infinite loop in CollapseEdge about manifold

[pca006132]

yes, I will add this after writing the fuzzing prototype

[elalish]

Thanks, I managed to simplify your first repro yesterday and I see where the problem is. Hopefully I'll have it fixed soon.

[elalish]

Thanks! Would you like to add this to mest_test? We can just put DISABLED_ in front of the name so the CI passes until we manage to debug it.

[pca006132]

Did some debugging with this (learning to use rr), it seems that current becomes -1 after executing line 248, and then just produce a cycle of 1, 37, 11, 18, 67, 21. I guess -1 means that the edge should be removed?

manifold/manifold/src/edge_op.cpp

Lines 245 to 249 in fadad77

	while (current != tri1edge[2]) {
	current = NextHalfedge(current);
	edges.push_back(current);
	current = halfedge[current].pairedHalfedge;
	}

[elalish]

Thanks! I'd say -1 generally means something went wrong (it should probably be asserted, ideally). I'll take a look; I usually have to draw out the topology on paper to figure out where my bad assumption is.

[pca006132]

it seems that the following change to master will also trigger this bug for Samples.Frame test:

diff --git a/manifold/src/impl.cpp b/manifold/src/impl.cpp
index 9b5513b..ea91a60 100644
--- a/manifold/src/impl.cpp
+++ b/manifold/src/impl.cpp
@@ -134,7 +134,7 @@ struct SwapHalfedges {
   Halfedge* halfedges;
   const TmpEdge* edges;
 
-  __host__ void operator()(int k) {
+  __host__ __device__ void operator()(int k) {
     const int i = 2 * k;
     const int j = i - 2;
     const TmpEdge thisEdge = edges[i];
@@ -519,10 +519,10 @@ void Manifold::Impl::CreateAndFixHalfedges(const VecDH<glm::ivec3>& triVerts) {
   // and fix it by swapping one of the identical edges, so it is important that
   // we have the edges paired according to their face.
   thrust::stable_sort(thrust::device, edge.begin(), edge.end());
-  thrust::for_each_n(thrust::host, countAt(0), halfedge_.size() / 2,
-                     LinkHalfedges({halfedge_.ptrH(), edge.cptrH()}));
-  thrust::for_each(thrust::host, countAt(1), countAt(halfedge_.size() / 2),
-                   SwapHalfedges({halfedge_.ptrH(), edge.cptrH()}));
+  thrust::for_each_n(thrust::device, countAt(0), halfedge_.size() / 2,
+                     LinkHalfedges({halfedge_.ptrD(), edge.cptrD()}));
+  thrust::for_each(thrust::device, countAt(1), countAt(halfedge_.size() / 2),
+                   SwapHalfedges({halfedge_.ptrD(), edge.cptrD()}));
 }