Fix T49813: crash after changing Alembic cache topology.

Crash is due by mismatching loops and faces counts between the Alembic
data and the Blender derivedmesh which does not appear so
straightforward to fix (the crash happens deep in the derivedmesh code).

So for now, try to detect if the topology has changed and if so, both
only read vertices (vertex colors and UVs won't be read, as tied to face
loops) and add a warning message in the modifier's UI to let the user
know.

8 files changed, 47 insertions, 16 deletions

diff --git a/source/blender/alembic/intern/abc_curves.cc b/source/blender/alembic/intern/abc_curves.cc
index 7e5ea3b1853..4ecb9d944f2 100644
--- a/source/blender/alembic/intern/abc_curves.cc
+++ b/source/blender/alembic/intern/abc_curves.cc
@@ -361,7 +361,7 @@ void read_curve_sample(Curve *cu, const ICurvesSchema &schema, const float time)
  * object directly and create a new DerivedMesh from that. Also we might need to
  * create new or delete existing NURBS in the curve.
  */
-DerivedMesh *AbcCurveReader::read_derivedmesh(DerivedMesh */*dm*/, const float time, int /*read_flag*/)
+DerivedMesh *AbcCurveReader::read_derivedmesh(DerivedMesh */*dm*/, const float time, int /*read_flag*/, const char **/*err_str*/)
 {
 	ISampleSelector sample_sel(time);
 	const ICurvesSchema::Sample sample = m_curves_schema.getValue(sample_sel);
diff --git a/source/blender/alembic/intern/abc_curves.h b/source/blender/alembic/intern/abc_curves.h
index 979ee8af639..2757d179a47 100644
--- a/source/blender/alembic/intern/abc_curves.h
+++ b/source/blender/alembic/intern/abc_curves.h
@@ -56,7 +56,7 @@ public:
 	bool valid() const;
 
 	void readObjectData(Main *bmain, float time);
-	DerivedMesh *read_derivedmesh(DerivedMesh *, const float time, int);
+	DerivedMesh *read_derivedmesh(DerivedMesh *, const float time, int read_flag, const char **err_str);
 };
 
 /* ************************************************************************** */
diff --git a/source/blender/alembic/intern/abc_mesh.cc b/source/blender/alembic/intern/abc_mesh.cc
index 5b282e3c5bb..395c3e666f7 100644
--- a/source/blender/alembic/intern/abc_mesh.cc
+++ b/source/blender/alembic/intern/abc_mesh.cc
@@ -896,7 +896,7 @@ void AbcMeshReader::readObjectData(Main *bmain, float time)
 	const ISampleSelector sample_sel(time);
 
 	DerivedMesh *dm = CDDM_from_mesh(mesh);
-	DerivedMesh *ndm = this->read_derivedmesh(dm, time, MOD_MESHSEQ_READ_ALL);
+	DerivedMesh *ndm = this->read_derivedmesh(dm, time, MOD_MESHSEQ_READ_ALL, NULL);
 
 	if (ndm != dm) {
 		dm->release(dm);
@@ -978,7 +978,7 @@ CDStreamConfig get_config(DerivedMesh *dm)
 	return config;
 }
 
-DerivedMesh *AbcMeshReader::read_derivedmesh(DerivedMesh *dm, const float time, int read_flag)
+DerivedMesh *AbcMeshReader::read_derivedmesh(DerivedMesh *dm, const float time, int read_flag, const char **err_str)
 {
 	ISampleSelector sample_sel(time);
 	const IPolyMeshSchema::Sample sample = m_schema.getValue(sample_sel);
@@ -1003,6 +1003,21 @@ DerivedMesh *AbcMeshReader::read_derivedmesh(DerivedMesh *dm, const float time,
 
 		settings.read_flag |= MOD_MESHSEQ_READ_ALL;
 	}
+	else {
+		/* If the face count changed (e.g. by triangulation), only read points.
+		 * This prevents crash from T49813
+		 * TODO(kevin): perhaps find a better way to do this? */
+		if (face_counts->size() != dm->getNumPolys(dm) ||
+		    face_indices->size() != dm->getNumLoops(dm))
+		{
+			settings.read_flag = MOD_MESHSEQ_READ_VERT;
+
+			if (err_str) {
+				*err_str = "Topology has changed, perhaps by triangulating the"
+				           " mesh. Only vertices will be read!";
+			}
+		}
+	}
 
 	CDStreamConfig config = get_config(new_dm ? new_dm : dm);
 	config.time = time;
@@ -1177,7 +1192,7 @@ void AbcSubDReader::readObjectData(Main *bmain, float time)
 	m_object->data = mesh;
 
 	DerivedMesh *dm = CDDM_from_mesh(mesh);
-	DerivedMesh *ndm = this->read_derivedmesh(dm, time, MOD_MESHSEQ_READ_ALL);
+	DerivedMesh *ndm = this->read_derivedmesh(dm, time, MOD_MESHSEQ_READ_ALL, NULL);
 
 	if (ndm != dm) {
 		dm->release(dm);
@@ -1257,7 +1272,7 @@ void read_subd_sample(ImportSettings *settings,
 	/* TODO: face sets */
 }
 
-DerivedMesh *AbcSubDReader::read_derivedmesh(DerivedMesh *dm, const float time, int read_flag)
+DerivedMesh *AbcSubDReader::read_derivedmesh(DerivedMesh *dm, const float time, int read_flag, const char **err_str)
 {
 	ISampleSelector sample_sel(time);
 	const ISubDSchema::Sample sample = m_schema.getValue(sample_sel);
@@ -1281,6 +1296,21 @@ DerivedMesh *AbcSubDReader::read_derivedmesh(DerivedMesh *dm, const float time,
 
 		settings.read_flag |= MOD_MESHSEQ_READ_ALL;
 	}
+	else {
+		/* If the face count changed (e.g. by triangulation), only read points.
+		 * This prevents crash from T49813
+		 * TODO(kevin): perhaps find a better way to do this? */
+		if (face_counts->size() != dm->getNumPolys(dm) ||
+		    face_indices->size() != dm->getNumLoops(dm))
+		{
+			settings.read_flag = MOD_MESHSEQ_READ_VERT;
+
+			if (err_str) {
+				*err_str = "Topology has changed, perhaps by triangulating the"
+				           " mesh. Only vertices will be read!";
+			}
+		}
+	}
 
 	/* Only read point data when streaming meshes, unless we need to create new ones. */
 	CDStreamConfig config = get_config(new_dm ? new_dm : dm);
diff --git a/source/blender/alembic/intern/abc_mesh.h b/source/blender/alembic/intern/abc_mesh.h
index 66e6585a3d3..28d17105480 100644
--- a/source/blender/alembic/intern/abc_mesh.h
+++ b/source/blender/alembic/intern/abc_mesh.h
@@ -102,7 +102,7 @@ public:
 
 	void readObjectData(Main *bmain, float time);
 
-	DerivedMesh *read_derivedmesh(DerivedMesh *dm, const float time, int read_flag);
+	DerivedMesh *read_derivedmesh(DerivedMesh *dm, const float time, int read_flag, const char **err_str);
 
 private:
 	void readFaceSetsSample(Main *bmain, Mesh *mesh, size_t poly_start,
@@ -128,7 +128,7 @@ public:
 	bool valid() const;
 
 	void readObjectData(Main *bmain, float time);
-	DerivedMesh *read_derivedmesh(DerivedMesh *dm, const float time, int read_flag);
+	DerivedMesh *read_derivedmesh(DerivedMesh *dm, const float time, int read_flag, const char **err_str);
 };
 
 void read_subd_sample(ImportSettings *settings,
diff --git a/source/blender/alembic/intern/abc_object.h b/source/blender/alembic/intern/abc_object.h
index 7ff927b4d33..d1bcbbe6cbe 100644
--- a/source/blender/alembic/intern/abc_object.h
+++ b/source/blender/alembic/intern/abc_object.h
@@ -165,10 +165,11 @@ public:
 
 	virtual void readObjectData(Main *bmain, float time) = 0;
 
-	virtual DerivedMesh *read_derivedmesh(DerivedMesh *dm, const float time, int read_flag)
+	virtual DerivedMesh *read_derivedmesh(DerivedMesh *dm, const float time, int read_flag, const char **err_str)
 	{
 		(void)time;
 		(void)read_flag;
+		(void)err_str;
 		return dm;
 	}
 
diff --git a/source/blender/alembic/intern/abc_points.cc b/source/blender/alembic/intern/abc_points.cc
index c16da621c77..6ddba350b0a 100644
--- a/source/blender/alembic/intern/abc_points.cc
+++ b/source/blender/alembic/intern/abc_points.cc
@@ -156,7 +156,7 @@ void AbcPointsReader::readObjectData(Main *bmain, float time)
 	Mesh *mesh = BKE_mesh_add(bmain, m_data_name.c_str());
 
 	DerivedMesh *dm = CDDM_from_mesh(mesh);
-	DerivedMesh *ndm = this->read_derivedmesh(dm, time, 0);
+	DerivedMesh *ndm = this->read_derivedmesh(dm, time, 0, NULL);
 
 	if (ndm != dm) {
 		dm->release(dm);
@@ -199,7 +199,7 @@ void read_points_sample(const IPointsSchema &schema,
 	read_mverts(config.mvert, positions, vnormals);
 }
 
-DerivedMesh *AbcPointsReader::read_derivedmesh(DerivedMesh *dm, const float time, int /*read_flag*/)
+DerivedMesh *AbcPointsReader::read_derivedmesh(DerivedMesh *dm, const float time, int /*read_flag*/, const char **/*err_str*/)
 {
 	ISampleSelector sample_sel(time);
 	const IPointsSchema::Sample sample = m_schema.getValue(sample_sel);
diff --git a/source/blender/alembic/intern/abc_points.h b/source/blender/alembic/intern/abc_points.h
index 54873eed346..792283f04d3 100644
--- a/source/blender/alembic/intern/abc_points.h
+++ b/source/blender/alembic/intern/abc_points.h
@@ -61,7 +61,7 @@ public:
 
 	void readObjectData(Main *bmain, float time);
 
-	DerivedMesh *read_derivedmesh(DerivedMesh *dm, const float time, int read_flag);
+	DerivedMesh *read_derivedmesh(DerivedMesh *dm, const float time, int read_flag, const char **err_str);
 };
 
 void read_points_sample(const Alembic::AbcGeom::IPointsSchema &schema,
diff --git a/source/blender/alembic/intern/alembic_capi.cc b/source/blender/alembic/intern/alembic_capi.cc
index e690a255505..d0eb9900f4f 100644
--- a/source/blender/alembic/intern/alembic_capi.cc
+++ b/source/blender/alembic/intern/alembic_capi.cc
@@ -816,7 +816,7 @@ DerivedMesh *ABC_read_mesh(CacheReader *reader,
 			return NULL;
 		}
 
-		return abc_reader->read_derivedmesh(dm, time, read_flag);
+		return abc_reader->read_derivedmesh(dm, time, read_flag, err_str);
 	}
 	else if (ISubD::matches(header)) {
 		if (ob->type != OB_MESH) {
@@ -824,7 +824,7 @@ DerivedMesh *ABC_read_mesh(CacheReader *reader,
 			return NULL;
 		}
 
-		return abc_reader->read_derivedmesh(dm, time, read_flag);
+		return abc_reader->read_derivedmesh(dm, time, read_flag, err_str);
 	}
 	else if (IPoints::matches(header)) {
 		if (ob->type != OB_MESH) {
@@ -832,7 +832,7 @@ DerivedMesh *ABC_read_mesh(CacheReader *reader,
 			return NULL;
 		}
 
-		return abc_reader->read_derivedmesh(dm, time, read_flag);
+		return abc_reader->read_derivedmesh(dm, time, read_flag, err_str);
 	}
 	else if (ICurves::matches(header)) {
 		if (ob->type != OB_CURVE) {
@@ -840,7 +840,7 @@ DerivedMesh *ABC_read_mesh(CacheReader *reader,
 			return NULL;
 		}
 
-		return abc_reader->read_derivedmesh(dm, time, read_flag);
+		return abc_reader->read_derivedmesh(dm, time, read_flag, err_str);
 	}
 
 	*err_str = "Unsupported object type: verify object path"; // or poke developer