Add new heuristic for handling OCAMLLIB

ocamlbuild -where will now ignore OCAMLLIB completely if it detects that
OCAMLBUILD_LIBDIR is not a lexical subdirectory of OCAML_LIBDIR. If it is
a subdirectory, then instead of returning $OCAMLLIB/ocamlbuild, it will
also include the difference between OCAMLBUILD_LIBDIR and OCAML_LIBDIR.

For example, if in this normal findlib local installation:
  OCAMLBUILD_LIBDIR=/home/dra/ocaml/lib/site-lib
  OCAML_LIBDIR=/home/dra/ocaml/lib

then ocamlbuild -where will return $OCAMLLIB/site-lib/ocamlbuild if
OCAMLLIB is set.

Changes

@@ -94,6 +94,14 @@ detailed changelog below for details.
 - #202: install license, changes and readme in opam's docdir for `odig`
   (Gabriel Scherer, request and review by Daniel Bünzli)
 
+* #240: new heuristic for handling the OCAMLLIB environment variable.
+  `ocamlbuild -where` will ignore OCAMLLIB completely if OCAMLBUILD_LIBDIR is
+  not a lexical subdirectory of OCAML_LIBDIR (i.e. an opam installation).
+  Otherwise, it now returns $OCAMLLIB with the difference between
+  OCAMLBUILD_LIBDIR and OCAML_LIBDIR appended (i.e. for a normal findlib
+  installation, it now returns $OCAMLLIB/site-lib/ocamlbuild)
+  (David Allsopp, review by Gabriel Schere)
+
 - "noautolink" tag for ocaml{c,opt}
   (Gabriel Scherer)
 

configure.make

@@ -94,6 +94,8 @@ src/ocamlbuild_config.ml:
 	echo ;\
 	echo 'let bindir = "$(OCAMLBUILD_BINDIR)"'; \
 	echo 'let libdir = "$(OCAMLBUILD_LIBDIR)"'; \
+	echo 'let ocaml_libdir = "$(abspath $(OCAML_LIBDIR))"'; \
+	echo 'let libdir_abs = "$(abspath $(OCAMLBUILD_LIBDIR))"'; \
 	echo 'let supports_shared_libraries = $(SUPPORTS_SHARED_LIBRARIES)';\
 	echo 'let a = "$(A)"'; \
 	echo 'let o = "$(O)"'; \

src/ocamlbuild_where.ml

@@ -11,10 +11,40 @@
 (*                                                                     *)
 (***********************************************************************)
 
-let bindir = ref Ocamlbuild_config.bindir;;
+module O = Ocamlbuild_config;;
+
+let bindir = ref O.bindir;;
 let libdir = ref begin
-  Filename.concat
-    (try Sys.getenv "OCAMLLIB"
-     with Not_found -> Ocamlbuild_config.libdir)
-    "ocamlbuild"
+  let root, suffix =
+    let ocaml_lib_len = String.length O.ocaml_libdir + 1 in
+    let lib_len = String.length O.libdir_abs in
+    (* Windows note: O.ocaml_libdir and O.libdir_abs have both been passed
+       through GNU make's abspath function and will be forward-slash normalised.
+       Filename.dir_sep is therefore not appropriate here. *)
+    if lib_len < ocaml_lib_len
+       || String.sub O.libdir_abs 0 ocaml_lib_len <> O.ocaml_libdir ^ "/" then
+      O.libdir, "ocamlbuild"
+    else
+      (* https://github.com/ocaml/ocamlbuild/issues/69. Only use OCAMLLIB if
+         the configured LIBDIR is a subdirectory (lexically) of OCAML_LIBDIR.
+         If it is, append the difference between LIBDIR and OCAML_LIBDIR to
+         OCAMLLIB. This allows `OCAMLLIB=/foo ocamlbuild -where` to return
+         /foo/site-lib/ocamlbuild for a findlib-based installation and also
+         to ignore OCAMLLIB in an opam-based installation (where setting
+         OCAMLLIB is already a strange thing to have done). *)
+      try
+        let normalise_slashes =
+          if Sys.os_type = "Win32" then
+            String.map (function '/' -> '\\' | c -> c)
+          else
+            function s -> s
+        in
+        let subroot =
+          String.sub O.libdir_abs ocaml_lib_len (lib_len - ocaml_lib_len)
+          |> normalise_slashes
+        in
+        Sys.getenv "OCAMLLIB", Filename.concat subroot "ocamlbuild"
+      with Not_found -> O.libdir, "ocamlbuild"
+  in
+  Filename.concat root suffix
 end;;