Fix T52816: regression can't open file in 2.79 (crash).

Tentative fix, since I cannot reproduce thenissue for some reason here
on linux.

Core of the problem is pretty clear though, thanks to Germano Cavalcante
(@mano-wii): another thread could try to use looptris data after worker
one had allocated it, but before it had actually computed looptris.

So now, we use a temp 'wip' pointer to store looptris being computed
(since this is protected by a mutex, other threads will have to wait on
it, no possibility for them to double-compute the looptris here).

This should probably be backported to 2.79a if done.

1 files changed, 3 insertions, 1 deletions

diff --git a/source/blender/blenkernel/BKE_DerivedMesh.h b/source/blender/blenkernel/BKE_DerivedMesh.h
index e2577689101..700bfe568f1 100644
--- a/source/blender/blenkernel/BKE_DerivedMesh.h
+++ b/source/blender/blenkernel/BKE_DerivedMesh.h
@@ -193,7 +193,9 @@ struct DerivedMesh {
 	 * \warning Typical access is done via #getLoopTriArray, #getNumLoopTri.
 	 */
 	struct {
-		struct MLoopTri *array;
+		/* WARNING! swapping between array (ready-to-be-used data) and array_wip (where data is actually computed)
+		 *          shall always be protected by same lock as one used for looptris computing. */
+		struct MLoopTri *array, *array_wip;
 		int num;
 		int num_alloc;
 	} looptris;