GPU: Make viewport not acquireable during rendering

This is a partial fix to the fact that rendering with EEVEE or other GL
render engines is currently blocking the whole UI when asking to redraw
a viewport.

This patch just bypasses the viewport bind (containing the Draw Context
lock) and the following drawing. There is an update tagging to not
loose a viewport update if there was one asked.

Other queries other than view redraw (such as selection depth drawing or
offscreen drawing) will still block the whole UI as they need immediate
data feedback.

Ping @Severin for the change in `WM_draw_region_viewport_bind()`.
I'm assuming this is not an issue because it's highly unlikely to
bring up this operator during rendering. But in this case, it would just
lock as usual.

The bypassing in `DRW_notify_view_update` might be a bit overparanoid.

2 files changed, 38 insertions, 17 deletions

diff --git a/source/blender/draw/intern/draw_manager.c b/source/blender/draw/intern/draw_manager.c
index ec3a5b3a7b1..88d05fcd928 100644
--- a/source/blender/draw/intern/draw_manager.c
+++ b/source/blender/draw/intern/draw_manager.c
@@ -1286,6 +1286,10 @@ void DRW_notify_view_update(const DRWUpdateContext *update_ctx)
 
   const bool gpencil_engine_needed = drw_gpencil_engine_needed(depsgraph, v3d);
 
+  if (G.is_rendering) {
+    return;
+  }
+
   /* XXX Really nasty locking. But else this could
    * be executed by the material previews thread
    * while rendering a viewport. */
diff --git a/source/blender/windowmanager/intern/wm_draw.c b/source/blender/windowmanager/intern/wm_draw.c
index 3c8474b1b6c..4b506564260 100644
--- a/source/blender/windowmanager/intern/wm_draw.c
+++ b/source/blender/windowmanager/intern/wm_draw.c
@@ -24,6 +24,7 @@
 #include "BLI_utildefines.h"
 
 #include "BKE_context.h"
+#include "BKE_global.h"
 #include "BKE_image.h"
 #include "BKE_main.h"
 #include "BKE_scene.h"
@@ -459,13 +460,24 @@ static void wm_draw_region_buffer_create(ARegion *region, bool stereo, bool use_
   }
 }
 
-static void wm_draw_region_bind(ARegion *region, int view)
+static bool wm_draw_region_bind(bContext *C, ARegion *region, int view)
 {
   if (!region->draw_buffer) {
-    return;
+    return true;
   }
 
   if (region->draw_buffer->viewport) {
+    if (G.is_rendering && C != NULL) {
+      Scene *scene = CTX_data_scene(C);
+      RenderEngineType *render_engine_type = RE_engines_find(scene->r.engine);
+      if (RE_engine_is_opengl(render_engine_type)) {
+        /* Do not try to acquire the viewport as this would be locking at the moment.
+         * But tag the viewport to update after the rendering finishes. */
+        GPU_viewport_tag_update(region->draw_buffer->viewport);
+        return false;
+      }
+    }
+
     GPU_viewport_bind(region->draw_buffer->viewport, view, &region->winrct);
   }
   else {
@@ -478,6 +490,7 @@ static void wm_draw_region_bind(ARegion *region, int view)
   }
 
   region->draw_buffer->bound_view = view;
+  return true;
 }
 
 static void wm_draw_region_unbind(ARegion *region)
@@ -700,9 +713,10 @@ static void wm_draw_window_offscreen(bContext *C, wmWindow *win, bool stereo)
             wm_draw_region_stereo_set(bmain, area, region, sview);
           }
 
-          wm_draw_region_bind(region, view);
-          ED_region_do_draw(C, region);
-          wm_draw_region_unbind(region);
+          if (wm_draw_region_bind(C, region, view)) {
+            ED_region_do_draw(C, region);
+            wm_draw_region_unbind(region);
+          }
         }
         if (use_viewport) {
           GPUViewport *viewport = region->draw_buffer->viewport;
@@ -711,9 +725,10 @@ static void wm_draw_window_offscreen(bContext *C, wmWindow *win, bool stereo)
       }
       else {
         wm_draw_region_buffer_create(region, false, use_viewport);
-        wm_draw_region_bind(region, 0);
-        ED_region_do_draw(C, region);
-        wm_draw_region_unbind(region);
+        if (wm_draw_region_bind(C, region, 0)) {
+          ED_region_do_draw(C, region);
+          wm_draw_region_unbind(region);
+        }
       }
 
       GPU_debug_group_end();
@@ -744,10 +759,11 @@ static void wm_draw_window_offscreen(bContext *C, wmWindow *win, bool stereo)
     }
 
     wm_draw_region_buffer_create(region, false, false);
-    wm_draw_region_bind(region, 0);
-    GPU_clear_color(0.0f, 0.0f, 0.0f, 0.0f);
-    ED_region_do_draw(C, region);
-    wm_draw_region_unbind(region);
+    if (wm_draw_region_bind(C, region, 0)) {
+      GPU_clear_color(0.0f, 0.0f, 0.0f, 0.0f);
+      ED_region_do_draw(C, region);
+      wm_draw_region_unbind(region);
+    }
 
     GPU_debug_group_end();
 
@@ -1102,10 +1118,11 @@ void wm_draw_region_test(bContext *C, ScrArea *area, ARegion *region)
   /* Function for redraw timer benchmark. */
   bool use_viewport = WM_region_use_viewport(area, region);
   wm_draw_region_buffer_create(region, false, use_viewport);
-  wm_draw_region_bind(region, 0);
-  ED_region_do_draw(C, region);
-  wm_draw_region_unbind(region);
-  region->do_draw = false;
+  if (wm_draw_region_bind(C, region, 0)) {
+    ED_region_do_draw(C, region);
+    wm_draw_region_unbind(region);
+    region->do_draw = false;
+  }
 }
 
 void WM_redraw_windows(bContext *C)
@@ -1141,7 +1158,7 @@ void WM_draw_region_viewport_ensure(ARegion *region, short space_type)
 
 void WM_draw_region_viewport_bind(ARegion *region)
 {
-  wm_draw_region_bind(region, 0);
+  wm_draw_region_bind(NULL, region, 0);
 }
 
 void WM_draw_region_viewport_unbind(ARegion *region)