Add link flags for external dependencies when using ctypes.

Required when the external lib is not statically linked, in which case
linker needs to be aware at runtime what libs to call. This is the case
for

- dune exec <file>.bc
- dune utop
- mdx tests

That is add to ocamlmklib, link flags such as -l<libname> -L<libdir>
from ctypes if info is available.

If dependent library are not installed in expected locations, env
variable such as LD_LIBRARY_PATH must be extended when required program
is run, depending on os. This may be fixed by adding relocatable paths to
link flags.

Signed-off-by: Frej Soya <frej.soya@gmail.com>

src/dune_rules/ctypes/ctypes_rules.ml

@@ -483,8 +483,7 @@ let gen_rules ~cctx ~(buildable : Buildable.t) ~loc ~scope ~dir ~sctx =
           ~c_types_includer_module))
 ;;
 
-let ctypes_cclib_flags sctx ~expander ~(buildable : Buildable.t) =
-  let standard = Action_builder.return [] in
+let ctypes_cclib_flags ?(standard = Action_builder.return []) sctx ~expander ~(buildable : Buildable.t)  =
   match buildable.ctypes with
   | None -> standard
   | Some ctypes ->

src/dune_rules/ctypes/ctypes_rules.mli

@@ -10,7 +10,8 @@ val gen_rules
   -> unit Memo.t
 
 val ctypes_cclib_flags
-  :  Super_context.t
+  : ?standard:(string list Action_builder.t)
+  ->  Super_context.t
   -> expander:Expander.t
   -> buildable:Dune_file.Buildable.t
   -> string list Action_builder.t

src/dune_rules/lib_rules.ml

@@ -331,9 +331,7 @@ let build_stubs lib ~cctx ~dir ~expander ~requires ~dir_contents ~vlib_stubs_o_f
         Cxx_flags.get_flags ~for_:Link (Context.build_context ctx)
       | _ -> Action_builder.return []
     in
-    let c_library_flags =
-      Expander.expand_and_eval_set expander lib.c_library_flags ~standard
-    in
+    let c_library_flags = Ctypes_rules.ctypes_cclib_flags sctx ~expander ~buildable:lib.buildable ~standard in
     let lib_o_files_for_all_modes = Mode.Map.Multi.for_all_modes o_files in
     let for_all_modes = List.rev_append vlib_stubs_o_files lib_o_files_for_all_modes in
     if Mode.Dict.Set.to_list modes.ocaml

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/dune

@@ -0,0 +1,4 @@
+(executable
+ (name example)
+ (modes byte)
+ (libraries examplelib))

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/dune-project

@@ -0,0 +1,3 @@
+(lang dune 3.10)
+
+(using ctypes 0.3)

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/example.ml

@@ -0,0 +1,2 @@
+let () =
+  Printf.printf "%d\n" (Examplelib.C.Functions.add2 2)

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/run.t

@@ -0,0 +1,24 @@
+Build an example library as a DLL and set up the environment so that it looks
+like a system/distro library that can be probed with pkg-config and dynamically
+loaded.
+
+Run the program in bytecode
+  $ LIBEX=$(realpath "$PWD/../libexample")
+
+ocamlrun + requires CAML_LD_LIBRARY_PATH such that dlopen can find
+dllexamplelib_stub.so
+
+Explictly set {DYLD,LD}_LIBRARY_PATH at runtime for this testcase, otherwise
+dlopen cannot find libexample.
+
+  $  PKG_CONFIG_PATH="$LIBEX/pkgconfig" PKG_CONFIG_ARGN="--define-prefix" DYLD_LIBRARY_PATH="$DYLD_LIBRARY_PATH:$LIBEX" LD_LIBRARY_PATH="$LD_LIBRARY_PATH:$LIBEX" CAML_LD_LIBRARY_PATH="$CAML_LD_LIBRARY_PATH:$PWD/_build/default/stubgen" dune exec ./example.bc
+  4
+
+Utop works with ctypes pkg-config external library.
+
+  $   DYLD_LIBRARY_PATH="$DYLD_LIBRARY_PATH:$LIBEX" LD_LIBRARY_PATH="$LD_LIBRARY_PATH:$LIBEX"  PKG_CONFIG_PATH="$LIBEX/pkgconfig" PKG_CONFIG_ARGN="--define-prefix" dune utop --display=short ./ -- example.ml
+    pkg-config stubgen/.pkg-config/libexample.cflags
+    pkg-config stubgen/.pkg-config/libexample.libs
+        ocamlc .utop/.utop.eobjs/byte/dune__exe__Utop.{cmi,cmo,cmt}
+        ocamlc .utop/utop.bc
+  4

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/stubgen/dune

@@ -0,0 +1,14 @@
+(library
+  (name examplelib)
+  (flags (:standard -w -9-27))
+  (ctypes
+    (external_library_name libexample)
+    (build_flags_resolver pkg_config)
+    (headers (include "example.h"))
+    (type_description
+      (instance Types)
+      (functor Type_description))
+    (function_description
+      (instance Functions)
+      (functor Function_description))
+    (generated_entry_point C)))

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/stubgen/function_description.ml

@@ -0,0 +1,8 @@
+open Ctypes
+
+module Types = Types_generated
+
+module Functions (F : Ctypes.FOREIGN) = struct
+  open F
+  let add2 = foreign "example_add2" (int @-> returning int)
+end

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/stubgen/type_description.ml

@@ -0,0 +1,3 @@
+module Types (F : Ctypes.TYPE) = struct
+
+end

test/blackbox-tests/test-cases/ctypes/dune

@@ -27,6 +27,7 @@
 
 (cram
  (applies_to
+  bytecode-stubs-external-lib
   lib-pkg_config
   lib-pkg_config-multiple-fd
   lib-external-name-need-mangling