[gc] add stop-the-world api (#42138)

It's sometimes useful to have a way for the runtime to stop the world.  Only works on SGen.

Co-authored-by: lambdageek <lambdageek@users.noreply.github.com>

src/mono/mono/metadata/boehm-gc.c

@@ -289,6 +289,18 @@ mono_gc_collection_count (int generation)
 	return GC_get_gc_no ();
 }
 
+void
+mono_gc_stop_world ()
+{
+	g_assert ("mono_gc_stop_world is not supported in Boehm");
+}
+
+void
+mono_gc_restart_world ()
+{
+	g_assert ("mono_gc_restart_world is not supported in Boehm");
+}
+
 /**
  * mono_gc_add_memory_pressure:
  * \param value amount of bytes

src/mono/mono/metadata/mono-gc.h

@@ -126,6 +126,12 @@ MONO_API int    mono_gc_walk_heap        (int flags, MonoGCReferences callback,
 MONO_API MONO_RT_EXTERNAL_ONLY void
 mono_gc_init_finalizer_thread (void);
 
+/*
+ * Only supported under SGen. These two with Sgen will take and release the LOCK_GC
+ */
+void mono_gc_stop_world (void);
+void mono_gc_restart_world (void);
+
 MONO_END_DECLS
 
 #endif /* __METADATA_MONO_GC_H__ */

src/mono/mono/metadata/null-gc.c

@@ -85,6 +85,18 @@ mono_gc_collection_count (int generation)
 	return 0;
 }
 
+void
+mono_gc_stop_world ()
+{
+	g_assert ("mono_gc_stop_world is not supported in null GC");
+}
+
+void
+mono_gc_restart_world ()
+{
+	g_assert ("mono_gc_restart_world is not supported in null GC");
+}
+
 void
 mono_gc_add_memory_pressure (gint64 value)
 {

src/mono/mono/metadata/sgen-mono.c

@@ -840,6 +840,20 @@ sgen_finish_concurrent_work (const char *reason, gboolean stw)
 	sgen_major_collector.finish_sweeping ();
 }
 
+void
+mono_gc_stop_world ()
+{
+	LOCK_GC;
+	sgen_stop_world (0, FALSE);
+}
+
+void
+mono_gc_restart_world ()
+{
+	sgen_restart_world (0, FALSE);
+	UNLOCK_GC;
+}
+
 /*
  * When appdomains are unloaded we can easily remove objects that have finalizers,
  * but all the others could still be present in random places on the heap.