[mutter] kms: Add assert to check that the main thread is blocked on impl task

[Jonas Ådahl <jadahl src gnome org>]

commit 35776c5d29c2e31a15ca44c0377722766b1b67db
Author: Jonas Ådahl <jadahl gmail com>
Date:   Tue Sep 3 19:12:10 2019 +0200

    kms: Add assert to check that the main thread is blocked on impl task
    
    This is so that we can have code in impl tasks that pokes at the main
    context objects.
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/743

 src/backends/native/meta-kms-private.h | 4 ++++
 src/backends/native/meta-kms.c         | 9 +++++++++
 2 files changed, 13 insertions(+)
---
diff --git a/src/backends/native/meta-kms-private.h b/src/backends/native/meta-kms-private.h
index fa632e1f7..4b6aaee7e 100644
--- a/src/backends/native/meta-kms-private.h
+++ b/src/backends/native/meta-kms-private.h
@@ -55,9 +55,13 @@ GSource * meta_kms_register_fd_in_impl (MetaKms             *kms,
 
 gboolean meta_kms_in_impl_task (MetaKms *kms);
 
+gboolean meta_kms_is_waiting_for_impl_task (MetaKms *kms);
+
 #define meta_assert_in_kms_impl(kms) \
   g_assert (meta_kms_in_impl_task (kms))
 #define meta_assert_not_in_kms_impl(kms) \
   g_assert (!meta_kms_in_impl_task (kms))
+#define meta_assert_is_waiting_for_kms_impl_task(kms) \
+  g_assert (meta_kms_is_waiting_for_impl_task (kms))
 
 #endif /* META_KMS_PRIVATE_H */
diff --git a/src/backends/native/meta-kms.c b/src/backends/native/meta-kms.c
index 296e63be8..91b260db3 100644
--- a/src/backends/native/meta-kms.c
+++ b/src/backends/native/meta-kms.c
@@ -154,6 +154,7 @@ struct _MetaKms
 
   MetaKmsImpl *impl;
   gboolean in_impl_task;
+  gboolean waiting_for_impl_task;
 
   GList *devices;
 
@@ -322,7 +323,9 @@ meta_kms_run_impl_task_sync (MetaKms              *kms,
   gboolean ret;
 
   kms->in_impl_task = TRUE;
+  kms->waiting_for_impl_task = TRUE;
   ret = func (kms->impl, user_data, error);
+  kms->waiting_for_impl_task = FALSE;
   kms->in_impl_task = FALSE;
 
   return ret;
@@ -440,6 +443,12 @@ meta_kms_in_impl_task (MetaKms *kms)
   return kms->in_impl_task;
 }
 
+gboolean
+meta_kms_is_waiting_for_impl_task (MetaKms *kms)
+{
+  return kms->waiting_for_impl_task;
+}
+
 static void
 meta_kms_update_states_in_impl (MetaKms *kms)
 {