From fc460351170478e712740ae1917a2e24803eba3b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sybren=20A=2E=20St=C3=BCvel?= <sybren@blender.org>
Date: Thu, 9 Sep 2021 14:27:00 +0200
Subject: Depsgraph: release GIL when evaluating the depsgraph

Evaluating the dependency graph potentially executes Python code when
evaluating drivers. In specific situations (see T91046) this could deadlock
Blender entirely. Temporarily releasing the GIL when evaluating the depsgraph
resolves this.

Calling the `BPy_BEGIN_ALLOW_THREADS` macro is relatively safe, as it's
a no-op when the current thread does not have the GIL.

Developed in collaboration with @sergey

Manifest task: T91046
---
 source/blender/depsgraph/CMakeLists.txt          |  7 +++++++
 source/blender/depsgraph/intern/eval/deg_eval.cc | 13 +++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/source/blender/depsgraph/CMakeLists.txt b/source/blender/depsgraph/CMakeLists.txt
index 3ad26c6f4db..41253117096 100644
--- a/source/blender/depsgraph/CMakeLists.txt
+++ b/source/blender/depsgraph/CMakeLists.txt
@@ -161,6 +161,13 @@ set(LIB
   bf_blenkernel
 )
 
+if(WITH_PYTHON)
+  add_definitions(-DWITH_PYTHON)
+  list(APPEND INC
+    ../python
+  )
+endif()
+
 blender_add_lib(bf_depsgraph "${SRC}" "${INC}" "${INC_SYS}" "${LIB}")
 
 if(WITH_GTESTS)
diff --git a/source/blender/depsgraph/intern/eval/deg_eval.cc b/source/blender/depsgraph/intern/eval/deg_eval.cc
index ad88cf656ad..c816c7b8db5 100644
--- a/source/blender/depsgraph/intern/eval/deg_eval.cc
+++ b/source/blender/depsgraph/intern/eval/deg_eval.cc
@@ -41,6 +41,10 @@
 #include "DEG_depsgraph.h"
 #include "DEG_depsgraph_query.h"
 
+#ifdef WITH_PYTHON
+#  include "BPY_extern.h"
+#endif
+
 #include "atomic_ops.h"
 
 #include "intern/depsgraph.h"
@@ -375,6 +379,11 @@ void deg_evaluate_on_refresh(Depsgraph *graph)
 
   graph->debug.begin_graph_evaluation();
 
+#ifdef WITH_PYTHON
+  /* Release the GIL so that Python drivers can be evaluated. See T91046. */
+  BPy_BEGIN_ALLOW_THREADS;
+#endif
+
   graph->is_evaluating = true;
   depsgraph_ensure_view_layer(graph);
   /* Set up evaluation state. */
@@ -415,6 +424,10 @@ void deg_evaluate_on_refresh(Depsgraph *graph)
   deg_graph_clear_tags(graph);
   graph->is_evaluating = false;
 
+#ifdef WITH_PYTHON
+  BPy_END_ALLOW_THREADS;
+#endif
+
   graph->debug.end_graph_evaluation();
 }
 
-- 
cgit v1.2.3