[gvfs] Accept XDG_RUNTIME_DIR/bus as a valid D-Bus session/user bus

[Ondrej Holy <oholy src gnome org>]

commit ac80e77f6e45a3b688238e1fbf5b8baba98b58b4
Author: Simon McVittie <simon mcvittie collabora co uk>
Date:   Fri Oct 23 18:51:09 2015 +0100

    Accept XDG_RUNTIME_DIR/bus as a valid D-Bus session/user bus
    
    These checks for DBUS_SESSION_BUS_ADDRESS were added to solve
    https://bugzilla.gnome.org/show_bug.cgi?id=526454,
    in which non-X11-session processes (for example a system service),
    or processes under su or similar inside an X11 session, could cause
    a dbus-daemon to be autolaunched via dbus-launch. If there was no
    X11 display to represent the lifetime of a session, the dbus-daemon
    would potentially run forever, causing a "leaked" process;
    additionally, other uses of D-Bus by the same uid would start more
    dbus-daemons.
    
    This becomes potentially problematic on systems with the "user bus"
    model introduced in dbus 1.10: libdbus, GDBus and sd-bus will now
    all try the per-uid socket XDG_RUNTIME_DIR/bus before attempting
    autolaunch, so if those are known to be the only implementations in
    use on a "legacy-free" system, setting DBUS_SESSION_BUS_ADDRESS is
    unnecessary. Check for that socket before giving up.
    
    XDG_RUNTIME_DIR/bus as implemented by dbus 1.10 with systemd avoids
    several of the down sides of autolaunching: it will never start more
    than one session bus per uid, and the socket and bus will automatically
    be cleaned up when the corresponding "systemd --user" exits.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=756420

 client/gdaemonvfs.c                 |    3 +-
 common/gvfsutils.c                  |   46 +++++++++++++++++++++++++++++++++++
 common/gvfsutils.h                  |    1 +
 monitor/proxy/gproxyvolumemonitor.c |    3 +-
 4 files changed, 51 insertions(+), 2 deletions(-)
---
diff --git a/client/gdaemonvfs.c b/client/gdaemonvfs.c
index caf846a..8a91570 100644
--- a/client/gdaemonvfs.c
+++ b/client/gdaemonvfs.c
@@ -40,6 +40,7 @@
 #include <glib/gi18n-lib.h>
 #include <glib/gstdio.h>
 #include <gvfsdbus.h>
+#include "gvfsutils.h"
 
 typedef struct  {
   char *type;
@@ -1501,7 +1502,7 @@ g_io_module_load (GIOModule *module)
    * without spawning private dbus instances.
    * See bug 526454.
    */
-  if (g_getenv ("DBUS_SESSION_BUS_ADDRESS") == NULL) 
+  if (!gvfs_have_session_bus ())
     return;
 
   /* Make this module resident so that we ground the common
diff --git a/common/gvfsutils.c b/common/gvfsutils.c
index 87718f3..47c3117 100644
--- a/common/gvfsutils.c
+++ b/common/gvfsutils.c
@@ -21,8 +21,15 @@
 
 #include <string.h>
 #include <glib.h>
+#include <glib/gstdio.h>
 #include "gvfsutils.h"
 
+#ifdef G_OS_UNIX
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+#endif
+
 /**
  * gvfs_randomize_string:
  * @str: the string to randomize
@@ -40,3 +47,42 @@ gvfs_randomize_string (char *str,
   for (i = 0; i < len; i++)
     str[i] = chars[g_random_int_range (0, strlen(chars))];
 }
+
+/**
+ * gvfs_have_session_bus:
+ *
+ * Returns: %TRUE if we can connect to a session or user bus without
+ *  triggering X11 autolaunching.
+ */
+gboolean
+gvfs_have_session_bus (void)
+{
+  if (g_getenv ("DBUS_SESSION_BUS_ADDRESS") != NULL)
+    return TRUE;
+
+#ifdef G_OS_UNIX
+    {
+      gboolean ret = FALSE;
+      gchar *bus;
+      GStatBuf buf;
+
+      bus = g_build_filename (g_get_user_runtime_dir (), "bus", NULL);
+
+      if (g_stat (bus, &buf) < 0)
+        goto out;
+
+      if (buf.st_uid != geteuid ())
+        goto out;
+
+      if ((buf.st_mode & S_IFMT) != S_IFSOCK)
+        goto out;
+
+      ret = TRUE;
+out:
+      g_free (bus);
+      return ret;
+    }
+#else
+  return FALSE;
+#endif
+}
diff --git a/common/gvfsutils.h b/common/gvfsutils.h
index aa7faf5..edac0b0 100644
--- a/common/gvfsutils.h
+++ b/common/gvfsutils.h
@@ -24,6 +24,7 @@ G_BEGIN_DECLS
 
 void         gvfs_randomize_string                  (char             *str,
                                                      int               len);
+gboolean     gvfs_have_session_bus                  (void);
 
 G_END_DECLS
 
diff --git a/monitor/proxy/gproxyvolumemonitor.c b/monitor/proxy/gproxyvolumemonitor.c
index a7466f0..d258546 100644
--- a/monitor/proxy/gproxyvolumemonitor.c
+++ b/monitor/proxy/gproxyvolumemonitor.c
@@ -45,6 +45,7 @@
 #include "gvfsmonitorimpl.h"
 #include "gvfsdbus.h"
 #include "gvfsdaemonprotocol.h"
+#include "gvfsutils.h"
 
 G_LOCK_DEFINE_STATIC(proxy_vm);
 
@@ -1412,7 +1413,7 @@ g_proxy_volume_monitor_setup_session_bus_connection (void)
    * without spawning private dbus instances.
    * See bug 526454.
    */
-  if (g_getenv ("DBUS_SESSION_BUS_ADDRESS") == NULL)
+  if (!gvfs_have_session_bus ())
     return FALSE;
 
   if (the_volume_monitors == NULL)