Set and persist $HOME on Windows in opam init

Windows setups will not normally have $HOME set, which causes opam init
to initialise in the cwd. Native Windows updated to query the user's
Documents folder and use that location instead.

Additionally, opam init offers to save $HOME permanently to the user's
registry, as having HOME set is important for OCaml (so that .ocamlinit
can be found) and also for related tools (Vim, for example, can display
and use ~\ paths if $HOME is set)

Signed-off-by: David Allsopp <david.allsopp@metastack.com>

src/client/opamClient.ml

@@ -523,6 +523,30 @@ let init
   let root_empty =
     not (OpamFilename.exists_dir root) || OpamFilename.dir_is_empty root in
 
+    if update_config <> `no && (OpamStd.Sys.(os () = Win32) && (try ignore (Sys.getenv "HOME"); false with Not_found -> true)) then begin
+      let home = OpamStd.Sys.home () in
+      let persistHomeDirectory home =
+        OpamConsole.msg "Persisting HOME (this may take a few seconds)...";
+        OpamStd.Win32.persistHomeDirectory home;
+        OpamConsole.msg " done\n" in
+      let get_action () =
+        if update_config = `yes || OpamCoreConfig.(!r.answer <> None) then
+          Some "y"
+        else
+          OpamConsole.read
+            "The HOME environment variable is not set, so OPAM has defaulted to:\n\
+            \  %s\n\
+             OCaml ideally requires $HOME to be set properly (in order to find .ocamlinit)\n\
+             Do you want OPAM to alter your persistent environment so that HOME is always set?\n\
+             (default is 'yes')\n\
+            \    [Y/n]" home in
+      match get_action () with
+      | Some ("y" | "Y" | "yes"  | "YES") ->
+          persistHomeDirectory home
+      | _ ->
+          ()
+    end;
+
   let gt, rt, default_compiler =
     if OpamFile.exists config_f then (
       OpamConsole.msg "Opam has already been initialized.\n";

src/core/opamStd.ml

@@ -785,7 +785,20 @@ module OpamSys = struct
         else default_columns
 
   let home =
-    let home = lazy (try Env.get "HOME" with Not_found -> Sys.getcwd ()) in
+    let home = lazy (
+      try
+        Env.get "HOME"
+      with Not_found ->
+        if Sys.os_type = "Win32" then
+          (*
+           * Windows setups will rarely have $HOME set, so cwd is a poor default. Instead, return
+           * the value of the user's My Documents folder.
+           *
+           * CSIDL_PERSONAL = 0x5; SHGFP_TYPE_CURRENT = 0x0
+           *)
+          Win32.shGetFolderPath 5 0
+        else
+          Sys.getcwd ()) in
     fun () -> Lazy.force home
 
   let etc () = "/etc"