Fix deadlock happening when using Save Buffers for render

Summary:
Issue was caused by the same tile being written twice to
the EXR file. This was happening because of partial update
of work-in-progress tiles was merging result to the final
render result in order to make color management pipeline
happy.

We need to avoid such a merges and keep memory usage as
low as possible when Save Buffers is enabled.

Now render pipeline will allocate special display buffer
in render layer which will contain combined pass in the
display space. This keeps memory usage as low as we can
do at this moment.

There's one weak thing which is changing color management
settings during rendering would lead to lossy conversion.
This is because render result's display buffer uses color
space from the time when rendering was invoked.

This is actually what was happening in previous release
already actually so not a big issue.

Reviewers: brecht

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D162

3 files changed, 15 insertions, 2 deletions

diff --git a/source/blender/render/extern/include/RE_pipeline.h b/source/blender/render/extern/include/RE_pipeline.h
index 7e8713566e5..4c15ddd6833 100644
--- a/source/blender/render/extern/include/RE_pipeline.h
+++ b/source/blender/render/extern/include/RE_pipeline.h
@@ -91,6 +91,8 @@ typedef struct RenderLayer {
 	float *rectf;		/* 4 float, standard rgba buffer (read not above!) */
 	float *acolrect;	/* 4 float, optional transparent buffer, needs storage for display updates */
 	float *scolrect;	/* 4 float, optional strand buffer, needs storage for display updates */
+	int *display_buffer;	/* 4 char, optional color managed display buffer which is used when
+	                         * Save Buffer is enabled to display combined pass of the screen. */
 	int rectx, recty;
 
 	/* optional saved endresult on disk */
diff --git a/source/blender/render/intern/source/external_engine.c b/source/blender/render/intern/source/external_engine.c
index fa72d5f4057..b9c89f449a7 100644
--- a/source/blender/render/intern/source/external_engine.c
+++ b/source/blender/render/intern/source/external_engine.c
@@ -207,6 +207,11 @@ RenderResult *RE_engine_begin_result(RenderEngine *engine, int x, int y, int w,
 
 	result = render_result_new(re, &disprect, 0, RR_USE_MEM, layername);
 
+	/* Copy EXR tile settings, so pipeline knows whether this is a result
+	 * for Save Buffers enabled rendering.
+	 */
+	result->do_exr_tile = re->result->do_exr_tile;
+
 	/* todo: make this thread safe */
 
 	/* can be NULL if we CLAMP the width or height to 0 */
@@ -263,8 +268,11 @@ void RE_engine_end_result(RenderEngine *engine, RenderResult *result, int cancel
 	}
 
 	if (!cancel || merge_results) {
-		if (re->result->do_exr_tile)
-			render_result_exr_file_merge(re->result, result);
+		if (re->result->do_exr_tile) {
+			if (!cancel) {
+				render_result_exr_file_merge(re->result, result);
+			}
+		}
 		else if (!(re->test_break(re->tbh) && (re->r.scemode & R_BUTS_PREVIEW)))
 			render_result_merge(re->result, result);
 
diff --git a/source/blender/render/intern/source/render_result.c b/source/blender/render/intern/source/render_result.c
index 487de42515d..5e8b030c3fa 100644
--- a/source/blender/render/intern/source/render_result.c
+++ b/source/blender/render/intern/source/render_result.c
@@ -71,6 +71,7 @@ void render_result_free(RenderResult *res)
 		/* acolrect and scolrect are optionally allocated in shade_tile, only free here since it can be used for drawing */
 		if (rl->acolrect) MEM_freeN(rl->acolrect);
 		if (rl->scolrect) MEM_freeN(rl->scolrect);
+		if (rl->display_buffer) MEM_freeN(rl->display_buffer);
 		
 		while (rl->passes.first) {
 			RenderPass *rpass = rl->passes.first;
@@ -501,6 +502,8 @@ RenderResult *render_result_new(Render *re, rcti *partrct, int crop, int savebuf
 		rl->recty = recty;
 		
 		if (rr->do_exr_tile) {
+			rl->display_buffer = MEM_mapallocN(rectx * recty * sizeof(unsigned int), "Combined display space rgba");
+
 			rl->exrhandle = IMB_exr_get_handle();
 
 			IMB_exr_add_channel(rl->exrhandle, rl->name, "Combined.R", 0, 0, NULL);