Allow X11 autolaunch to be disabled even if the headers/libraries are there

In an embedded system where the D-Bus session is a core part of the
environment, like Maemo, accidentally auto-launching a second session bus
(for instance for a concurrent ssh session) is a bad idea - it can lead
to a "split brain" situation where half the applications in the GUI are
using a different bus. In these controlled environments, it'd be useful
to prevent autolaunch from ever happening.

(As a side benefit, the changes to configure.in also mean that packagers
can explicitly --enable-x11-autolaunch, to make sure that failure to find
X will make compilation fail cleanly.)

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=19997
Bug-NB: NB#219964

4 files changed, 47 insertions, 22 deletions

diff --git a/README b/README
index be4aede2..6c0c39d0 100644
--- a/README
+++ b/README
@@ -105,7 +105,7 @@ to the cmake program are these (use -D<key>=<value> on command line)
 
     CMAKE_BUILD_TYPE                   set dbus build mode - one of Debug|Release|RelWithDebInfo|MinSizeRel
     DBUS_BUILD_TESTS                   enable unit test code  default=ON
-    DBUS_BUILD_X11                     Build X11-dependent code default=ON
+    DBUS_BUILD_X11                     Build with X11 autolaunch support default=ON
     HAVE_CONSOLE_OWNER_FILE            enable console owner file (solaris only) ) default=ON
     DBUS_DISABLE_ASSERTS               Disable assertion checking default=OFF
     DBUS_DISABLE_CHECKS                Disable public API sanity checking default=OFF
diff --git a/cmake/CMakeLists.txt b/cmake/CMakeLists.txt
index ae053355..13cdb741 100644
--- a/cmake/CMakeLists.txt
+++ b/cmake/CMakeLists.txt
@@ -345,7 +345,7 @@ OPTION(DBUS_HAVE_ATOMIC_INT    "Some atomic integer implementation present" ${at
 OPTION(DBUS_USE_ATOMIC_INT_486 "Use atomic integer implementation for 486" ${atomic_int_486})
 
 if(X11_FOUND)
-  OPTION(DBUS_BUILD_X11 "Build X11-dependent code " ON)
+  OPTION(DBUS_BUILD_X11 "Build with X11 autolaunch support " ON)
 endif(X11_FOUND)
 
 # test binary names
diff --git a/configure.in b/configure.in
index b043018f..3e40931b 100644
--- a/configure.in
+++ b/configure.in
@@ -1051,34 +1051,48 @@ AC_SUBST(DBUS_TEST_CFLAGS)
 AC_SUBST(DBUS_TEST_LIBS)
 
 ### X11 detection
-if test x$dbus_win = xyes ; then
-   enable_x11=no
-else
-AC_PATH_XTRA
+DBUS_X_LIBS=
+DBUS_X_CFLAGS=
+
+AC_ARG_ENABLE([x11-autolaunch],
+  AS_HELP_STRING([--enable-x11-autolaunch], [build with X11 auto-launch support]),
+  [], [enable_x11_autolaunch=auto])
+
+if test "x$dbus_win" = xyes; then
+    if test "x$enable_x11_autolaunch" = xyes; then
+        AC_MSG_ERROR([X11 auto-launch is not supported on Windows])
+    fi
 
-## for now enable_x11 just tracks have_x11,
-## there's no --enable-x11
-if test x$no_x = xyes ; then
-   have_x11=no
-   enable_x11=no
+    enable_x11_autolaunch=no
+fi
+
+if test "x$enable_x11_autolaunch" = xno; then
+    have_x11=no
 else
-   have_x11=yes
-   enable_x11=yes
+    AC_PATH_XTRA
+
+    if test "x$no_x" = xyes; then
+        have_x11=no
+    else
+        have_x11=yes
+        DBUS_X_LIBS="$X_LIBS $X_PRE_LIBS -lX11 $X_EXTRA_LIBS"
+        DBUS_X_CFLAGS="$X_CFLAGS"
+    fi
 fi
 
-if test x$enable_x11 = xyes ; then
-   AC_DEFINE(DBUS_BUILD_X11,1,[Build X11-dependent code])
-   DBUS_X_LIBS="$X_LIBS $X_PRE_LIBS -lX11 $X_EXTRA_LIBS"
-   DBUS_X_CFLAGS="$X_CFLAGS"
+if test "x$enable_x11_autolaunch,$have_x11" = xyes,no; then
+    AC_MSG_ERROR([X11 auto-launch requires X headers/libraries])
 else
-   DBUS_X_LIBS=
-   DBUS_X_CFLAGS=
+    # move from "auto" to "yes" or "no" if necessary
+    enable_x11_autolaunch="$have_x11"
 fi
 
-AC_SUBST(DBUS_X_CFLAGS)
-AC_SUBST(DBUS_X_LIBS)
+if test "x$enable_x11_autolaunch" = xyes ; then
+   AC_DEFINE([DBUS_BUILD_X11], [1], [Define to enable X11 auto-launch])
 fi
 
+AC_SUBST([DBUS_X_CFLAGS])
+AC_SUBST([DBUS_X_LIBS])
 
 #### gcc warning flags
 
diff --git a/dbus/dbus-sysdeps-unix.c b/dbus/dbus-sysdeps-unix.c
index ff902ff1..e9ea2535 100644
--- a/dbus/dbus-sysdeps-unix.c
+++ b/dbus/dbus-sysdeps-unix.c
@@ -3257,6 +3257,11 @@ _dbus_get_autolaunch_address (const char *scope,
                               DBusString *address,
                               DBusError  *error)
 {
+#ifdef DBUS_BUILD_X11
+  /* Perform X11-based autolaunch. (We also support launchd-based autolaunch,
+   * but that's done elsewhere, and if it worked, this function wouldn't
+   * be called.) */
+  const char *display;
   static char *argv[6];
   int i;
   DBusString uuid;
@@ -3273,7 +3278,7 @@ _dbus_get_autolaunch_address (const char *scope,
   if (display == NULL || display[0] == '\0')
     {
       dbus_set_error_const (error, DBUS_ERROR_NOT_SUPPORTED,
-          "Unable to autolaunch a dbus-daemon without DISPLAY set");
+          "Unable to autolaunch a dbus-daemon without a $DISPLAY for X11");
       return FALSE;
     }
 
@@ -3312,6 +3317,12 @@ _dbus_get_autolaunch_address (const char *scope,
  out:
   _dbus_string_free (&uuid);
   return retval;
+#else
+  dbus_set_error_const (error, DBUS_ERROR_NOT_SUPPORTED,
+      "Using X11 for dbus-daemon autolaunch was disabled at compile time, "
+      "set your DBUS_SESSION_BUS_ADDRESS instead");
+  return FALSE;
+#endif
 }
 
 /**