ok exe ->
-      run_capture_exn t ~dir:(Filename.dirname exe) (command_line exe [])
-      |> String.split_lines
-      |> List.map ~f:(fun s ->
-        let var, data = String.lsplit2_exn s ~on:'=' in
-        (var,
-         match String.lsplit2_exn data ~on:':' with
-         | "b", s -> Value.Switch (bool_of_string s)
-         | "i", s -> Int    (int_of_string s)
-         | "s", s -> String s
-         | _ -> assert false))
+    match pp_c_prog t ?c_flags code with
+    | Error _ -> die "failed to compile program"
+    | Ok pped_lines ->
+      String.split_lines pped_lines
+      |> List.filter_map ~f:(fun l ->
+        try
+          Scanf.sscanf l "--- configurator %S=%[ibs]:%s" (fun name typ v ->
+            Some (
+              ( name
+              , match typ with
+              | "b" -> Value.Switch (bool_of_string v)
+              | "i" -> Int (int_of_string v)
+              | "s" -> String v
+              | _ -> assert false)
+            )
+          )
+        with
+        | End_of_file
+        | Scanf.Scan_failure _ -> None
+      )
 
   let gen_header_file t ~fname ?protection_var vars =
     let protection_var =

src/configurator/v1.mli

@@ -48,7 +48,7 @@ module C_define : sig
     (** Define extra code be used with extracting values below. Note that the
         compiled code is never executed. *)
     -> ?c_flags:   string list
-    -> ?link_flags:string list
+    -> ?link_flags:string list (** @deprecated, this argument is ignored *)
     -> includes:   string list
     -> (string * Type.t ) list
     -> (string * Value.t) list