Split off the extension importing stuff into a new library, 'ShellJS'

This allows us to create a separate utility to import things from
shell extensions that does not have the entire Shell stack built up

https://bugzilla.gnome.org/show_bug.cgi?id=668429

src/shell-js.c

@@ -0,0 +1,81 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
+
+#include "config.h"
+
+#include "shell-js.h"
+
+#include <jsapi.h>
+#include <gio/gio.h>
+#include <gjs/gjs.h>
+#include <gjs/gjs-module.h>
+
+/**
+ * shell_js_add_extension_importer:
+ * @target_object_script: JavaScript code evaluating to a target object
+ * @target_property: Name of property to use for importer
+ * @directory: Source directory:
+ * @error: A #GError
+ *
+ * This function sets a property named @target_property on the object
+ * resulting from the evaluation of @target_object_script code, which
+ * acts as a GJS importer for directory @directory.
+ *
+ * Returns: %TRUE on success
+ */
+gboolean
+shell_js_add_extension_importer (const char  *target_object_script,
+                                 const char  *target_property,
+                                 const char  *directory,
+                                 GError     **error)
+{
+  jsval target_object;
+  GList *contexts;
+  JSContext *context;
+  char *search_path[2] = { 0, 0 };
+  gboolean ret = FALSE;
+
+  /* Take the first GjsContext from all of them --
+   * we should only ever have one context, so this
+   * should be alright. */
+  contexts = gjs_context_get_all ();
+  context = gjs_context_get_native_context (contexts->data);
+  g_list_free_full (contexts, g_object_unref);
+
+  JS_BeginRequest (context);
+
+  /* This is a bit of a hack; ideally we'd be able to pass our target
+   * object directly into this function, but introspection doesn't
+   * support that at the moment.  Instead evaluate a string to get it. */
+  if (!JS_EvaluateScript(context,
+                         JS_GetGlobalObject(context),
+                         target_object_script,
+                         strlen (target_object_script),
+                         "<target_object_script>",
+                         0,
+                         &target_object))
+    {
+      char *message;
+      gjs_log_exception(context,
+                        &message);
+      g_set_error(error,
+                  G_IO_ERROR,
+                  G_IO_ERROR_FAILED,
+                  "%s", message ? message : "(unknown)");
+      g_free(message);
+      goto out;
+    }
+
+  if (!JSVAL_IS_OBJECT (target_object))
+    {
+      g_error ("shell_js_add_extension_importer: invalid target object");
+      goto out;
+    }
+
+  search_path[0] = (char*)directory;
+  gjs_define_importer (context, JSVAL_TO_OBJECT (target_object), target_property, (const char **)search_path, FALSE);
+  ret = TRUE;
+
+ out:
+  JS_EndRequest (context);
+  return ret;
+}