load and choose what backend to use; also write a simple null backend

The thinking is that if someone wants to turn off PolicyKit, they
simply drop a file /etc/polkit-1/nullbackend.conf.d/99-i-hate-polkit.conf
with the contents

[Configuration]
priority=1000

This also provides a good cut-n-paste template / example etc. of how
to implement a PolicyKit backend.

5 files changed, 72 insertions, 2 deletions

diff --git a/src/polkitbackend/Makefile.am b/src/polkitbackend/Makefile.am
index 5f10f9e..a505fa7 100644
--- a/src/polkitbackend/Makefile.am
+++ b/src/polkitbackend/Makefile.am
@@ -84,3 +84,4 @@ clean-local :
 install-exec-hook:
 	mkdir -p $(DESTDIR)$(localstatedir)/lib/polkit-1
 	-chmod 600 $(DESTDIR)$(localstatedir)/lib/polkit-1
+	mkdir -p $(DESTDIR)$(libdir)/polkit-1/backends
diff --git a/src/polkitbackend/polkitbackend.h b/src/polkitbackend/polkitbackend.h
index 3da728d..6ff0439 100644
--- a/src/polkitbackend/polkitbackend.h
+++ b/src/polkitbackend/polkitbackend.h
@@ -22,6 +22,8 @@
 #ifndef __POLKIT_BACKEND_H
 #define __POLKIT_BACKEND_H
 
+#include <polkit/polkit.h>
+
 #define _POLKIT_BACKEND_INSIDE_POLKIT_BACKEND_H 1
 #include <polkitbackend/polkitbackendtypes.h>
 #include <polkitbackend/polkitbackendauthority.h>
diff --git a/src/polkitbackend/polkitbackendauthority.c b/src/polkitbackend/polkitbackendauthority.c
index f81fba8..bf40283 100644
--- a/src/polkitbackend/polkitbackendauthority.c
+++ b/src/polkitbackend/polkitbackendauthority.c
@@ -26,6 +26,7 @@
 #include <polkit/polkit.h>
 #include <polkit/polkitprivate.h>
 #include "polkitbackendauthority.h"
+#include "polkitbackendlocalauthority.h"
 
 #include "polkitbackendprivate.h"
 
@@ -85,7 +86,8 @@ polkit_backend_authority_system_bus_name_owner_changed (PolkitBackendAuthority
 
   klass = POLKIT_BACKEND_AUTHORITY_GET_CLASS (authority);
 
-  klass->system_bus_name_owner_changed (authority, name, old_owner, new_owner);
+  if (klass->system_bus_name_owner_changed != NULL)
+    klass->system_bus_name_owner_changed (authority, name, old_owner, new_owner);
 }
 
 /**
@@ -1261,3 +1263,55 @@ polkit_backend_register_authority (PolkitBackendAuthority   *authority,
   g_object_unref (server);
   return FALSE;
 }
+
+
+/**
+ * polkit_backend_authority_get:
+ *
+ * Loads all #GIOModule<!-- -->s from <literal>$(libdir)/polkit-1/backends</literal> to determine
+ * what implementation of #PolkitBackendAuthority to use. Then instantiates an object of the
+ * implementation with the highest priority and unloads all other modules.
+ *
+ * Returns: A #PolkitBackendAuthority. Free with g_object_unref().
+ **/
+PolkitBackendAuthority *
+polkit_backend_authority_get (void)
+{
+  static GIOExtensionPoint *ep = NULL;
+  static volatile GType local_authority_type = G_TYPE_INVALID;
+  GList *modules;
+  GList *authority_implementations;
+  GType authority_type;
+  PolkitBackendAuthority *authority;
+
+  /* define the extension point */
+  if (ep == NULL)
+    {
+      ep = g_io_extension_point_register (POLKIT_BACKEND_AUTHORITY_EXTENSION_POINT_NAME);
+      g_io_extension_point_set_required_type (ep, POLKIT_BACKEND_TYPE_AUTHORITY);
+    }
+
+  /* make sure local types are registered */
+  if (local_authority_type == G_TYPE_INVALID)
+    {
+      local_authority_type = POLKIT_BACKEND_TYPE_LOCAL_AUTHORITY;
+    }
+
+  /* load all modules */
+  modules = g_io_modules_load_all_in_directory (PACKAGE_LIB_DIR "/polkit-1/backends");
+
+  /* find all extensions; we have at least one here since we've registered the local backend */
+  authority_implementations = g_io_extension_point_get_extensions (ep);
+
+  /* the returned list is sorted according to priority so just take the highest one */
+  authority_type = g_io_extension_get_type ((GIOExtension*) authority_implementations->data);
+  authority = POLKIT_BACKEND_AUTHORITY (g_object_new (authority_type, NULL));
+
+  /* unload all modules; the module our instantiated authority is in won't be unloaded because
+   * we've instantiated a reference to a type in this module
+   */
+  g_list_foreach (modules, (GFunc) g_type_module_unuse, NULL);
+  g_list_free (modules);
+
+  return authority;
+}
diff --git a/src/polkitbackend/polkitbackendauthority.h b/src/polkitbackend/polkitbackendauthority.h
index 25c0ce0..e08360b 100644
--- a/src/polkitbackend/polkitbackendauthority.h
+++ b/src/polkitbackend/polkitbackendauthority.h
@@ -41,6 +41,13 @@ G_BEGIN_DECLS
 typedef struct _PolkitBackendAuthorityClass    PolkitBackendAuthorityClass;
 
 /**
+ * POLKIT_BACKEND_AUTHORITY_EXTENSION_POINT_NAME:
+ *
+ * Extension point name for authority backend implementations.
+ */
+#define POLKIT_BACKEND_AUTHORITY_EXTENSION_POINT_NAME "polkit-backend-authority-1"
+
+/**
  * PolkitBackendAuthority:
  *
  * The #PolkitBackendAuthority struct should not be accessed directly.
@@ -258,6 +265,8 @@ gboolean polkit_backend_authority_authentication_agent_response (PolkitBackendAu
 
 /* --- */
 
+PolkitBackendAuthority *polkit_backend_authority_get (void);
+
 gboolean polkit_backend_register_authority (PolkitBackendAuthority   *authority,
                                             const gchar              *well_known_name,
                                             const gchar              *object_path,
diff --git a/src/polkitbackend/polkitbackendlocalauthority.c b/src/polkitbackend/polkitbackendlocalauthority.c
index 15a7f8f..c735c8c 100644
--- a/src/polkitbackend/polkitbackendlocalauthority.c
+++ b/src/polkitbackend/polkitbackendlocalauthority.c
@@ -220,7 +220,11 @@ action_pool_changed (PolkitBackendActionPool *action_pool,
 
 /* ---------------------------------------------------------------------------------------------------- */
 
-G_DEFINE_TYPE (PolkitBackendLocalAuthority, polkit_backend_local_authority, POLKIT_BACKEND_TYPE_AUTHORITY);
+G_DEFINE_TYPE_WITH_CODE (PolkitBackendLocalAuthority, polkit_backend_local_authority,POLKIT_BACKEND_TYPE_AUTHORITY,
+                         g_io_extension_point_implement (POLKIT_BACKEND_AUTHORITY_EXTENSION_POINT_NAME,
+                                                         g_define_type_id,
+                                                         "local-files " PACKAGE_VERSION,
+                                                         0));
 
 #define POLKIT_BACKEND_LOCAL_AUTHORITY_GET_PRIVATE(o) (G_TYPE_INSTANCE_GET_PRIVATE ((o), POLKIT_BACKEND_TYPE_LOCAL_AUTHORITY, PolkitBackendLocalAuthorityPrivate))