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.

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;

source/blender/blenkernel/intern/DerivedMesh.c

@@ -494,6 +494,8 @@ 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)
 {
@@ -501,18 +503,22 @@ 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;
 		}
 

source/blender/blenkernel/intern/cdderivedmesh.c

@@ -1919,12 +1919,16 @@ void CDDM_recalc_looptri(DerivedMesh *dm)
 	const unsigned int totloop = dm->numLoopData;
 
 	DM_ensure_looptri_data(dm);
+	BLI_assert(cddm->dm.looptris.array_wip != NULL);
 
 	BKE_mesh_recalc_looptri(
 	        cddm->mloop, cddm->mpoly,
 	        cddm->mvert,
 	        totloop, totpoly,
-	        cddm->dm.looptris.array);
+	        cddm->dm.looptris.array_wip);
+
+	BLI_assert(cddm->dm.looptris.array == NULL);
+	SWAP(MLoopTri *, cddm->dm.looptris.array, cddm->dm.looptris.array_wip);
 }
 
 static void cdDM_free_internal(CDDerivedMesh *cddm)

source/blender/blenkernel/intern/editderivedmesh.c

@@ -641,8 +641,9 @@ static void emDM_recalcLoopTri(DerivedMesh *dm)
 	int i;
 
 	DM_ensure_looptri_data(dm);
-	mlooptri = dm->looptris.array;
+	mlooptri = dm->looptris.array_wip;
 
+	BLI_assert(mlooptri != NULL);
 	BLI_assert(poly_to_tri_count(dm->numPolyData, dm->numLoopData) == dm->looptris.num);
 	BLI_assert(tottri == dm->looptris.num);
 
@@ -659,6 +660,9 @@ static void emDM_recalcLoopTri(DerivedMesh *dm)
 		        BM_elem_index_get(ltri[2]));
 		lt->poly = BM_elem_index_get(ltri[0]->f);
 	}
+
+	BLI_assert(dm->looptris.array == NULL);
+	SWAP(MLoopTri *, dm->looptris.array, dm->looptris.array_wip);
 }
 
 static void emDM_foreachMappedVert(

source/blender/blenkernel/intern/subsurf_ccg.c

@@ -4482,8 +4482,9 @@ static void ccgDM_recalcLoopTri(DerivedMesh *dm)
 	int i, poly_index;
 
 	DM_ensure_looptri_data(dm);
-	mlooptri = dm->looptris.array;
+	mlooptri = dm->looptris.array_wip;
 
+	BLI_assert(mlooptri != NULL);
 	BLI_assert(poly_to_tri_count(dm->numPolyData, dm->numLoopData) == dm->looptris.num);
 	BLI_assert(tottri == dm->looptris.num);
 
@@ -4502,6 +4503,9 @@ static void ccgDM_recalcLoopTri(DerivedMesh *dm)
 		lt->tri[2] = (poly_index * 4) + 2;
 		lt->poly = poly_index;
 	}
+
+	BLI_assert(dm->looptris.array == NULL);
+	SWAP(MLoopTri *, dm->looptris.array, dm->looptris.array_wip);
 }
 
 static void ccgDM_calcNormals(DerivedMesh *dm)