[mutter/wip/carlosg/rt-scheduler: 1/2] core: Drop all capabilities before running the main loop

[Carlos Garnacho <carlosg src gnome org>]

commit b49640c4e40bf4566e14750383a619b421705bc0
Author: Carlos Garnacho <carlosg gnome org>
Date:   Wed Feb 27 18:58:52 2019 +0100

    core: Drop all capabilities before running the main loop
    
    Add an optional dependency on libcap-ng, if the library is detected
    drop all capabilities by default, in order to allow packagers/users
    to do "setcap CAP_SYS_NICE=+ep `which gnome-shell`" and let it set
    higher priorities it wouldn't be allowed to.
    
    Examples are: EGL_IMG_context_priority, SCHED_RR.
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/460

 config.h.meson    | 3 +++
 meson.build       | 5 +++++
 meson_options.txt | 6 ++++++
 src/core/main.c   | 9 +++++++++
 src/meson.build   | 1 +
 5 files changed, 24 insertions(+)
---
diff --git a/config.h.meson b/config.h.meson
index 70681d774..ee301752c 100644
--- a/config.h.meson
+++ b/config.h.meson
@@ -60,3 +60,6 @@
 
 /* Default rules for allowing Xwayland grabs */
 #mesondefine XWAYLAND_GRAB_DEFAULT_ACCESS_RULES
+
+/* Defined if libcap-ng is available */
+#mesondefine HAVE_LIBCAPNG
diff --git a/meson.build b/meson.build
index 7de4eba55..d42611a66 100644
--- a/meson.build
+++ b/meson.build
@@ -29,6 +29,7 @@ libstartup_notification_req = '>= 0.7'
 libcanberra_req = '>= 0.26'
 libwacom_req = '>= 0.13'
 atk_req = '>= 2.5.3'
+libcapng_req = '>= 0.7.9'
 
 # optional version requirements
 udev_req = '>= 228'
@@ -120,6 +121,7 @@ xinerama_dep = dependency('xinerama')
 ice_dep = dependency('ice')
 atk_dep = dependency('atk', version: atk_req)
 libcanberra_dep = dependency('libcanberra', version: libcanberra_req)
+libcapng_dep = dependency('libcap-ng', required: get_option('libcap'))
 
 # For now always require X11 support
 have_x11 = true
@@ -249,6 +251,7 @@ endif
 have_cogl_tests = get_option('cogl_tests')
 have_clutter_tests = get_option('clutter_tests')
 have_installed_tests = get_option('installed_tests')
+have_libcap = libcapng_dep.found()
 
 have_tests = get_option('tests')
 if have_tests
@@ -336,6 +339,7 @@ cdata.set('HAVE_LIBWACOM', have_libwacom)
 cdata.set('HAVE_SM', have_sm)
 cdata.set('HAVE_STARTUP_NOTIFICATION', have_startup_notification)
 cdata.set('HAVE_INTROSPECTION', have_introspection)
+cdata.set('HAVE_LIBCAPNG', have_libcap)
 
 xkb_base = xkeyboard_config_dep.get_pkgconfig_variable('xkb_base')
 cdata.set_quoted('XKB_BASE', xkb_base)
@@ -401,6 +405,7 @@ output = [
   '        SM....................... ' + have_sm.to_string(),
   '        Startup notification..... ' + have_startup_notification.to_string(),
   '        Introspection............ ' + have_introspection.to_string(),
+  '        libcap................... ' + have_libcap.to_string(),
   '',
   '    Tests:',
   '',
diff --git a/meson_options.txt b/meson_options.txt
index add49c866..9f58feba6 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -140,3 +140,9 @@ option('xwayland_grab_default_access_rules',
   value: 'gnome-boxes,remote-viewer,virt-viewer,virt-manager,vinagre,vncviewer,Xephyr',
   description: 'Comma delimited list of applications ressources or class allowed to issue X11 grabs in 
Xwayland'
 )
+
+option('libcap',
+  type: 'feature',
+  value: 'auto',
+  description: 'Enable libcap support'
+)
diff --git a/src/core/main.c b/src/core/main.c
index e8464720f..898f49f47 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -66,6 +66,10 @@
 #include <girepository.h>
 #endif
 
+#ifdef HAVE_LIBCAPNG
+#include <cap-ng.h>
+#endif
+
 #if defined(HAVE_NATIVE_BACKEND) && defined(HAVE_WAYLAND)
 #include <systemd/sd-login.h>
 #endif /* HAVE_WAYLAND && HAVE_NATIVE_BACKEND */
@@ -680,6 +684,11 @@ meta_run (void)
   if (!meta_display_open ())
     meta_exit (META_EXIT_ERROR);
 
+#ifdef HAVE_LIBCAPNG
+  capng_clear (CAPNG_SELECT_BOTH);
+  capng_apply (CAPNG_SELECT_BOTH);
+#endif
+
   g_main_loop_run (meta_main_loop);
 
   meta_finalize ();
diff --git a/src/meson.build b/src/meson.build
index 8779c956e..75cfc22a6 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -18,6 +18,7 @@ mutter_pkg_deps = [
   gsettings_desktop_schemas_dep,
   gtk3_dep,
   pango_dep,
+  libcapng_dep,
 ]
 
 mutter_pkg_private_deps = [