Python/Context: do not allow any UI context access from threads like render

or baking. This basically means you will only have access to bpy.data and
bpy.context.scene, not current window, active object, etc, as those are not
thread safe anyway and were likely to cause issues already.

This fixes #30858, where the UI would lose buttons due to context getting
corrupted when editing objects in pre/post render or using luxrender. The
context access they did (indirectly) was only using the current scene or
data so they still work.

source/blender/blenkernel/intern/context.c

@@ -44,6 +44,7 @@
 
 #include "BLI_listbase.h"
 #include "BLI_string.h"
+#include "BLI_threads.h"
 #include "BLI_utildefines.h"
 
 #include "BKE_context.h"
@@ -245,6 +246,10 @@ static void *ctx_wm_python_context_get(const bContext *C, const char *member, vo
 	(void)C, (void)member;
 #endif
 
+	/* don't allow UI context access from non-main threads */
+	if (!BLI_thread_is_main())
+		return NULL;
+
 	return fall_through;
 }
 
@@ -264,6 +269,11 @@ static int ctx_data_get(bContext *C, const char *member, bContextDataResult *res
 //			return 1;
 	}
 #endif
+
+	/* don't allow UI context access from non-main threads */
+	if (!BLI_thread_is_main())
+		return done;
+
 	/* we check recursion to ensure that we do not get infinite
 	 * loops requesting data from ourselfs in a context callback */