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, 9 insertions, 3 deletions

diff --git a/source/blender/blenkernel/intern/DerivedMesh.c b/source/blender/blenkernel/intern/DerivedMesh.c
index 12d0c68f4d8..ace79f4125b 100644
--- a/source/blender/blenkernel/intern/DerivedMesh.c
+++ b/source/blender/blenkernel/intern/DerivedMesh.c
@@ -501,6 +501,8 @@ void DM_ensure_tessface(DerivedMesh *dm)
 
 /**
  * Ensure the array is large enough
+ *
+ * /note This function must always be thread-protected by caller. It should only be used by internal code.
  */
 void DM_ensure_looptri_data(DerivedMesh *dm)
 {
@@ -508,18 +510,22 @@ void DM_ensure_looptri_data(DerivedMesh *dm)
 	const unsigned int totloop = dm->numLoopData;
 	const int looptris_num = poly_to_tri_count(totpoly, totloop);
 
+	BLI_assert(dm->looptris.array_wip == NULL);
+
+	SWAP(MLoopTri *, dm->looptris.array, dm->looptris.array_wip);
+
 	if ((looptris_num > dm->looptris.num_alloc) ||
 	    (looptris_num < dm->looptris.num_alloc * 2) ||
 	    (totpoly == 0))
 	{
-		MEM_SAFE_FREE(dm->looptris.array);
+		MEM_SAFE_FREE(dm->looptris.array_wip);
 		dm->looptris.num_alloc = 0;
 		dm->looptris.num = 0;
 	}
 
 	if (totpoly) {
-		if (dm->looptris.array == NULL) {
-			dm->looptris.array = MEM_mallocN(sizeof(*dm->looptris.array) * looptris_num, __func__);
+		if (dm->looptris.array_wip == NULL) {
+			dm->looptris.array_wip = MEM_mallocN(sizeof(*dm->looptris.array_wip) * looptris_num, __func__);
 			dm->looptris.num_alloc = looptris_num;
 		}