discover.ml: don't run the opam binary

Instead, pass information about the presence of conf-libev down from
opam on the discover.ml command line.

See #761 (comment).

.gitignore

@@ -1,5 +1,5 @@
 _build
-src/unix/lwt_config
+src/unix/discover_arguments
 *.flag
 
 # OPAM 2.0 local switches.

Makefile

@@ -85,7 +85,7 @@ clean:
 	dune clean
 	find . -name '.merlin' | xargs rm -f
 	rm -fr docs/api
-	rm -f src/jbuild-ignore src/unix/lwt_config
+	rm -f src/unix/discover_arguments
 	for TEST in `ls -d test/packaging/*/*` ; \
 	do \
 	    $(MAKE) -wC $$TEST clean ; \

lwt.opam

@@ -46,6 +46,8 @@ conflicts: [
 ]
 
 build: [
+  ["dune" "exec" "src/unix/config/discover.exe" "--root" "." "--" "--save"
+    "--use-libev" "%{conf-libev:installed}%"]
   ["dune" "build" "-p" name "-j" jobs]
 ]
 

src/unix/config/discover.ml

@@ -26,14 +26,17 @@
 
     This program tries to detect everything automatically. If it is not behaving
     correctly, its behavior can be tweaked by passing it arguments. There are
-    three ways to do so:
+    four ways to do so:
 
     - By editing [src/unix/dune], to pass arguments to [discover.exe] on the
       command line.
     - By setting the environment variable [LWT_DISCOVER_ARGUMENTS]. The syntax
       is the same as the command line.
     - By writing a file [src/unix/discover_arguments]. The syntax is again the
       same as the command line.
+    - By running [dune exec src/unix/config/discover.exe -- --save] with
+      additional arguments. Those arguments will be written to
+      [src/unix/discover_arguments] for the build to use later.
 
     The possible arguments can be found by running
 
@@ -103,7 +106,7 @@ struct
       "BOOLEAN whether to use the libev backend by default";
 
     "--verbose", Arg.Set verbose,
-      "BOOLEAN show results of feature detection"
+      "BOOLEAN show results of feature detection";
   ]
 
   let environment_variable = "LWT_DISCOVER_ARGUMENTS"
@@ -390,17 +393,6 @@ struct
     pretty_name = "libev";
     macro_name = "HAVE_LIBEV";
     detect = fun context ->
-      let detect_opam_conf_libev () =
-        let result =
-          Configurator.Process.run context
-            "opam" ["config"; "var"; "conf-libev:installed"]
-        in
-        match result.stdout, result.exit_code with
-        | "true\n", 0 -> true
-        | "false\n", 0 -> false
-        | _, _ -> failwith "opam exited with an error code, or isn't installed."
-      in
-
       let detect_esy_wants_libev () =
         match Sys.getenv "cur__target_dir" with
         | exception Not_found -> None
@@ -416,15 +408,13 @@ struct
         | Some argument ->
           argument
         | None ->
-          try detect_opam_conf_libev ()
-          with _ ->
-            match detect_esy_wants_libev () with
-            | Some result ->
-              result
-            | None ->
-              (* we're not under esy *)
-              let os = Configurator.ocaml_config_var_exn context "os_type" in
-              os <> "Win32" && !Arguments.android_target <> Some true
+          match detect_esy_wants_libev () with
+          | Some result ->
+            result
+          | None ->
+            (* we're not under esy *)
+            let os = Configurator.ocaml_config_var_exn context "os_type" in
+            os <> "Win32" && !Arguments.android_target <> Some true
       in
 
       if not should_look_for_libev then
@@ -812,6 +802,15 @@ end
 
 
 let () =
+  begin match List.partition ((=) "--save") (Array.to_list Sys.argv) with
+  | ["--save"], rest ->
+    Configurator.Flags.write_lines
+      "src/unix/discover_arguments" [String.concat " " (List.tl rest)];
+    exit 0
+  | _ ->
+    ()
+  end;
+
   Configurator.main ~args:Arguments.args ~name:"lwt" begin fun context ->
     (* Parse arguments from additional sources. *)
     Arguments.parse_environment_variable ();