Load X resources from a settings file like other programs on Haiku

* doc/emacs/haiku.texi (Haiku Basics): Document how X resources
are discovered on Haiku.
* src/haiku_support.cc (class Emacs): Load settings file.
(be_find_setting): New function.
* src/haiku_support.h (be_find_setting): New prototype.
* src/haikuterm.c (get_string_resource): Look in the settings
file if there's nothing in the in-memory resource database.

doc/emacs/haiku.texi

@@ -96,6 +96,19 @@ the nil value, and Emacs will use its own implementation of tooltips.
 the menu bar, so help text in the menu bar will display in the echo
 area instead.
 
+@cindex X resources on Haiku
+  Unlike the X window system, Haiku does not have a system-wide
+resource database.  Since many important options are are specified via
+X resources (@pxref{X Resources}), an emulation is provided: upon
+startup, Emacs will load a file named @file{GNU Emacs} inside the user
+configuration directory (normally @file{/boot/home/config/settings}),
+which should be a flattened system message where keys and values are
+both strings, and correspond to attributes and their values
+respectively.
+
+You can create such a file with the tool @code{xmlbmessage}, which is
+free software.
+
 @subsection What to do when Emacs crashes
 @cindex crashes, Haiku
 @cindex haiku debugger

src/haiku_support.cc

@@ -291,8 +291,25 @@ map_normal (uint32_t kc, uint32_t *ch)
 class Emacs : public BApplication
 {
 public:
+  BMessage settings;
+  bool settings_valid_p = false;
+
   Emacs () : BApplication ("application/x-vnd.GNU-emacs")
   {
+    BPath settings_path;
+
+    if (find_directory (B_USER_SETTINGS_DIRECTORY, &settings_path) != B_OK)
+      return;
+
+    settings_path.Append (PACKAGE_NAME);
+
+    BEntry entry (settings_path.Path ());
+    BFile settings_file (&entry, B_READ_ONLY | B_CREATE_FILE);
+
+    if (settings.Unflatten (&settings_file) != B_OK)
+      return;
+
+    settings_valid_p = true;
   }
 
   void
@@ -3131,3 +3148,23 @@ BWindow_set_override_redirect (void *window, bool override_redirect_p)
       w->UnlockLooper ();
     }
 }
+
+/* Find a resource by the name NAME inside the settings file.  The
+   string returned is in UTF-8 encoding, and will stay allocated as
+   long as the BApplication (a.k.a display) is alive.  */
+const char *
+be_find_setting (const char *name)
+{
+  Emacs *app = (Emacs *) be_app;
+  const char *value;
+
+  /* Note that this is thread-safe since the constructor of `Emacs'
+     runs in the main thread.  */
+  if (!app->settings_valid_p)
+    return NULL;
+
+  if (app->settings.FindString (name, 0, &value) != B_OK)
+    return NULL;
+
+  return value;
+}

src/haiku_support.h

@@ -855,6 +855,9 @@ extern "C"
   extern void
   BWindow_set_override_redirect (void *window, bool override_redirect_p);
 
+  extern const char *
+  be_find_setting (const char *name);
+
 #ifdef __cplusplus
   extern void *
   find_appropriate_view_for_draw (void *vw);

src/haikuterm.c

@@ -107,6 +107,8 @@ haiku_delete_terminal (struct terminal *terminal)
 static const char *
 get_string_resource (void *ignored, const char *name, const char *class)
 {
+  const char *native;
+
   if (!name)
     return NULL;
 
@@ -115,6 +117,9 @@ get_string_resource (void *ignored, const char *name, const char *class)
   if (!NILP (lval))
     return SSDATA (XCDR (lval));
 
+  if ((native = be_find_setting (name)))
+    return native;
+
   return NULL;
 }